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 <H1><a name="Ruby">35 SWIG and Ruby</a></H1>
 <!-- INDEX -->
 <div class="sectiontoc">
 <ul>
 <li><a href="#Ruby_nn2">Preliminaries</a>
 <ul>
 <li><a href="#Ruby_nn3">Running SWIG</a>
 <li><a href="#Ruby_nn4">Getting the right header files</a>
 <li><a href="#Ruby_nn5">Compiling a dynamic module</a>
 <li><a href="#Ruby_nn6">Using your module</a>
 <li><a href="#Ruby_nn7">Static linking</a>
 <li><a href="#Ruby_nn8">Compilation of C++ extensions</a>
 </ul>
 <li><a href="#Ruby_nn9">Building Ruby Extensions under Windows 95/NT</a>
 <ul>
 <li><a href="#Ruby_nn10">Running SWIG from Developer Studio</a>
 </ul>
 <li><a href="#Ruby_nn11">The Ruby-to-C/C++ Mapping</a>
 <ul>
 <li><a href="#Ruby_nn12">Modules</a>
 <li><a href="#Ruby_nn13">Functions</a>
 <li><a href="#Ruby_nn14">Variable Linking</a>
 <li><a href="#Ruby_nn15">Constants</a>
 <li><a href="#Ruby_nn16">Pointers</a>
 <li><a href="#Ruby_nn17">Structures</a>
 <li><a href="#Ruby_nn18">C++ classes</a>
 <li><a href="#Ruby_nn19">C++ Inheritance</a>
 <li><a href="#Ruby_nn20">C++ Overloaded Functions</a>
 <li><a href="#Ruby_nn21">C++ Operators</a>
 <li><a href="#Ruby_nn22">C++ namespaces</a>
 <li><a href="#Ruby_nn23">C++ templates</a>
 <li><a href="#Ruby_nn23_1">C++ Standard Template Library (STL)</a>
 <li><a href="#Ruby_C_STL_Functors">C++ STL Functors</a>
 <li><a href="#Ruby_C_Iterators">C++ STL Iterators</a>
 <li><a href="#Ruby_nn24">C++ Smart Pointers</a>
 <ul>
 <li><a href="#Ruby_smart_pointers_shared_ptr">The shared_ptr Smart Pointer</a>
 <li><a href="#Ruby_smart_pointers_generic">Generic Smart Pointers</a>
 </ul>
 <li><a href="#Ruby_nn25">Cross-Language Polymorphism</a>
 <ul>
 <li><a href="#Ruby_nn26">Exception Unrolling</a>
 </ul>
 </ul>
 <li><a href="#Ruby_nn27">Naming</a>
 <ul>
 <li><a href="#Ruby_nn28">Defining Aliases</a>
 <li><a href="#Ruby_nn29">Predicate Methods</a>
 <li><a href="#Ruby_nn30">Bang Methods</a>
 <li><a href="#Ruby_nn31">Getters and Setters</a>
 </ul>
 <li><a href="#Ruby_nn32">Input and output parameters</a>
 <li><a href="#Ruby_nn33">Exception handling </a>
 <ul>
 <li><a href="#Ruby_nn34">Using the %exception directive </a>
 <li><a href="#Ruby_nn34_2">Handling Ruby Blocks </a>
 <li><a href="#Ruby_nn35">Raising exceptions </a>
 <li><a href="#Ruby_nn36">Exception classes </a>
 </ul>
 <li><a href="#Ruby_nn37">Typemaps</a>
 <ul>
 <li><a href="#Ruby_nn38">What is a typemap?</a>
 <li><a href="#Ruby_Typemap_scope">Typemap scope</a>
 <li><a href="#Ruby_Copying_a_typemap">Copying a typemap</a>
 <li><a href="#Ruby_Deleting_a_typemap">Deleting a typemap</a>
 <li><a href="#Ruby_Placement_of_typemaps">Placement of typemaps</a>
 <li><a href="#Ruby_nn39">Ruby typemaps</a>
 <ul>
 <li><a href="#Ruby_in_typemap">"in" typemap</a>
 <li><a href="#Ruby_typecheck_typemap">"typecheck" typemap</a>
 <li><a href="#Ruby_out_typemap">"out" typemap</a>
 <li><a href="#Ruby_arginit_typemap">"arginit" typemap</a>
 <li><a href="#Ruby_default_typemap">"default" typemap</a>
 <li><a href="#Ruby_check_typemap">"check" typemap</a>
 <li><a href="#Ruby_argout_typemap_">"argout" typemap</a>
 <li><a href="#Ruby_freearg_typemap_">"freearg" typemap</a>
 <li><a href="#Ruby_newfree_typemap">"newfree" typemap</a>
 <li><a href="#Ruby_memberin_typemap">"memberin" typemap</a>
 <li><a href="#Ruby_varin_typemap">"varin" typemap</a>
 <li><a href="#Ruby_varout_typemap_">"varout" typemap</a>
 <li><a href="#Ruby_throws_typemap">"throws" typemap</a>
 <li><a href="#Ruby_directorin_typemap">directorin typemap</a>
 <li><a href="#Ruby_directorout_typemap">directorout typemap</a>
 <li><a href="#Ruby_directorargout_typemap">directorargout typemap</a>
 <li><a href="#Ruby_ret_typemap">ret typemap</a>
 <li><a href="#Ruby_globalin_typemap">globalin typemap</a>
 </ul>
 <li><a href="#Ruby_nn40">Typemap variables</a>
 <li><a href="#Ruby_nn41">Useful Functions</a>
 <ul>
 <li><a href="#Ruby_nn42">C Datatypes to Ruby Objects</a>
 <li><a href="#Ruby_nn43">Ruby Objects to C Datatypes</a>
 <li><a href="#Ruby_nn44">Macros for VALUE</a>
 <li><a href="#Ruby_nn45">Exceptions</a>
 <li><a href="#Ruby_nn46">Iterators</a>
 </ul>
 <li><a href="#Ruby_nn47">Typemap Examples</a>
 <li><a href="#Ruby_nn48">Converting a Ruby array to a char **</a>
 <li><a href="#Ruby_nn49">Collecting arguments in a hash</a>
 <li><a href="#Ruby_nn50">Pointer handling</a>
 <ul>
 <li><a href="#Ruby_nn51">Ruby Datatype Wrapping</a>
 </ul>
 <li><a href="#Ruby_nn52">Example: STL Vector to Ruby Array</a>
 </ul>
 <li><a href="#Ruby_nn65">Docstring Features</a>
 <ul>
 <li><a href="#Ruby_nn66">Module docstring</a>
 <li><a href="#Ruby_nn67">%feature("autodoc")</a>
 <ul>
 <li><a href="#Ruby_nn68">%feature("autodoc", "0")</a>
 <li><a href="#Ruby_autodoc1">%feature("autodoc", "1")</a>
 <li><a href="#Ruby_autodoc2">%feature("autodoc", "2")</a>
 <li><a href="#Ruby_feature_autodoc3">%feature("autodoc", "3")</a>
 <li><a href="#Ruby_nn70">%feature("autodoc", "docstring")</a>
 </ul>
 <li><a href="#Ruby_nn71">%feature("docstring")</a>
 </ul>
 <li><a href="#Ruby_nn53">Advanced Topics</a>
 <ul>
 <li><a href="#Ruby_operator_overloading">Operator overloading</a>
 <li><a href="#Ruby_nn55">Creating Multi-Module Packages</a>
 <li><a href="#Ruby_nn56">Specifying Mixin Modules</a>
 </ul>
 <li><a href="#Ruby_nn57">Memory Management</a>
 <ul>
 <li><a href="#Ruby_nn58">Mark and Sweep Garbage Collector </a>
 <li><a href="#Ruby_nn59">Object Ownership</a>
 <li><a href="#Ruby_nn60">Object Tracking</a>
 <li><a href="#Ruby_nn61">Mark Functions</a>
 <li><a href="#Ruby_nn62">Free Functions</a>
 <li><a href="#Ruby_nn63">Embedded Ruby and the C++ Stack</a>
 </ul>
 </ul>
 </div>
 <!-- INDEX -->


 <p>This chapter describes SWIG's support of Ruby.</p>

 <H2><a name="Ruby_nn2">35.1 Preliminaries</a></H2>


 <p> SWIG 4.0 is known to work with Ruby versions 1.9 and later.
 Given the choice, you should use the latest stable version of Ruby. You
 should also determine if your system supports shared libraries and
 dynamic loading. SWIG will work with or without dynamic loading, but
 the compilation process will vary. </p>

 <p>This chapter covers most SWIG features, but in less depth than
 is found in earlier chapters. At the very least, make sure you also
 read the "<a href="SWIG.html#SWIG">SWIG Basics</a>"
 chapter. It is also assumed that the reader has a basic understanding
 of Ruby. </p>

 <H3><a name="Ruby_nn3">35.1.1 Running SWIG</a></H3>


 <p> To build a Ruby module, run SWIG using the <tt>-ruby</tt>
 option:</p>

 <div class="code shell">
 <pre>$ swig -ruby example.i
 </pre>
 </div>

 <p> If building a C++ extension, add the <tt>-c++</tt>
 option: </p>

 <div class="code shell">
 <pre>$ swig -c++ -ruby example.i
 </pre>
 </div>

 <p> This creates a file <tt>example_wrap.c</tt> (<tt>example_wrap.cxx</tt>
 if compiling a C++ extension) that contains all of the code needed to
 build a Ruby extension module. To finish building the module, you need
 to compile this file and link it with the rest of your program. </p>

 <H3><a name="Ruby_nn4">35.1.2 Getting the right header files</a></H3>


 <p> In order to compile the wrapper code, the compiler needs the <tt>ruby.h</tt>
 header file and its dependencies, notably <tt>ruby/config.h</tt> which is
 found in a different, architecture-dependent, directory. The best way to find
 the compiler options needed to compile the code is to ask Ruby itself:</p>

 <div class="code shell">
 <pre>$ ruby -rrbconfig -e 'puts "-I#{RbConfig::CONFIG[%q{rubyhdrdir}]} -I#{RbConfig::CONFIG[%q{rubyarchhdrdir}]}"'
 -I/usr/include/ruby-2.1.0 -I/usr/include/x86_64-linux-gnu/ruby-2.1.0
 </pre>
 </div>

 <H3><a name="Ruby_nn5">35.1.3 Compiling a dynamic module</a></H3>


 <p> Ruby extension modules are typically compiled into shared
 libraries that the interpreter loads dynamically at runtime. Since the
 exact commands for doing this vary from platform to platform, your best
 bet is to follow the steps described in the <tt>README.EXT</tt>
 file from the Ruby distribution: </p>

 <ol>
   <li>
     <p>Create a file called <tt>extconf.rb</tt> that
 looks like the following:</p>
     <div class="code targetlang">
     <pre>require 'mkmf'
 create_makefile('example')</pre>
     </div>
   </li>
   <li>
     <p>Type the following to build the extension:</p>
     <div class="code shell">
     <pre>
 $ ruby extconf.rb
 $ make
 $ make install
     </pre>
     </div>
   </li>
 </ol>

 <p> Of course, there is the problem that mkmf does not work
 correctly on all platforms, e.g, HPUX. If you need to add your own make
 rules to the file that <tt>extconf.rb</tt> produces, you
 can add this: </p>

 <div class="code targetlang">
 <pre>open("Makefile", "a") { |mf|
  puts &lt;&lt;EOM
  # Your make rules go here
  EOM
 }
 </pre>
 </div>

 <p> to the end of the <tt>extconf.rb</tt> file. If
 for some reason you don't want to use the standard approach, you'll
 need to determine the correct compiler and linker flags for your build
 platform. For example, assuming you have code you need to link to in a file
 called <tt>example.c</tt>, a typical sequence of commands for the Linux
 operating system would look something like this: </p>

 <div class="code shell">
 <pre>$ swig -ruby example.i
 $ gcc -O2 -fPIC -c example.c
 $ gcc -O2 -fPIC -c example_wrap.c -I/usr/include/ruby-2.1.0
 $ gcc -shared example.o example_wrap.o -o example.so
 </pre>
 </div>

 <p>
 The -fPIC option tells GCC to generate position-independent code (PIC)
 which is required for most architectures (it's not vital on x86, but
 still a good idea as it allows code pages from the library to be shared between
 processes).  Other compilers may need a different option specified instead of
 -fPIC.
 </p>

 <p>
 If in doubt, consult the
 manual pages for your compiler and linker to determine the correct set
 of options. You might also check the <a href="https://github.com/swig/swig/wiki">SWIG Wiki</a>
 for additional information. </p>

 <H3><a name="Ruby_nn6">35.1.4 Using your module</a></H3>


 <p> Ruby <i>module</i> names must be capitalized,
 but the convention for Ruby <i>feature</i> names is to use
 lowercase names. So, for example, the <b>Etc</b> extension
 module is imported by requiring the <b>etc</b> feature: </p>

 <div class="code targetlang">
 <pre># The feature name begins with a lowercase letter...
 require 'etc'

 # ... but the module name begins with an uppercase letter
 puts "Your login name: #{Etc.getlogin}"
 </pre>
 </div>

 <p> To stay consistent with this practice, you should always
 specify a <b>lowercase</b> module name with SWIG's <tt>%module</tt>
 directive. SWIG will automatically correct the resulting Ruby module
 name for your extension. So for example, a SWIG interface file that
 begins with: </p>

 <div class="code">
 <pre>%module example</pre>
 </div>

 <p> will result in an extension module using the feature name
 "example" and Ruby module name "Example". </p>

 <H3><a name="Ruby_nn7">35.1.5 Static linking</a></H3>


 <p> An alternative approach to dynamic linking is to rebuild the
 Ruby interpreter with your extension module added to it. In the past,
 this approach was sometimes necessary due to limitations in dynamic
 loading support on certain machines. However, the situation has
 improved greatly over the last few years and you should not consider
 this approach unless there is really no other option. </p>

 <p>The usual procedure for adding a new module to Ruby involves
 finding the Ruby source, adding an entry to the <tt>ext/Setup</tt>
 file, adding your directory to the list of extensions in the file, and
 finally rebuilding Ruby. </p>

 <H3><a name="Ruby_nn8">35.1.6 Compilation of C++ extensions</a></H3>


 <p> On most machines, C++ extension modules should be linked
 using the C++ compiler. For example: </p>

 <div class="code shell">
 <pre>
 $ swig -c++ -ruby example.i
 $ g++ -fPIC -c example.cxx
 $ g++ -fPIC -c example_wrap.cxx -I/usr/include/ruby-2.1.0
 $ g++ -shared example.o example_wrap.o -o example.so
 </pre>
 </div>

 <p> If you've written an <tt>extconf.rb</tt> script
 to automatically generate a <tt>Makefile</tt> for your C++
 extension module, keep in mind that (as of this writing) Ruby still
 uses <tt>gcc</tt> and not <tt>g++</tt> as its
 linker. As a result, the required C++ runtime library support will not
 be automatically linked into your extension module and it may fail to
 load on some platforms. A workaround for this problem is use the <tt>mkmf</tt>
 module's <tt>append_library()</tt> method to add one of
 the C++ runtime libraries to the list of libraries linked into your
 extension, e.g. </p>

 <div class="code targetlang">
 <pre>require 'mkmf'
 $libs = append_library($libs, "supc++")
 create_makefile('example')</pre>
 </div>

 <H2><a name="Ruby_nn9">35.2 Building Ruby Extensions under Windows 95/NT</a></H2>


 <p> Building a SWIG extension to Ruby under Windows 95/NT is
 roughly similar to the process used with Unix. Normally, you will want
 to produce a DLL that can be loaded into the Ruby interpreter. For all
 recent versions of Ruby, the procedure described above (i.e. using an <tt>extconf.rb</tt>
 script) will work with Windows as well; you should be able to build
 your code into a DLL by typing: </p>

 <div class="code shell">
 <pre>
 C:\swigtest&gt; ruby extconf.rb
 C:\swigtest&gt; nmake
 C:\swigtest&gt; nmake install
 </pre>
 </div>

 <p> The remainder of this section covers the process of compiling
 SWIG-generated Ruby extensions with Microsoft Visual C++ 6 (i.e. within
 the Developer Studio IDE, instead of using the command line tools). In
 order to build extensions, you may need to download the source
 distribution to the Ruby package, as you will need the Ruby header
 files. </p>

 <H3><a name="Ruby_nn10">35.2.1 Running SWIG from Developer Studio</a></H3>


 <p> If you are developing your application within Microsoft
 developer studio, SWIG can be invoked as a custom build option. The
 process roughly follows these steps : </p>
 <ul>
   <li> Open up a new workspace and use the AppWizard to select a
 DLL project. </li>
   <li> Add both the SWIG interface file (the .i file), any
 supporting C files, and the name of the wrapper file that will be
 created by SWIG (i.e. <tt>example_wrap.c</tt>). Note : If
 using C++, choose a different suffix for the wrapper file such as <tt>example_wrap.cxx</tt>.
 Don't worry if the wrapper file doesn't exist yet--Developer Studio
 will keep a reference to it around. </li>
   <li> Select the SWIG interface file and go to the settings
 menu. Under settings, select the "Custom Build" option. </li>
   <li> Enter "SWIG" in the description field. </li>
   <li> Enter "<tt>swig -ruby -o
 $(ProjDir)\$(InputName)_wrap.c $(InputPath)</tt>" in the "Build
 command(s) field". You may have to include the path to swig.exe. </li>
   <li> Enter "<tt>$(ProjDir)\$(InputName)_wrap.c</tt>"
 in the "Output files(s) field". </li>
   <li> Next, select the settings for the entire project and go to
 the C/C++ tab and select the Preprocessor category. Add NT=1 to the
 Preprocessor definitions. This must be set else you will get
 compilation errors. Also add IMPORT to the preprocessor definitions,
 else you may get runtime errors. Also add the include directories for
 your Ruby installation under "Additional include directories". </li>
   <li> Next, select the settings for the entire project and go to
 the Link tab and select the General category. Set the name of the
 output file to match the name of your Ruby module (i.e.. example.dll).
 Next add the Ruby library file to your link libraries under
 Object/Library modules. For example "mswin32-ruby16.lib. You also need
 to add the path to the library under the Input tab - Additional library
 path. </li>
   <li> Build your project. </li>
 </ul>

 <p> Now, assuming all went well, SWIG will be automatically
 invoked when you build your project. Any changes made to the interface
 file will result in SWIG being automatically invoked to produce a new
 version of the wrapper file. To run your new Ruby extension, simply run
 Ruby and use the <tt>require</tt> command as normal. For
 example if you have this ruby file run.rb:</p>

 <div class="code targetlang">
 <pre># file: run.rb
 require 'Example'

 # Call a c function
 print "Foo = ", Example.Foo, "\n"</pre>
 </div>

 <p> Ensure the dll just built is in your path or current
 directory, then run the Ruby script from the DOS/Command prompt: </p>

 <div class="code shell">
 <pre>
 C:\swigtest&gt; ruby run.rb
 Foo = 3.0
 </pre>
 </div>

 <H2><a name="Ruby_nn11">35.3 The Ruby-to-C/C++ Mapping</a></H2>


 <p> This section describes the basics of how SWIG maps C or C++
 declarations in your SWIG interface files to Ruby constructs. </p>

 <H3><a name="Ruby_nn12">35.3.1 Modules</a></H3>


 <p> The SWIG <tt>%module</tt> directive specifies
 the name of the Ruby module. If you specify: </p>

 <div class="code">
 <pre>%module example</pre>
 </div>

 <p> then everything is wrapped into a Ruby module named <tt>Example</tt>
 that is nested directly under the global module. You can specify a more
 deeply nested module by specifying the fully-qualified module name in
 quotes, e.g. </p>

 <div class="code">
 <pre>%module "foo::bar::spam"</pre>
 </div>

 <p> An alternate method of specifying a nested module name is to
 use the <tt>-prefix</tt>
 option on the SWIG command line. The prefix that you specify with this
 option will be prepended to the module name specified with the <tt>%module</tt>
 directive in your SWIG interface file. So for example, this declaration
 at the top of your SWIG interface file:
 </p>

 <div class="code">
 <pre>%module "foo::bar::spam"</pre>
 </div>

 <p> will result in a nested module name of <tt>Foo::Bar::Spam</tt>,
 but you can achieve the <span style="font-style: italic;">same</span>
 effect by specifying:
 </p>

 <div class="code">
 <pre>%module spam</pre>
 </div>

 <p> and then running SWIG with the <tt>-prefix</tt> command
 line option:
 </p>

 <div class="code shell">
 <pre>
 $ swig -ruby -prefix "foo::bar::" example.i
 </pre>
 </div>

 <p> Starting with SWIG 1.3.20, you can also choose to wrap
 everything into the global module by specifying the <tt>-globalmodule</tt>
 option on the SWIG command line, i.e. </p>

 <div class="code shell">
 <pre>
 $ swig -ruby -globalmodule example.i
 </pre>
 </div>

 <p> Note that this does not relieve you of the requirement of
 specifying the SWIG module name with the <tt>%module</tt>
 directive (or the <tt>-module</tt> command-line option) as
 described earlier. </p>

 <p>When choosing a module name, do not use the same name as a
 built-in Ruby command or standard module name, as the results may be
 unpredictable. Similarly, if you're using the <tt>-globalmodule</tt>
 option to wrap everything into the global module, take care that the
 names of your constants, classes and methods don't conflict with any of
 Ruby's built-in names. </p>

 <H3><a name="Ruby_nn13">35.3.2 Functions</a></H3>


 <p> Global functions are wrapped as Ruby module methods. For
 example, given the SWIG interface file <tt>example.i</tt>:
 </p>

 <div class="code">
 <pre>%module example

 int fact(int n);</pre>
 </div>

 <p> and C source file <tt>example.c</tt>: </p>

 <div class="code">
 <pre>int fact(int n) {
   if (n == 0)
   return 1;
   return (n * fact(n-1));
 }</pre>
 </div>

 <p> SWIG will generate a method <i>fact</i> in the <i>Example</i>
 module that can be used like so: </p>

 <div class="code targetlang">
 <pre>$ <b>irb</b>
 irb(main):001:0&gt; <b>require 'example'</b>
 true
 irb(main):002:0&gt; <b>Example.fact(4)</b>
 24</pre>
 </div>

 <H3><a name="Ruby_nn14">35.3.3 Variable Linking</a></H3>


 <p> C/C++ global variables are wrapped as a pair of singleton
 methods for the module: one to get the value of the global variable and
 one to set it. For example, the following SWIG interface file declares
 two global variables: </p>

 <div class="code">
 <pre>// SWIG interface file with global variables
 %module example
 ...
 %inline %{
   extern int variable1;
   extern double Variable2;
 %}
 ...</pre>
 </div>

 <p> Now look at the Ruby interface:</p>

 <div class="code targetlang">
 <pre>$ <b>irb</b>
 irb(main):001:0&gt; <b>require 'Example'</b>
 true
 irb(main):002:0&gt; <b>Example.variable1 = 2</b>
 2
 irb(main):003:0&gt; <b>Example.Variable2 = 4 * 10.3</b>
 41.2
 irb(main):004:0&gt; <b>Example.Variable2</b>
 41.2</pre>
 </div>

 <p> If you make an error in variable assignment, you will receive
 an error message. For example: </p>

 <div class="code targetlang">
 <pre>irb(main):005:0&gt; <b>Example.Variable2 = "hello"</b>
 TypeError: no implicit conversion to float from string
 from (irb):5:in `Variable2='
 from (irb):5</pre>
 </div>

 <p> If a variable is declared as <tt>const</tt>, it
 is wrapped as a read-only variable. Attempts to modify its value will
 result in an error. </p>

 <p>To make ordinary variables read-only, you can also use the <tt>%immutable</tt>
 directive. For example: </p>

 <div class="code">
 <pre>%immutable;
 %inline %{
   extern char *path;
 %}
 %mutable;</pre>
 </div>

 <p> The <tt>%immutable</tt> directive stays in
 effect until it is explicitly disabled using <tt>%mutable</tt>.
 </p>

 <p>Note: When SWIG is invoked with the <tt>-globalmodule</tt> option in
 effect, the C/C++ global variables will be translated into Ruby global
 variables. Type-checking and the optional read-only characteristic are
 available in the same way as described above. However the example would
 then have to be modified and executed in the following way:

 <div class="code targetlang">
 <pre>$ <b>irb</b>
 irb(main):001:0&gt; <b>require 'Example'</b>
 true
 irb(main):002:0&gt; <b>$variable1 = 2</b>
 2
 irb(main):003:0&gt; <b>$Variable2 = 4 * 10.3</b>
 41.2
 irb(main):004:0&gt; <b>$Variable2</b>
 41.2</pre>
 </div>

 <H3><a name="Ruby_nn15">35.3.4 Constants</a></H3>


 <p> C/C++ constants are wrapped as module constants initialized
 to the appropriate value. To create a constant, use <tt>#define</tt>
 or the <tt>%constant</tt> directive. For example: </p>

 <div class="code">
 <pre>#define PI 3.14159
 #define VERSION "1.0"

 %constant int FOO = 42;
 %constant const char *path = "/usr/local";

 const int BAR = 32;</pre>
 </div>

 <p> Remember to use the :: operator in Ruby to get at these
 constant values, e.g. </p>

 <div class="code targetlang">
 <pre>$ <b>irb</b>
 irb(main):001:0&gt; <b>require 'Example'</b>
 true
 irb(main):002:0&gt; <b>Example::PI</b>
 3.14159</pre>
 </div>

 <H3><a name="Ruby_nn16">35.3.5 Pointers</a></H3>


 <p> "Opaque" pointers to arbitrary C/C++ types (i.e. types that
 aren't explicitly declared in your SWIG interface file) are wrapped as
 data objects. So, for example, consider a SWIG interface file
 containing only the declarations: </p>

 <div class="code">
 <pre>Foo *get_foo();
 void set_foo(Foo *foo);</pre>
 </div>

 <p> For this case, the <i>get_foo()</i> method
 returns an instance of an internally generated Ruby class: </p>

 <div class="code targetlang">
 <pre>irb(main):001:0&gt; <b>foo = Example::get_foo()</b>
 #&lt;SWIG::TYPE_p_Foo:0x402b1654&gt;</pre>
 </div>

 <p> A <tt>NULL</tt> pointer is always represented by
 the Ruby <tt>nil</tt> object. </p>

 <H3><a name="Ruby_nn17">35.3.6 Structures</a></H3>


 <p> C/C++ structs are wrapped as Ruby classes, with accessor
 methods (i.e. "getters" and "setters") for all of the struct members.
 For example, this struct declaration: </p>

 <div class="code">
 <pre>struct Vector {
   double x, y;
 };</pre>
 </div>

 <p> gets wrapped as a <tt>Vector</tt> class, with
 Ruby instance methods <tt>x</tt>, <tt>x=</tt>,
 <tt>y</tt> and <tt>y=</tt>. These methods can
 be used to access structure data from Ruby as follows: </p>

 <div class="code targetlang">
 <pre>$ <b>irb</b>
 irb(main):001:0&gt; <b>require 'Example'</b>
 true
 irb(main):002:0&gt; <b>f = Example::Vector.new</b>
 #&lt;Example::Vector:0x4020b268&gt;
 irb(main):003:0&gt; <b>f.x = 10</b>
 nil
 irb(main):004:0&gt; <b>f.x</b>
 10.0</pre>
 </div>

 <p> Similar access is provided for unions and the public data
 members of C++ classes.</p>

 <p><tt>const</tt> members of a structure are
 read-only. Data members can also be forced to be read-only using the <tt>%immutable</tt>
 directive (in C++, <tt>private</tt> may also be used). For
 example: </p>

 <div class="code">
 <pre>struct Foo {
   ...
   %immutable;
   int x; /* Read-only members */
   char *name;
   %mutable;
   ...
 };</pre>
 </div>

 <p> When <tt>char *</tt> members of a structure are
 wrapped, the contents are assumed to be dynamically allocated using <tt>malloc</tt>
 or <tt>new</tt> (depending on whether or not SWIG is run
 with the <tt>-c++</tt> option). When the structure member
 is set, the old contents will be released and a new value created. If
 this is not the behavior you want, you will have to use a typemap
 (described shortly). </p>

 <p>Array members are normally wrapped as read-only. For example,
 this code: </p>

 <div class="code">
 <pre>struct Foo {
   int x[50];
 };</pre>
 </div>

 <p> produces a single accessor function like this: </p>

 <div class="code">
 <pre>int *Foo_x_get(Foo *self) {
   return self-&gt;x;
 };</pre>
 </div>

 <p> If you want to set an array member, you will need to supply a
 "memberin" typemap described in the <a href="#Ruby_memberin_typemap">section on typemaps</a>.
 As a special case, SWIG does generate code to set array members of type
 <tt>char</tt> (allowing you to store a Ruby string in the
 structure). </p>

 <p>When structure members are wrapped, they are handled as
 pointers. For example, </p>

 <div class="code">
 <pre>struct Foo {
   ...
 };

 struct Bar {
   Foo f;
 };</pre>
 </div>

 <p> generates accessor functions such as this: </p>

 <div class="code">
 <pre>Foo *Bar_f_get(Bar *b) {
   return &amp;b-&gt;f;
 }

 void Bar_f_set(Bar *b, Foo *val) {
   b-&gt;f = *val;
 }</pre>
 </div>

 <H3><a name="Ruby_nn18">35.3.7 C++ classes</a></H3>


 <p> Like structs, C++ classes are wrapped by creating a new Ruby
 class of the same name with accessor methods for the public class
 member data. Additionally, public member functions for the class are
 wrapped as Ruby instance methods, and public static member functions
 are wrapped as Ruby singleton methods. So, given the C++ class
 declaration: </p>

 <div class="code">
 <pre>class List {
 public:
   List();
   ~List();
   int search(char *item);
   void insert(char *item);
   void remove(char *item);
   char *get(int n);
   int length;
   static void print(List *l);
 };</pre>
 </div>

 <p> SWIG would create a <tt>List</tt> class with: </p>

 <ul>
   <li> instance methods <i>search</i>, <i>insert</i>,
     <i>remove</i>, and <i>get</i>; </li>
   <li> instance methods <i>length</i> and <i>length=</i>
 (to get and set the value of the <i>length</i> data
 member); and, </li>
   <li> a <i>print</i> singleton method for the
 class. </li>
 </ul>

 <p> In Ruby, these functions are used as follows: </p>

 <div class="code targetlang">
 <pre>require 'Example'

 l = Example::List.new

 l.insert("Ale")
 l.insert("Stout")
 l.insert("Lager")
 Example.print(l)
 l.length()
 ----- produces the following output
 Lager
 Stout
 Ale
 3</pre>
 </div>

 <H3><a name="Ruby_nn19">35.3.8 C++ Inheritance</a></H3>


 <p> The SWIG type-checker is fully aware of C++ inheritance.
 Therefore, if you have classes like this: </p>

 <div class="code">
 <pre>class Parent {
   ...
 };

 class Child : public Parent {
   ...
 };</pre>
 </div>

 <p> those classes are wrapped into a hierarchy of Ruby classes
 that reflect the same inheritance structure. All of the usual Ruby
 utility methods work normally: </p>

 <div class="code">
 <pre>irb(main):001:0&gt; <b>c = Child.new</b>
 #&lt;Bar:0x4016efd4&gt;
 irb(main):002:0&gt; <b>c.instance_of? Child</b>
 true
 irb(main):003:0&gt; <b>b.instance_of? Parent</b>
 false
 irb(main):004:0&gt; <b>b.is_a? Child</b>
 true
 irb(main):005:0&gt; <b>b.is_a? Parent</b>
 true
 irb(main):006:0&gt; <b>Child &lt; Parent</b>
 true
 irb(main):007:0&gt; <b>Child &gt; Parent</b>
 false</pre>
 </div>

 <p> Furthermore, if you have a function like this: </p>

 <div class="code">
 <pre>void spam(Parent *f);</pre>
 </div>

 <p> then the function <tt>spam()</tt> accepts <tt>Parent</tt>*
 or a pointer to any class derived from <tt>Parent</tt>. </p>

 <p>Until recently, the Ruby module for SWIG didn't support
 multiple inheritance, and this is still the default behavior. This
 doesn't mean that you can't wrap C++ classes which inherit from
 multiple base classes; it simply means that only the <b>first</b>
 base class listed in the class declaration is considered, and any
 additional base classes are ignored. As an example, consider a SWIG
 interface file with a declaration like this: </p>

 <div class="code">
 <pre>class Derived : public Base1, public Base2
 {
   ...
 };</pre>
 </div>

 <p> For this case, the resulting Ruby class (<tt>Derived</tt>)
 will only consider <tt>Base1</tt> as its superclass. It
 won't inherit any of <tt>Base2</tt>'s member functions or
 data and it won't recognize <tt>Base2</tt> as an
 "ancestor" of <tt>Derived</tt> (i.e. the <em>is_a?</em>
 relationship would fail). When SWIG processes this interface file,
 you'll see a warning message like: </p>

 <div class="code shell">
 <pre>example.i:5: Warning 802: Warning for Derived: Base Base2 ignored.
 Multiple inheritance is not supported in Ruby.</pre>
 </div>

 <p> Starting with SWIG 1.3.20, the Ruby module for SWIG provides
 limited support for multiple inheritance. Because the approach for
 dealing with multiple inheritance introduces some limitations, this is
 an optional feature that you can activate with the <tt>-minherit</tt>
 command-line option: </p>

 <div class="code shell">
 <pre>
 $ swig -c++ -ruby -minherit example.i
 </pre>
 </div>

 <p> Using our previous example, if your SWIG interface file
 contains a declaration like this: </p>

 <div class="code">
 <pre>class Derived : public Base1, public Base2
 {
   ...
 };</pre>
 </div>

 <p> and you run SWIG with the <tt>-minherit</tt>
 command-line option, then you will end up with a Ruby class <tt>Derived</tt>
 that appears to "inherit" the member data and functions from both <tt>Base1</tt>
 and <tt>Base2</tt>. What actually happens is that three
 different top-level classes are created, with Ruby's <tt>Object</tt>
 class as their superclass. Each of these classes defines a nested
 module named <tt>Impl</tt>, and it's in these nested <tt>Impl</tt>
 modules that the actual instance methods for the classes are defined,
 i.e. </p>

 <div class="code targetlang">
 <pre>class Base1
   module Impl
   # Define Base1 methods here
   end
   include Impl
 end

 class Base2
   module Impl
   # Define Base2 methods here
   end
   include Impl
 end

 class Derived
   module Impl
   include Base1::Impl
   include Base2::Impl
   # Define Derived methods here
   end
   include Impl
 end</pre>
 </div>

 <p> Observe that after the nested <tt>Impl</tt>
 module for a class is defined, it is mixed-in to the class itself. Also
 observe that the <tt>Derived::Impl</tt> module first
 mixes-in its base classes' <tt>Impl</tt> modules, thus
 "inheriting" all of their behavior. </p>

 <p>The primary drawback is that, unlike the default mode of
 operation, neither <tt>Base1</tt> nor <tt>Base2</tt>
 is a true superclass of <tt>Derived</tt> anymore: </p>

 <div class="code targetlang">
 <pre>obj = Derived.new
 obj.is_a? Base1 # this will return false...
 obj.is_a? Base2 # ... and so will this</pre>
 </div>

 <p> In most cases, this is not a serious problem since objects of
 type <tt>Derived</tt> will otherwise behave as though they
 inherit from both <tt>Base1</tt> and <tt>Base2</tt>
 (i.e. they exhibit <a href="http://c2.com/cgi/wiki?DuckTyping">"Duck
 Typing"</a>). </p>

 <H3><a name="Ruby_nn20">35.3.9 C++ Overloaded Functions</a></H3>


 <p> C++ overloaded functions, methods, and constructors are
 mostly supported by SWIG. For example, if you have two functions like
 this: </p>

 <div class="code">
 <pre>void foo(int);
 void foo(char *c);</pre>
 </div>

 <p> You can use them in Ruby in a straightforward manner: </p>

 <div class="code targetlang">
 <pre>irb(main):001:0&gt; <b>foo(3)</b> # foo(int)
 irb(main):002:0&gt; <b>foo("Hello")</b> # foo(char *c)</pre>
 </div>

 <p>Similarly, if you have a class like this, </p>

 <div class="code">
 <pre>class Foo {
 public:
   Foo();
   Foo(const Foo &amp;);
   ...
 };</pre>
 </div>

 <p>you can write Ruby code like this:</p>

 <div class="code targetlang">
 <pre>irb(main):001:0&gt; <b>f = Foo.new</b> # Create a Foo
 irb(main):002:0&gt; <b>g = Foo.new(f)</b> # Copy f</pre>
 </div>

 <p> Overloading support is not quite as flexible as in C++.
 Sometimes there are methods that SWIG can't disambiguate. For example: </p>

 <div class="code">
 <pre>void spam(int);
 void spam(short);</pre>
 </div>

 <p>or</p>

 <div class="code">
 <pre>void foo(Bar *b);
 void foo(Bar &amp;b);</pre>
 </div>

 <p> If declarations such as these appear, you will get a warning
 message like this: </p>

 <div class="code shell">
 <pre>
 example.i:12: Warning 509: Overloaded method spam(short) effectively ignored,
 example.i:11: Warning 509: as it is shadowed by spam(int).
 </pre>
 </div>

 <p> To fix this, you either need to ignore or rename one of the
 methods. For example: </p>

 <div class="code">
 <pre>%rename(spam_short) spam(short);
 ...
 void spam(int);
 void spam(short); // Accessed as spam_short</pre>
 </div>

 <p>or</p>

 <div class="code">
 <pre>%ignore spam(short);
 ...
 void spam(int);
 void spam(short); // Ignored</pre>
 </div>

 <p> SWIG resolves overloaded functions and methods using a
 disambiguation scheme that ranks and sorts declarations according to a
 set of type-precedence rules. The order in which declarations appear in
 the input does not matter except in situations where ambiguity
 arises--in this case, the first declaration takes precedence. </p>

 <p>Please refer to the <a href="SWIGPlus.html#SWIGPlus">"SWIG
 and C++"</a> chapter for more information about overloading. </p>

 <H3><a name="Ruby_nn21">35.3.10 C++ Operators</a></H3>


 <p> For the most part, overloaded operators are handled
 automatically by SWIG and do not require any special treatment on your
 part. So if your class declares an overloaded addition operator, e.g. </p>

 <div class="code">
 <pre>class Complex {
   ...
   Complex operator+(Complex &amp;);
   ...
 };</pre>
 </div>

 <p> the resulting Ruby class will also support the addition (+)
 method correctly. </p>

 <p>For cases where SWIG's built-in support is not sufficient, C++
 operators can be wrapped using the <tt>%rename</tt>
 directive (available on SWIG 1.3.10 and later releases). All you need
 to do is give the operator the name of a valid Ruby identifier. For
 example: </p>

 <div class="code">
 <pre>%rename(add_complex) operator+(Complex &amp;, Complex &amp;);
 ...
 Complex operator+(Complex &amp;, Complex &amp;);</pre>
 </div>

 <p>Now, in Ruby, you can do this:</p>

 <div class="code targetlang">
 <pre>a = Example::Complex.new(2, 3)
 b = Example::Complex.new(4, -1)
 c = Example.add_complex(a, b)</pre>
 </div>

 <p> More details about wrapping C++ operators into Ruby operators
 is discussed in the <a href="#Ruby_operator_overloading">section
 on operator overloading</a>. </p>

 <H3><a name="Ruby_nn22">35.3.11 C++ namespaces</a></H3>


 <p> SWIG is aware of C++ namespaces, but namespace names do not
 appear in the module nor do namespaces result in a module that is
 broken up into submodules or packages. For example, if you have a file
 like this, </p>

 <div class="code">
 <pre>%module example

 namespace foo {
   int fact(int n);
   struct Vector {
     double x, y, z;
   };
 };</pre>
 </div>

 <p>it works in Ruby as follows:</p>

 <div class="code targetlang">
 <pre>irb(main):001:0&gt; <b>require 'example'</b>
 true
 irb(main):002:0&gt; <b>Example.fact(3)</b>
 6
 irb(main):003:0&gt; <b>v = Example::Vector.new</b>
 #&lt;Example::Vector:0x4016f4d4&gt;
 irb(main):004:0&gt; <b>v.x = 3.4</b>
 3.4
 irb(main):004:0&gt; <b>v.y</b>
 0.0</pre>
 </div>

 <p> If your program has more than one namespace, name conflicts
 (if any) can be resolved using <tt>%rename</tt> For
 example: </p>

 <div class="code">
 <pre>%rename(Bar_spam) Bar::spam;

 namespace Foo {
   int spam();
 }

 namespace Bar {
   int spam();
 }</pre>
 </div>

 <p> If you have more than one namespace and your want to keep
 their symbols separate, consider wrapping them as separate SWIG
 modules. For example, make the module name the same as the namespace
 and create extension modules for each namespace separately. If your
 program utilizes thousands of small deeply nested namespaces each with
 identical symbol names, well, then you get what you deserve. </p>

 <H3><a name="Ruby_nn23">35.3.12 C++ templates</a></H3>


 <p> C++ templates don't present a huge problem for SWIG. However,
 in order to create wrappers, you have to tell SWIG to create wrappers
 for a particular template instantiation. To do this, you use the <tt>%template</tt>
 directive. For example: </p>

 <div class="code">
 <pre>%module example

 %{
 #include "pair.h"
 %}

 template&lt;class T1, class T2&gt;
 struct pair {
   typedef T1 first_type;
   typedef T2 second_type;
   T1 first;
   T2 second;
   pair();
   pair(const T1&amp;, const T2&amp;);
   ~pair();
 };

 %template(Pairii) pair&lt;int, int&gt;;</pre>
 </div>

 <p>In Ruby:</p>

 <div class="code targetlang">
 <pre>irb(main):001:0&gt; <b>require 'example'</b>
 true
 irb(main):002:0&gt; <b>p = Example::Pairii.new(3, 4)</b>
 #&lt;Example:Pairii:0x4016f4df&gt;
 irb(main):003:0&gt; <b>p.first</b>
 3
 irb(main):004:0&gt; <b>p.second</b>
 4</pre>
 </div>

 <H3><a name="Ruby_nn23_1">35.3.13 C++ Standard Template Library (STL)</a></H3>


 <p> On a related note, the standard SWIG library contains a
 number of modules that provide typemaps for standard C++ library
 classes (such as <tt>std::pair</tt>, <tt>std::string</tt>
 and <tt>std::vector</tt>). These library modules don't
 provide wrappers around the templates themselves, but they do make it
 convenient for users of your extension module to pass Ruby objects
 (such as arrays and strings) to wrapped C++ code that expects instances
 of standard C++ templates. For example, suppose the C++ library you're
 wrapping has a function that expects a vector of floats: </p>

 <div class="code">
 <pre>%module example

 float sum(const std::vector&lt;float&gt;&amp; values);</pre>
 </div>

 <p> Rather than go through the hassle of writing an "in" typemap
 to convert an array of Ruby numbers into a
 std::vector&lt;float&gt;, you can just use the <tt>std_vector.i</tt>
 module from the standard SWIG library: </p>

 <div class="code">
 <pre>%module example

 %include std_vector.i
 float sum(const std::vector&lt;float&gt;&amp; values);</pre>
 </div>

 <p>Ruby's STL wrappings provide additional methods to make them
 behave more similarly to Ruby's native classes.</p>

 <p>Thus, you can do, for example:</p>

 <div class="targetlang">
 <pre>v = IntVector.new
 v &lt;&lt; 2
 v &lt;&lt; 3
 v &lt;&lt; 4
 v.each { |x| puts x }

 =&gt; 2
 3
 4
 v.delete_if { |x| x == 3 }
 =&gt; [2, 4]</pre>
 </div>

 <p>The SWIG Ruby module provides also the ability for all the STL
 containers to carry around Ruby native objects (Fixnum, Classes, etc)
 making them act almost like Ruby's own Array, Hash, etc.  To
 do
 that, you need to define a container that contains a swig::GC_VALUE,
 like:</p>

 <div class="code"><pre>
 %module nativevector

 %{
 std::vector&lt; swig::GC_VALUE &gt; NativeVector;
 %}

 %template(NativeVector) std::vector&lt; swig::GC_VALUE &gt;;
 </pre>
 </div>

 <p>This vector can then contain any Ruby object, making them
 almost identical to Ruby's own Array class.</p>

 <div class="targetlang">
 <pre>require 'nativevector'
 include NativeVector

 v = NativeVector.new
 v &lt;&lt; 1
 v &lt;&lt; [1, 2]
 v &lt;&lt; 'hello'

 class A; end

 v &lt;&lt; A.new

 puts v
 =&gt; [1, [1, 2], 'hello', #&lt;A:0x245325&gt;]
 </pre>
 </div>

 <p>Obviously, there is a lot more to template wrapping than
 shown in these examples. More details can be found in the <a href="SWIGPlus.html#SWIGPlus">SWIG and C++</a>
 chapter.</p>

 <H3><a name="Ruby_C_STL_Functors">35.3.14 C++ STL Functors</a></H3>


 <p>Some containers in the STL allow you to modify their default
 behavior by using so called functors or function objects.
 Functors are often just a very simple struct with <tt>operator()</tt>
 redefined or an actual C/C++ function. This allows you, for
 example, to always keep the sort order of a STL container to your
 liking.</p>

 <p>The Ruby STL mappings allows you to modify those containers
 that
 support functors using Ruby procs or methods, instead.
 Currently,
 this includes <tt>std::set</tt>,
 <tt>set::map</tt>,
 <tt>std::multiset</tt>
 and <tt>std::multimap</tt>.</p>

 <p>The functors in swig are called <tt>swig::UnaryFunction</tt>
 and <tt>swig::BinaryFunction</tt>.

 For C++ predicates (ie. functors that must return bool as a result) <tt>swig::UnaryPredicate</tt>
 and <tt>swig::BinaryPredicate</tt>
 are provided.</p>

 <p>As an example, if given this swig file:</p>

 <div class="code"><pre>
 %module intset;

 %include &lt;std_set.i&gt;

 %template(IntSet) std::set&lt; int, swig::BinaryPredicate &gt;;
 </pre></div>

 <p>You can then use the set from Ruby with or without a proc
 object as a predicate:</p>

 <div class="targetlang"><pre>
 require 'intset'
 include Intset

 # Default sorting behavior defined in C++
 a = IntSet.new
 a &lt;&lt; 1
 a &lt;&lt; 2
 a &lt;&lt; 3
 a
 <b>=&gt; [1, 2, 3]</b>

 # Custom sorting behavior defined by a Ruby proc
 b = IntSet.new( proc { |a, b| a &gt; b } )
 b &lt;&lt; 1
 b &lt;&lt; 2
 b &lt;&lt; 3
 b
 <b>=&gt;  [3, 2, 1]</b>
 </pre>
 </div>

 <H3><a name="Ruby_C_Iterators">35.3.15 C++ STL Iterators</a></H3>


 <p>The STL is well known for the use of iterators. There
 are a number of iterators possible with different properties, but in
 general there are two main categories: const iterators and non-const
 iterators. The const iterators can access and not modify the
 values they point at, while the non-const iterators can both read and
 modify the values.</p>

 <p>The Ruby STL wrappings support both type of iterators by using
 a proxy class in-between. This proxy class is <tt>swig::Iterator</tt> or
 <tt>swig::ConstIterator</tt>. Derived from them are template
 classes that need to be initialized with the actual iterator for the
 container you are wrapping and often times with the beginning and
 ending points of the iteration range.</p>

 <p>The SWIG STL library already provides typemaps to all the
 standard containers to do this wrapping automatically for you, but if
 you have your own STL-like iterator, you will need to write your own
 typemap for them. For out typemaps, the special functions <tt>make_const_iterator</tt> and <tt>make_nonconst_iterator</tt> are provided.</p>

 <p>These can be used either like:</p>

 <div class="code"><pre>
 make_const_iterator( iterator, rubyclass );
 make_const_iterator( iterator, iterator_begin, iterator_end, rubyclass );
 </pre></div>

 <p>The iterators support a <tt>next()</tt> and <tt>previous()</tt> member function to
 just change the iterator without returning anything. <tt>previous()</tt>
 should obviously only be used for bidirectional iterators. You
 can also advance the iterator multiple steps by using standard math
 operations like <tt>+=</tt>.</p>

 <p>The
 value the iterator points at can be accessed with <tt>value()</tt> -- this is equivalent to dereferencing it with <tt>*i</tt>.
   For non-const iterators, a <tt>value=()</tt> function
 is also provided which allows you to change the value pointed by the
 iterator.  This is equivalent to the C++ construct of dereferencing and assignment, like <tt>*i = something</tt>. </p>

 <p>Thus, given say a vector class of doubles defined as:</p>

 <div class="code">
 <pre>
 %module doublevector

 %include std_vector.i

 %template(DoubleVector) std::vector&lt;double&gt;;
 </pre>
 </div>

 <p>Its iterator can then be used from Ruby like:</p>

 <div class="targetlang">
 <pre>
 require 'doublevector'
 include Doublevector

 v = DoubleVector.new
 v &lt;&lt; 1
 v &lt;&lt; 2
 v &lt;&lt; 3

 #
 # an elaborate and less efficient way of doing v.map! { |x| x+2 }
 #
 i = v.begin
 e = v.end
 while i != e
   val = i.value
   val += 2
   i.value = val
   i.next
 end
 i
 <b>&gt;&gt; [3, 4, 5 ]</b>
 </pre>
 </div>

 <p>If you'd rather have STL classes without any iterators, you should define <tt>-DSWIG_NO_EXPORT_ITERATOR_METHODS</tt> when running swig.</p>

 <H3><a name="Ruby_nn24">35.3.16 C++ Smart Pointers</a></H3>


 <H4><a name="Ruby_smart_pointers_shared_ptr">35.3.16.1 The shared_ptr Smart Pointer</a></H4>


 <p>
 The C++11 standard provides <tt>std::shared_ptr</tt> which was derived from the Boost
 implementation, <tt>boost::shared_ptr</tt>.
 Both of these are available for Ruby in the SWIG library and usage is outlined
 in the <a href="Library.html#Library_std_shared_ptr">shared_ptr smart pointer</a> library section.
 </p>


 <H4><a name="Ruby_smart_pointers_generic">35.3.16.2 Generic Smart Pointers</a></H4>


 <p> In certain C++ programs, it is common to use classes that
 have been wrapped by so-called "smart pointers." Generally, this
 involves the use of a template class that implements <tt>operator-&gt;()</tt>
 like this: </p>

 <div class="code">
 <pre>template&lt;class T&gt; class SmartPtr {
   ...
   T *operator-&gt;();
   ...
 }</pre>
 </div>

 <p>Then, if you have a class like this, </p>

 <div class="code">
 <pre>class Foo {
 public:
   int x;
   int bar();
 };</pre>
 </div>

 <p>A smart pointer would be used in C++ as follows:</p>

 <div class="code">
 <pre>SmartPtr&lt;Foo&gt; p = CreateFoo(); // Created somehow (not shown)
 ...
 p-&gt;x = 3; // Foo::x
 int y = p-&gt;bar(); // Foo::bar</pre>
 </div>

 <p> To wrap this in Ruby, simply tell SWIG about the <tt>SmartPtr</tt>
 class and the low-level <tt>Foo</tt> object. Make sure you
 instantiate <tt>SmartPtr</tt> using <tt>%template</tt>
 if necessary. For example: </p>

 <div class="code">
 <pre>%module example
 ...
 %template(SmartPtrFoo) SmartPtr&lt;Foo&gt;;
 ...</pre>
 </div>

 <p>Now, in Ruby, everything should just "work":</p>

 <div class="code targetlang">
 <pre>irb(main):001:0&gt; <b>p = Example::CreateFoo()</b> # Create a smart-pointer somehow
 #&lt;Example::SmartPtrFoo:0x4016f4df&gt;
 irb(main):002:0&gt; <b>p.x = 3</b> # Foo::x
 3
 irb(main):003:0&gt; <b>p.bar()</b> # Foo::bar</pre>
 </div>

 <p> If you ever need to access the underlying pointer returned by
 <tt>operator-&gt;()</tt> itself, simply use the <tt>__deref__()</tt>
 method. For example: </p>

 <div class="code targetlang">
 <pre>irb(main):004:0&gt; <b>f = p.__deref__()</b> # Returns underlying Foo *</pre>
 </div>

 <H3><a name="Ruby_nn25">35.3.17 Cross-Language Polymorphism</a></H3>


 <p> SWIG's Ruby module supports cross-language polymorphism
 (a.k.a. the "directors" feature) similar to that for SWIG's Python
 module. Rather than duplicate the information presented in the <a href="Python.html#Python">Python</a> chapter, this
 section just notes the differences that you need to be aware of when
 using this feature with Ruby. </p>

 <H4><a name="Ruby_nn26">35.3.17.1 Exception Unrolling</a></H4>


 <p> Whenever a C++ director class routes one of its virtual
 member function calls to a Ruby instance method, there's always the
 possibility that an exception will be raised in the Ruby code. By
 default, those exceptions are ignored, which simply means that the
 exception will be exposed to the Ruby interpreter. If you would like to
 change this behavior, you can use the <tt>%feature("director:except")</tt>
 directive to indicate what action should be taken when a Ruby exception
 is raised. The following code should suffice in most cases: </p>

 <div class="code">
 <pre>%feature("director:except") {
   throw Swig::DirectorMethodException($error);
 }</pre>
 </div>

 <p> When this feature is activated, the call to the Ruby instance
 method is "wrapped" using the <tt>rb_rescue2()</tt>
 function from Ruby's C API. If any Ruby exception is raised, it will be
 caught here and a C++ exception is raised in its place. </p>

 <H2><a name="Ruby_nn27">35.4 Naming</a></H2>


 <p>Ruby has several common naming conventions. Constants are
 generally
 in upper case, module and class names are in camel case and methods are
 in lower case with underscores. For example: </p>

 <div class="code">
 <ul>
   <li><strong>MATH::PI</strong> is a constant name</li>
   <li><strong>MyClass</strong> is a class name</li>
   <li><strong>my_method</strong> is a method name</li>
 </ul>
 </div>

 <p>Prior to version 1.3.28, SWIG did not support these Ruby
 conventions. The only modifications it made to names was to capitalize
 the first letter of constants (which includes module and class names).</p>

 <p>SWIG 1.3.28 introduces the new -autorename command line
 parameter.
 When this parameter is specified, SWIG will automatically change
 constant, class and method names to conform with the standard Ruby
 naming conventions. For example: </p>

 <div class="code shell">
 <pre>$ swig -ruby -autorename example.i
 </pre>
 </div>

 <p>To disable renaming use the -noautorename command line option.</p>

 <p>Since this change significantly changes the wrapper code
 generated
 by SWIG, it is turned off by default in SWIG 1.3.28. However, it is
 planned to become the default option in future releases.</p>

 <H3><a name="Ruby_nn28">35.4.1 Defining Aliases</a></H3>


 <p> It's a fairly common practice in the Ruby built-ins and
 standard library to provide aliases for method names. For example, <em>Array#size</em>
 is an alias for <em>Array#length</em>. If you would like
 to provide an alias for one of your class' instance methods, one
 approach is to use SWIG's <tt>%extend</tt> directive to
 add a new method of the aliased name that calls the original function.
 For example: </p>

 <div class="code">
 <pre>class MyArray {
 public:
   // Construct an empty array
   MyArray();

   // Return the size of this array
   size_t length() const;
 };

 %extend MyArray {
   // MyArray#size is an alias for MyArray#length
   size_t size() const {
     return $self-&gt;length();
   }
 }
  </pre>
 </div>

 <p> A better solution is to use the <tt>%alias</tt>
 directive (unique to SWIG's Ruby module). The previous example could
 then be rewritten as: </p>

 <div class="code">
 <pre>// MyArray#size is an alias for MyArray#length
 %alias MyArray::length "size";

 class MyArray {
 public:
   // Construct an empty array
   MyArray();

   // Return the size of this array
   size_t length() const;
 };</pre>
 </div>

 <p> Multiple aliases can be associated with a method by providing
 a comma-separated list of aliases to the <tt>%alias</tt>
 directive, e.g. </p>

 <div class="code">
 <pre>%alias MyArray::length "amount, quantity, size";</pre>
 </div>

 <p> From an end-user's standpoint, there's no functional
 difference between these two approaches; i.e. they should get the same
 result from calling either <em>MyArray#size</em> or <em>MyArray#length</em>.
 However, when the <tt>%alias</tt> directive is used, SWIG
 doesn't need to generate all of the wrapper code that's usually
 associated with added methods like our <em>MyArray::size()</em>
 example. </p>

 <p>Note that the <tt>%alias</tt> directive is
 implemented using SWIG's "features" mechanism and so the same name
 matching rules used for other kinds of features apply (see the chapter
 on <a href="Customization.html#Customization">"Customization
 Features"</a>) for more details).</p>

 <H3><a name="Ruby_nn29">35.4.2 Predicate Methods</a></H3>


 <p> Ruby methods that return a boolean value and end in a
 question mark
 are known as predicate methods. Examples of predicate methods in
 standard Ruby classes include <em>Array#empty?</em> (which
 returns <tt>true</tt> for an array containing no elements)
 and <em>Object#instance_of?</em> (which returns <tt>true</tt>
 if the object is an instance of the specified class). For consistency
 with Ruby conventions, methods that return boolean values should be
 marked as predicate methods.</p>

 <p>One cumbersome solution to this problem is to rename the
 method (using SWIG's <tt>%rename</tt> directive) and
 provide a custom typemap that converts the function's actual return
 type to Ruby's <tt>true</tt> or <tt>false</tt>.
 For example: </p>

 <div class="code">
 <pre>%rename("is_it_safe?") is_it_safe();

 %typemap(out) int is_it_safe "$result = ($1 != 0) ? Qtrue : Qfalse;";

 int is_it_safe();</pre>
 </div>

 <p> A better solution is to use the <tt>%predicate</tt>
 directive (unique to SWIG's Ruby module) to designate a method as a
 predicate method. For the previous example, this would look like: </p>

 <div class="code">
 <pre>%predicate is_it_safe();

 int is_it_safe();</pre>
 </div>

 <p>This method would be invoked from Ruby code like this:</p>

 <div class="code targetlang">
 <pre>irb(main):001:0&gt; <b>Example::is_it_safe?</b>
 true</pre>
 </div>

 <p> The <tt>%predicate</tt> directive is implemented
 using SWIG's "features" mechanism and so the same name matching rules
 used for other kinds of features apply (see the chapter on <a href="Customization.html#Customization">"Customization
 Features"</a>) for more details). </p>

 <H3><a name="Ruby_nn30">35.4.3 Bang Methods</a></H3>


 <p> Ruby methods that modify an object in-place and end in an
 exclamation mark are known as bang methods. An example of a bang method
 is <em>Array#sort!</em> which changes the ordering of
 items in an array. Contrast this with <em>Array#sort</em>,
 which returns a copy of the array with the items sorted instead of
 modifying the original array. For consistency with Ruby conventions,
 methods that modify objects in place should be marked as bang methods.</p>

 <p>Bang methods can be marked using the <tt>%bang</tt>
 directive which is unique to the Ruby module and was introduced in SWIG
 1.3.28. For example:</p>

 <div class="code">
 <pre>%bang sort(int arr[]);

 int sort(int arr[]); </pre>
 </div>

 <p>This method would be invoked from Ruby code like this:</p>

 <div class="code">
 <pre>irb(main):001:0&gt; <b>Example::sort!(arr)</b></pre>
 </div>

 <p> The <tt>%bang</tt> directive is implemented
 using SWIG's "features" mechanism and so the same name matching rules
 used for other kinds of features apply (see the chapter on <a href="Customization.html#Customization">"Customization
 Features"</a>) for more details). </p>

 <H3><a name="Ruby_nn31">35.4.4 Getters and Setters</a></H3>


 <p> Often times a C++ library will expose properties through
 getter and setter methods. For example:</p>

 <div class="code">
 <pre>class Foo {
   Foo() {}
   int getValue() { return value_; }
   void setValue(int value) { value_ = value; }

 private:
   int value_;
 };</pre>
 </div>

 <p>By default, SWIG will expose these methods to Ruby as <tt>get_value</tt>
 and <tt>set_value.</tt> However, it more natural for these
 methods to be exposed in Ruby as <tt>value</tt> and <tt>value=.
 </tt> That allows the methods to be used like this:</p>

 <div class="code">
 <pre>irb(main):001:0&gt; <b>foo = Foo.new()</b>
 irb(main):002:0&gt; <b>foo.value = 5</b>
 irb(main):003:0&gt; <b>puts foo.value</b></pre>
 </div>

 <p> This can be done by using the %rename directive:</p>

 <div class="code">
 <pre>%rename("value") Foo::getValue();
 %rename("value=") Foo::setValue(int value);</pre>
 </div>

 <H2><a name="Ruby_nn32">35.5 Input and output parameters</a></H2>


 <p> A common problem in some C programs is handling parameters
 passed as simple pointers. For example: </p>

 <div class="code">
 <pre>void add(int x, int y, int *result) {
   *result = x + y;
 }</pre>
 </div>

 <p>
 or
 </p>

 <div class="code">
 <pre>
 int sub(int *x, int *y) {
   return *x-*y;
 }</pre>
 </div>

 <p> The easiest way to handle these situations is to use the <tt>typemaps.i</tt>
 file. For example: </p>

 <div class="code">
 <pre>%module Example
 %include "typemaps.i"

 void add(int, int, int *OUTPUT);
 int sub(int *INPUT, int *INPUT);</pre>
 </div>

 <p>In Ruby, this allows you to pass simple values. For example:</p>

 <div class="code targetlang">
 <pre>a = Example.add(3, 4)
 puts a
 7
 b = Example.sub(7, 4)
 puts b
 3</pre>
 </div>

 <p> Notice how the <tt>INPUT</tt> parameters allow
 integer values to be passed instead of pointers and how the <tt>OUTPUT</tt>
 parameter creates a return result. </p>

 <p>If you don't want to use the names <tt>INPUT</tt>
 or <tt>OUTPUT</tt>, use the <tt>%apply</tt>
 directive. For example: </p>

 <div class="code">
 <pre>%module Example
 %include "typemaps.i"

 %apply int *OUTPUT { int *result };
 %apply int *INPUT { int *x, int *y};

 void add(int x, int y, int *result);
 int sub(int *x, int *y);</pre>
 </div>

 <p> If a function mutates one of its parameters like this, </p>

 <div class="code">
 <pre>void negate(int *x) {
  *x = -(*x);
 }</pre>
 </div>

 <p>you can use <tt>INOUT</tt> like this:</p>

 <div class="code">
 <pre>%include "typemaps.i"
 ...
 void negate(int *INOUT);</pre>
 </div>

 <p>In Ruby, a mutated parameter shows up as a return value. For
 example:</p>

 <div class="code targetlang">
 <pre>a = Example.negate(3)
 print a
 -3</pre>
 </div>

 <p> The most common use of these special typemap rules is to
 handle functions that return more than one value. For example,
 sometimes a function returns a result as well as a special error code: </p>

 <div class="code">
 <pre>/* send message, return number of bytes sent, success code, and error_code */
 int send_message(char *text, int *success, int *error_code);</pre>
 </div>

 <p> To wrap such a function, simply use the <tt>OUTPUT</tt>
 rule above. For example: </p>

 <div class="code">
 <pre>%module example
 %include "typemaps.i"
 ...
 int send_message(char *, int *OUTPUT, int *OUTPUT);</pre>
 </div>

 <p> When used in Ruby, the function will return an array of
 multiple values. </p>

 <div class="code targetlang">
 <pre>bytes, success, error_code = send_message("Hello World")
 if not success
   print "error #{error_code} : in send_message"
 else
   print "Sent", bytes
 end</pre>
 </div>

 <p> Another way to access multiple return values is to use the <tt>%apply</tt>
 rule. In the following example, the parameters rows and columns are
 related to SWIG as <tt>OUTPUT</tt> values through the use
 of <tt>%apply</tt> </p>

 <div class="code">
 <pre>%module Example
 %include "typemaps.i"
 %apply int *OUTPUT { int *rows, int *columns };
 ...
 void get_dimensions(Matrix *m, int *rows, int*columns);</pre>
 </div>

 <p>In Ruby:</p>

 <div class="code targetlang">
 <pre>r, c = Example.get_dimensions(m)</pre>
 </div>

 <H2><a name="Ruby_nn33">35.6 Exception handling </a></H2>


 <H3><a name="Ruby_nn34">35.6.1 Using the %exception directive </a></H3>


 <p>The SWIG <tt>%exception</tt> directive can be
 used to define a user-definable exception handler that can convert
 C/C++ errors into Ruby exceptions. The chapter on <a href="Customization.html#Customization">Customization
 Features</a> contains more details, but suppose you have a C++
 class like the following : </p>

 <div class="code">
 <pre>class DoubleArray {
 private:
   int n;
   double *ptr;
 public:
   // Create a new array of fixed size
   DoubleArray(int size) {
     ptr = new double[size];
     n = size;
   }

   // Destroy an array
   ~DoubleArray() {
     delete ptr;
   }

   // Return the length of the array
   int length() {
     return n;
   }

   // Get an array item and perform bounds checking.
   double getitem(int i) {
     if ((i &gt;= 0) &amp;&amp; (i &lt; n))
       return ptr[i];
     else
       throw RangeError();
   }

   // Set an array item and perform bounds checking.
   void setitem(int i, double val) {
     if ((i &gt;= 0) &amp;&amp; (i &lt; n))
       ptr[i] = val;
     else {
       throw RangeError();
     }
   }
 };</pre>
 </div>

 <p> Since several methods in this class can throw an exception
 for an out-of-bounds access, you might want to catch this in the Ruby
 extension by writing the following in an interface file: </p>

 <div class="code">
 <pre>%exception {
   try {
     $action
   }
   catch (const RangeError&amp;) {
     static VALUE cpperror = rb_define_class("CPPError", rb_eStandardError);
     rb_raise(cpperror, "Range error.");
   }
 }

 class DoubleArray {
   ...
 };</pre>
 </div>

 <p> The exception handling code is inserted directly into
 generated wrapper functions. When an exception handler is defined,
 errors can be caught and used to gracefully raise a Ruby exception
 instead of forcing the entire program to terminate with an uncaught
 error. </p>

 <p>As shown, the exception handling code will be added to every
 wrapper function. Because this is somewhat inefficient, you might
 consider refining the exception handler to only apply to specific
 methods like this: </p>

 <div class="code">
 <pre>%exception getitem {
   try {
     $action
   } catch (const RangeError&amp;) {
     static VALUE cpperror = rb_define_class("CPPError", rb_eStandardError);
     rb_raise(cpperror, "Range error in getitem.");
   }
 }

 %exception setitem {
   try {
     $action
   } catch (const RangeError&amp;) {
     static VALUE cpperror = rb_define_class("CPPError", rb_eStandardError);
     rb_raise(cpperror, "Range error in setitem.");
   }
 }</pre>
 </div>

 <p> In this case, the exception handler is only attached to
 methods and functions named <tt>getitem</tt> and <tt>setitem</tt>.
 </p>

 <p>Since SWIG's exception handling is user-definable, you are not
 limited to C++ exception handling. See the chapter on <a href="Customization.html#Customization">Customization
 Features</a> for more examples.</p>

 <H3><a name="Ruby_nn34_2">35.6.2 Handling Ruby Blocks </a></H3>


 <p>One of the highlights of Ruby and most of its standard library
 is
 the use of blocks, which allow the easy creation of continuations and
 other niceties.  Blocks in ruby are also often used to
 simplify the passing of many arguments to a class.</p>

 <p>In order to make your class constructor support blocks, you
 can take advantage of the %exception directive, which will get run
 after the C++ class' constructor was called. </p>

 <p>For example, this yields the class over after its
 construction:
 </p>

 <div class="code">
 <pre>class Window
 {
 public:
   Window(int x, int y, int w, int h);
   // .... other methods here ....
 };

 // Add support for yielding self in the Class' constructor.
 %exception Window::Window {
   $action
   if (rb_block_given_p()) {
     rb_yield(self);
   }
 }</pre>
 </div>

 <p> Then, in ruby, it can be used like:</p>

 <div class="targetlang"><pre>
 Window.new(0, 0, 360, 480) { |w|
   w.color = Fltk::RED
   w.border = false
 }
 </pre>
 </div>

 <p>For other methods, you can usually use a dummy parameter with
 a special in typemap, like:</p>

 <div class="code" ><pre>
 //
 // original function was:
 //
 // void func(int x);

 %typemap(in, numinputs=0) int RUBY_YIELD_SELF {
   if ( !rb_block_given_p() )
     rb_raise("No block given");
   return rb_yield(self);
 }

 %extend {
   void func(int x, int RUBY_YIELD_SELF );
 }
 </pre>
 </div>

 <p>For more information on typemaps, see <a href="#Ruby_nn37">Typemaps</a>.</p>

 <H3><a name="Ruby_nn35">35.6.3 Raising exceptions </a></H3>


 <p>There are three ways to raise exceptions from C++ code to
 Ruby. </p>

 <p>The first way is to use <tt>SWIG_exception(int code,
 const char *msg)</tt>. The following table shows the mappings
 from SWIG error codes to Ruby exceptions:</p>

 <div class="diagram">
 <table class="diagram" summary="Mapping between SWIG error codes and Ruby exceptions." border="1" width="80%">
   <tbody>
     <tr>
       <td class="diagram">
       <div>SWIG_MemoryError</div>
       </td>
       <td>
       <div>rb_eNoMemError</div>
       </td>
     </tr>
     <tr>
       <td class="diagram">
       <div>SWIG_IOError</div>
       </td>
       <td>
       <div>rb_eIOError</div>
       </td>
     </tr>
     <tr>
       <td class="diagram">
       <div>SWIG_RuntimeError</div>
       </td>
       <td>
       <div>rb_eRuntimeError</div>
       </td>
     </tr>
     <tr>
       <td class="diagram">
       <div>SWIG_IndexError</div>
       </td>
       <td>
       <div>rb_eIndexError</div>
       </td>
     </tr>
     <tr>
       <td class="diagram">
       <div>SWIG_TypeError</div>
       </td>
       <td>
       <div>rb_eTypeError</div>
       </td>
     </tr>
     <tr>
       <td class="diagram">
       <div>SWIG_DivisionByZero</div>
       </td>
       <td>
       <div>rb_eZeroDivError</div>
       </td>
     </tr>
     <tr>
       <td class="diagram">
       <div>SWIG_OverflowError</div>
       </td>
       <td>
       <div>rb_eRangeError</div>
       </td>
     </tr>
     <tr>
       <td class="diagram">
       <div>SWIG_SyntaxError</div>
       </td>
       <td>
       <div>rb_eSyntaxError</div>
       </td>
     </tr>
     <tr>
       <td class="diagram">
       <div>SWIG_ValueError</div>
       </td>
       <td>
       <div>rb_eArgError</div>
       </td>
     </tr>
     <tr>
       <td class="diagram">
       <div>SWIG_SystemError</div>
       </td>
       <td>
       <div>rb_eFatal</div>
       </td>
     </tr>
     <tr>
       <td class="diagram">
       <div>SWIG_AttributeError</div>
       </td>
       <td>
       <div>rb_eRuntimeError</div>
       </td>
     </tr>
     <tr>
       <td class="diagram">
       <div>SWIG_NullReferenceError</div>
       </td>
       <td>
       <div>rb_eNullReferenceError*</div>
       </td>
     </tr>
     <tr>
       <td class="diagram">
       <div>SWIG_ObjectPreviouslyDeletedError</div>
       </td>
       <td>
       <div>rb_eObjectPreviouslyDeleted*</div>
       </td>
     </tr>
     <tr>
       <td class="diagram">
       <div>SWIG_UnknownError</div>
       </td>
       <td>
       <div>rb_eRuntimeError</div>
       </td>
     </tr>
     <tr class="diagram">
       <td colspan="2">
       <div>* These error classes are created by
 SWIG and are not built-in Ruby exception classes </div>
       </td>
     </tr>
   </tbody>
 </table>
 </div>

 <p>The second way to raise errors is to use <tt>SWIG_Raise(obj,
 type, desc)</tt>.
 Obj is a C++ instance of an exception class, type is a string
 specifying the type of exception (for example, "MyError") and desc is
 the SWIG description of the exception class. For example: </p>

 <div class="code"><pre>
 %raise(SWIG_NewPointerObj(e, SWIGTYPE_p_AssertionFailedException, 0), ":AssertionFailedException", SWIGTYPE_p_AssertionFailedException);
 </pre></div>

 <p>This is useful when you want to pass the current exception
 object
 directly to Ruby, particularly when the object is an instance of class
 marked as an <tt>%exceptionclass</tt> (see the next
 section for more information).</p>

 <p>Last, you can raise an exception by directly calling Ruby's C
 api. This is done by invoking the <tt>rb_raise()</tt>
 function. The first argument passed to <tt>rb_raise()</tt>
 is the exception type. You can raise a custom exception type or one of
 the built-in Ruby exception types.</p>

 <H3><a name="Ruby_nn36">35.6.4 Exception classes </a></H3>


 <p>Starting with SWIG 1.3.28, the Ruby module supports the <tt>%exceptionclass</tt>
 directive, which is used to identify C++ classes that are used as
 exceptions. Classes that are marked with the <tt>%exceptionclass</tt>
 directive are exposed in Ruby as child classes of <tt>rb_eRuntimeError</tt>.
 This allows C++ exceptions to be directly mapped to Ruby exceptions,
 providing for a more natural integration between C++ code and Ruby code.</p>

 <div class="code">
 <pre>%exceptionclass CustomError;

 %inline %{
   class CustomError { };

   class Foo {
   public:
     void test() { throw CustomError; }
   };
 %}</pre>
 </div>

 <p>From Ruby you can now call this method like this: </p>

 <div class="code targetlang">
 <pre>foo = Foo.new
 begin
   foo.test()
 rescue CustomError =&gt; e
   puts "Caught custom error"
 end </pre>
 </div>

 <p>For another example look at swig/Examples/ruby/exception_class.
 </p>

 <H2><a name="Ruby_nn37">35.7 Typemaps</a></H2>


 <p> This section describes how you can modify SWIG's default
 wrapping behavior for various C/C++ datatypes using the <tt>%typemap</tt>
 directive. This is an advanced topic that assumes familiarity with the
 Ruby C API as well as the material in the "<a href="Typemaps.html#Typemaps">Typemaps</a>"
 chapter.
 </p>

 <p>Before proceeding, it should be stressed that typemaps are not
 a required part of using SWIG---the default wrapping behavior is enough
 in most cases. Typemaps are only used if you want to change some aspect
 of the primitive C-Ruby interface.</p>

 <H3><a name="Ruby_nn38">35.7.1 What is a typemap?</a></H3>


 <p> A typemap is nothing more than a code generation rule that is
 attached to a specific C datatype. The general form of this declaration
 is as follows ( parts enclosed in [...] are optional
 ): </p>

 <div class="code">
 <pre>
 %typemap( method [, modifiers...] ) typelist code;
 </pre>
 </div>

 <p><em> method</em> is a simply a name that specifies
 what kind of typemap is being defined. It is usually a name like <tt>"in"</tt>,
 <tt>"out"</tt>, or <tt>"argout"</tt> (or its
 director variations). The purpose of these methods is described later.</p>

 <p><em> modifiers</em> is an optional comma separated
 list of <tt>
 name="value"</tt> values. These are sometimes to attach extra
 information to a typemap and is often target-language dependent.</p>

 <p><em> typelist</em> is a list of the C++ type
 patterns that the typemap will match. The general form of this list is
 as follows:</p>

 <div class="diagram">
 <pre>typelist : typepattern [, typepattern, typepattern, ... ] ;

 typepattern : type [ (parms) ]
   | type name [ (parms) ]
   | ( typelist ) [ (parms) ]</pre>
 </div>

 <p> Each type pattern is either a simple type, a simple type and
 argument name, or a list of types in the case of multi-argument
 typemaps. In addition, each type pattern can be parameterized with a
 list of temporary variables (parms). The purpose of these variables
 will be explained shortly.</p>

 <p><em>code</em> specifies the C code used in the
 typemap. It can take any one of the following forms:</p>

 <div class="diagram">
 <pre>code : { ... }
   | " ... "
   | %{ ... %}</pre>
 </div>

 <p>For example, to convert integers
 from Ruby to C, you might define a typemap like this: </p>

 <div class="code">
 <pre>%module example

 %typemap(in) int {
   $1 = (int) NUM2INT($input);
   printf("Received an integer : %d\n", $1);
 }

 %inline %{
   extern int fact(int n);
 %}</pre>
 </div>

 <p> Typemaps are always associated with some specific aspect of
 code generation. In this case, the "in" method refers to the conversion
 of input arguments to C/C++. The datatype <tt>int</tt> is
 the datatype to which the typemap will be applied. The supplied C code
 is used to convert values. In this code a number of special variables
 prefaced by a <tt>$</tt> are used. The <tt>$1</tt>
 variable is placeholder for a local variable of type <tt>int</tt>.
 The <tt>$input</tt> variable is the input Ruby object. </p>

 <p>When this example is compiled into a Ruby module, the
 following sample code: </p>

 <div class="code targetlang">
 <pre>require 'example'

 puts Example.fact(6)</pre>
 </div>

 <p>prints the result:</p>

 <div class="code shell">
 <pre>
 Received an integer : 6
 720
 </pre>
 </div>

 <p> In this example, the typemap is applied to all occurrences of
 the <tt>int</tt> datatype. You can refine this by
 supplying an optional parameter name. For example: </p>

 <div class="code">
 <pre>%module example

 %typemap(in) int n {
   $1 = (int) NUM2INT($input);
   printf("n = %d\n", $1);
 }

 %inline %{
   extern int fact(int n);
 %}</pre>
 </div>

 <p> In this case, the typemap code is only attached to arguments
 that exactly match "<tt>int n</tt>". </p>

 <p>The application of a typemap to specific datatypes and
 argument names involves more than simple text-matching--typemaps are
 fully integrated into the SWIG type-system. When you define a typemap
 for <tt>int</tt>, that typemap applies to <tt>int</tt>
 and qualified variations such as <tt>const int</tt>. In
 addition, the typemap system follows <tt>typedef</tt>
 declarations. For example: </p>

 <div class="code">
 <pre>%typemap(in) int n {
   $1 = (int) NUM2INT($input);
   printf("n = %d\n", $1);
 }

 typedef int Integer;
 extern int fact(Integer n); // Above typemap is applied</pre>
 </div>

 <p> However, the matching of <tt>typedef</tt> only
 occurs in one direction. If you defined a typemap for <tt>Integer</tt>,
 it is not applied to arguments of type <tt>int</tt>. </p>

 <p>Typemaps can also be defined for groups of consecutive
 arguments. For example: </p>

 <div class="code">
 <pre>%typemap(in) (char *str, int len) {
   $1 = StringValuePtr($input);
   $2 = (int) RSTRING($input)-&gt;len;
 };

 int count(char c, char *str, int len);</pre>
 </div>

 <p> When a multi-argument typemap is defined, the arguments are
 always handled as a single Ruby object. This allows the function <tt>count</tt>
 to be used as follows (notice how the length parameter is omitted): </p>

 <div class="code targetlang">
 <pre>puts Example.count('o', 'Hello World')
 2</pre>
 </div>

 <H3><a name="Ruby_Typemap_scope">35.7.2 Typemap scope</a></H3>


 <p> Once defined, a typemap remains in effect for all of the
 declarations that follow. A typemap may be redefined for different
 sections of an input file. For example:</p>

 <div class="code">
 <pre>// typemap1
 %typemap(in) int {
   ...
 }

 int fact(int); // typemap1
 int gcd(int x, int y); // typemap1

 // typemap2
 %typemap(in) int {
   ...
 }

 int isprime(int); // typemap2</pre>
 </div>

 <p> One exception to the typemap scoping rules pertains to the <tt>
 %extend</tt> declaration. <tt>%extend</tt> is used
 to attach new declarations to a class or structure definition. Because
 of this, all of the declarations in an <tt>%extend</tt>
 block are subject to the typemap rules that are in effect at the point
 where the class itself is defined. For example:</p>

 <div class="code">
 <pre>class Foo {
   ...
 };

 %typemap(in) int {
   ...
 }

 %extend Foo {
   int blah(int x); // typemap has no effect. Declaration is attached to Foo which
   // appears before the %typemap declaration.
 };</pre>
 </div>

 <H3><a name="Ruby_Copying_a_typemap">35.7.3 Copying a typemap</a></H3>


 <p> A typemap is copied by using assignment. For example:</p>

 <div class="code">
 <pre>%typemap(in) Integer = int;</pre>
 </div>

 <p> or this:</p>

 <div class="code">
 <pre>%typemap(in) Integer, Number, int32_t = int;</pre>
 </div>

 <p> Types are often managed by a collection of different
 typemaps. For example:</p>

 <div class="code">
 <pre>%typemap(in) int { ... }
 %typemap(out) int { ... }
 %typemap(varin) int { ... }
 %typemap(varout) int { ... }</pre>
 </div>

 <p> To copy all of these typemaps to a new type, use <tt>%apply</tt>.
 For example:</p>

 <div class="code">
 <pre>%apply int { Integer }; // Copy all int typemaps to Integer
 %apply int { Integer, Number }; // Copy all int typemaps to both Integer and Number</pre>
 </div>

 <p> The patterns for <tt>%apply</tt> follow the same
 rules as for <tt>
 %typemap</tt>. For example:</p>

 <div class="code">
 <pre>%apply int *output { Integer *output }; // Typemap with name
 %apply (char *buf, int len) { (char *buffer, int size) }; // Multiple arguments</pre>
 </div>

 <H3><a name="Ruby_Deleting_a_typemap">35.7.4 Deleting a typemap</a></H3>


 <p> A typemap can be deleted by simply defining no code. For
 example:</p>

 <div class="code">
 <pre>%typemap(in) int; // Clears typemap for int
 %typemap(in) int, long, short; // Clears typemap for int, long, short
 %typemap(in) int *output; </pre>
 </div>

 <p> The <tt>%clear</tt> directive clears all
 typemaps for a given type. For example:</p>

 <div class="code">
 <pre>%clear int; // Removes all types for int
 %clear int *output, long *output;</pre>
 </div>

 <p><b> Note:</b> Since SWIG's default behavior is
 defined by typemaps, clearing a fundamental type like <tt>int</tt>
 will make that type unusable unless you also define a new set of
 typemaps immediately after the clear operation.</p>

 <H3><a name="Ruby_Placement_of_typemaps">35.7.5 Placement of typemaps</a></H3>


 <p> Typemap declarations can be declared in the global scope,
 within a C++ namespace, and within a C++ class. For example:</p>

 <div class="code">
 <pre>%typemap(in) int {
   ...
 }

 namespace std {
   class string;
   %typemap(in) string {
     ...
   }
 }

 class Bar {
 public:
   typedef const int &amp; const_reference;
   %typemap(out) const_reference {
     ...
   }
 };</pre>
 </div>

 <p> When a typemap appears inside a namespace or class, it stays
 in effect until the end of the SWIG input (just like before). However,
 the typemap takes the local scope into account. Therefore, this code</p>

 <div class="code">
 <pre>namespace std {
   class string;
   %typemap(in) string {
     ...
   }
 }</pre>
 </div>

 <p> is really defining a typemap for the type <tt>std::string</tt>.
 You could have code like this:</p>

 <div class="code">
 <pre>namespace std {
   class string;
   %typemap(in) string { /* std::string */
   ...
   }
 }

 namespace Foo {
   class string;
   %typemap(in) string { /* Foo::string */
   ...
   }
 }</pre>
 </div>

 <p> In this case, there are two completely distinct typemaps that
 apply to two completely different types (<tt>std::string</tt>
 and <tt>
 Foo::string</tt>).</p>

 <p> It should be noted that for scoping to work, SWIG has to know
 that <tt>
 string</tt> is a typename defined within a particular namespace.
 In this example, this is done using the class declaration <tt>class
 string</tt>
 .</p>

 <H3><a name="Ruby_nn39">35.7.6 Ruby typemaps</a></H3>


 <p>The following list details all of the typemap methods that
 can be used by the Ruby module: </p>

 <H4><a name="Ruby_in_typemap">35.7.6.1 "in" typemap</a></H4>


 <p>Converts Ruby objects to input
 function arguments. For example:
 </p>

 <div class="code">
 <pre>%typemap(in) int {
   $1 = NUM2INT($input);
 }</pre>
 </div>

 <p> The following special variables are available:</p>

 <div class="diagram">
 <table border="1" cellpadding="2" cellspacing="2" width="100%" summary="Special variables - in typemap">
   <tbody>
     <tr>
       <td>$input </td>
       <td> Input object
 holding value to be converted.</td>
     </tr>
     <tr>
       <td>$symname </td>
       <td> Name of
 function/method being wrapped</td>
     </tr>
     <tr>
       <td>$1...n </td>
       <td> Argument being
 sent to the function</td>
     </tr>
     <tr>
       <td>$1_name </td>
       <td> Name of the
 argument (if provided)</td>
     </tr>
     <tr>
       <td>$1_type </td>
       <td> The actual C
 datatype matched by the typemap.</td>
     </tr>
     <tr>
       <td>$1_ltype </td>
       <td> The assignable
 version of the C datatype matched by the typemap.</td>
     </tr>
   </tbody>
 </table>
 </div>

 <p> This is probably the most commonly redefined typemap because
 it can be used to implement customized conversions.</p>

 <p> In addition, the "in" typemap allows the number of converted
 arguments to be specified. For example:</p>

 <div class="code">
 <pre>// Ignored argument.
 %typemap(in, numinputs=0) int *out (int temp) {
   $1 = &amp;temp;
 }</pre>
 </div>

 <p> At this time, only zero or one arguments may be converted.</p>

 <H4><a name="Ruby_typecheck_typemap">35.7.6.2 "typecheck" typemap</a></H4>


 <p> The "typecheck" typemap is used to support overloaded
 functions and methods. It merely checks an argument to see whether or
 not it matches a specific type. For example:</p>

 <div class="code">
 <pre>%typemap(typecheck, precedence=SWIG_TYPECHECK_INTEGER) int {
   $1 = FIXNUM_P($input) ? 1 : 0;
 }</pre>
 </div>

 <p> For typechecking, the $1 variable is always a simple integer
 that is set to 1 or 0 depending on whether or not the input argument is
 the correct type.</p>

 <p> If you define new "in" typemaps<em> and</em> your
 program uses overloaded methods, you should also define a collection of
 "typecheck" typemaps. More details about this follow in a later section
 on "Typemaps and Overloading."</p>

 <H4><a name="Ruby_out_typemap">35.7.6.3 "out" typemap</a></H4>


 <p>Converts return value of a C function
 to a Ruby object.</p>

 <div class="code">
 <pre>%typemap(out) int {
   $result = INT2NUM( $1 );
 }
 </pre></div>

 <p> The following special variables are available.</p>

 <div class="diagram">
 <table border="1" cellpadding="2" cellspacing="2" width="100%" summary="Special variables - out typemap">
   <tbody>
     <tr>
       <td>$result </td>
       <td> Result object
 returned to target language.</td>
     </tr>
     <tr>
       <td>$symname </td>
       <td> Name of
 function/method being wrapped</td>
     </tr>
     <tr>
       <td>$1...n </td>
       <td> Argument being
 wrapped</td>
     </tr>
     <tr>
       <td>$1_name </td>
       <td> Name of the
 argument (if provided)</td>
     </tr>
     <tr>
       <td>$1_type </td>
       <td> The actual C
 datatype matched by the typemap.</td>
     </tr>
     <tr>
       <td>$1_ltype </td>
       <td> The assignable
 version of the C datatype matched by the typemap.</td>
     </tr>
   </tbody>
 </table>
 </div>

 <H4><a name="Ruby_arginit_typemap">35.7.6.4 "arginit" typemap</a></H4>


 <p> The "arginit" typemap is used to set the initial value of a
 function argument--before any conversion has occurred. This is not
 normally necessary, but might be useful in highly specialized
 applications. For example:</p>

 <div class="code">
 <pre>// Set argument to NULL before any conversion occurs
 %typemap(arginit) int *data {
   $1 = NULL;
 }</pre>
 </div>

 <H4><a name="Ruby_default_typemap">35.7.6.5 "default" typemap</a></H4>


 <p> The "default" typemap is used to turn an argument into a
 default argument. For example:</p>

 <div class="code">
 <pre>%typemap(default) int flags {
   $1 = DEFAULT_FLAGS;
 }
 ...
 int foo(int x, int y, int flags);</pre>
 </div>

 <p> The primary use of this typemap is to either change the
 wrapping of default arguments or specify a default argument in a
 language where they aren't supported (like C). Target languages that do
 not support optional arguments, such as Java and C#, effectively ignore
 the value specified by this typemap as all arguments must be given.</p>

 <p> Once a default typemap has been applied to an argument, all
 arguments that follow must have default values. See the <a href="SWIG.html#SWIG_default_args">
 Default/optional arguments</a> section for further information on
 default argument wrapping.</p>

 <H4><a name="Ruby_check_typemap">35.7.6.6 "check" typemap</a></H4>


 <p> The "check" typemap is used to supply value checking code
 during argument conversion. The typemap is applied<em> after</em>
 arguments have been converted. For example:</p>

 <div class="code">
 <pre>%typemap(check) int positive {
   if ($1 &lt;= 0) {
     SWIG_exception(SWIG_ValueError, "Expected positive value.");
   }
 }</pre>
 </div>

 <H4><a name="Ruby_argout_typemap_">35.7.6.7 "argout" typemap</a></H4>


 <p> The "argout" typemap is used to return values from arguments.
 This is most commonly used to write wrappers for C/C++ functions that
 need to return multiple values. The "argout" typemap is almost always
 combined with an "in" typemap---possibly to ignore the input value. For
 example:</p>

 <div class="code">
 <pre>/* Set the input argument to point to a temporary variable */
 %typemap(in, numinputs=0) int *out (int temp) {
   $1 = &amp;temp;
 }

 %typemap(argout, fragment="output_helper") int *out {
   // Append output value $1 to $result (assuming a single integer in this case)
   $result = output_helper( $result, INT2NUM(*$1) );
 }</pre>
 </div>

 <p> The following special variables are available.</p>

 <div class="diagram">
 <table border="1" cellpadding="2" cellspacing="2" width="100%" summary="Special variables - argout typemap">
   <tbody>
     <tr>
       <td>$result </td>
       <td> Result object
 returned to target language.</td>
     </tr>
     <tr>
       <td>$input </td>
       <td> The original
 input object passed.</td>
     </tr>
     <tr>
       <td>$symname </td>
       <td> Name of
 function/method being wrapped.</td>
     </tr>
   </tbody>
 </table>
 </div>

 <p> The code supplied to the "argout" typemap is always placed
 after the "out" typemap. If multiple return values are used, the extra
 return values are often appended to return value of the function.</p>

 <p>Output helper is a fragment that usually defines a macro to
 some function like SWIG_Ruby_AppendOutput.</p>

 <p> See the <tt>typemaps.i</tt> library for examples.</p>

 <H4><a name="Ruby_freearg_typemap_">35.7.6.8 "freearg" typemap</a></H4>


 <p> The "freearg" typemap is used to cleanup argument data. It is
 only used when an argument might have allocated resources that need to
 be cleaned up when the wrapper function exits. The "freearg" typemap
 usually cleans up argument resources allocated by the "in" typemap. For
 example:</p>

 <div class="code">
 <pre>// Get a list of integers
 %typemap(in) int *items {
   int nitems = Length($input);
   $1 = (int *) malloc(sizeof(int)*nitems);
 }
 // Free the list
 %typemap(freearg) int *items {
   free($1);
 }</pre>
 </div>

 <p> The "freearg" typemap inserted at the end of the wrapper
 function, just before control is returned back to the target language.
 This code is also placed into a special variable <tt>$cleanup</tt>
 that may be used in other typemaps whenever a wrapper function needs to
 abort prematurely.</p>

 <H4><a name="Ruby_newfree_typemap">35.7.6.9 "newfree" typemap</a></H4>


 <p> The "newfree" typemap is used in conjunction with the <tt>%newobject</tt>
 directive and is used to deallocate memory used by the return result of
 a function. For example:</p>

 <div class="code">
 <pre>%typemap(newfree) string * {
   delete $1;
 }
 %typemap(out) string * {
   $result = PyString_FromString($1-&gt;c_str());
 }
 ...

 %newobject foo;
 ...
 string *foo();</pre>
 </div>

 <p> See <a href="Customization.html#Customization_ownership">Object
 ownership and %newobject</a> for further details.</p>

 <H4><a name="Ruby_memberin_typemap">35.7.6.10 "memberin" typemap</a></H4>


 <p> The "memberin" typemap is used to copy data from<em> an
 already converted input value</em> into a structure member. It is
 typically used to handle array members and other special cases. For
 example:</p>

 <div class="code">
 <pre>%typemap(memberin) int [4] {
   memmove($1, $input, 4*sizeof(int));
 }</pre>
 </div>

 <p> It is rarely necessary to write "memberin" typemaps---SWIG
 already provides a default implementation for arrays, strings, and
 other objects.</p>

 <H4><a name="Ruby_varin_typemap">35.7.6.11 "varin" typemap</a></H4>


 <p> The "varin" typemap is used to convert objects in the target
 language to C for the purposes of assigning to a C/C++ global variable.
 This is implementation specific.</p>

 <H4><a name="Ruby_varout_typemap_">35.7.6.12 "varout" typemap</a></H4>


 <p> The "varout" typemap is used to convert a C/C++ object to an
 object in the target language when reading a C/C++ global variable.
 This is implementation specific.</p>

 <H4><a name="Ruby_throws_typemap">35.7.6.13 "throws" typemap</a></H4>


 <p> The "throws" typemap is only used when SWIG parses a C++
 method with an exception specification or has the <tt>%catches</tt>
 feature attached to the method. It provides a default mechanism for
 handling C++ methods that have declared the exceptions they will throw.
 The purpose of this typemap is to convert a C++ exception into an error
 or exception in the target language. It is slightly different to the
 other typemaps as it is based around the exception type rather than the
 type of a parameter or variable. For example:</p>

 <div class="code">
 <pre>%typemap(throws) const char * %{
   rb_raise(rb_eRuntimeError, $1);
   SWIG_fail;
 %}
 void bar() throw (const char *);</pre>
 </div>

 <p> As can be seen from the generated code below, SWIG generates
 an exception handler with the catch block comprising the "throws"
 typemap content.</p>

 <div class="code">
 <pre>...
 try {
   bar();
 }
 catch(char const *_e) {
   rb_raise(rb_eRuntimeError, _e);
   SWIG_fail;
 }
 ...</pre>
 </div>

 <p> Note that if your methods do not have an exception
 specification yet they do throw exceptions, SWIG cannot know how to
 deal with them. For a neat way to handle these, see the <a href="Customization.html#Customization_exception">Exception
 handling with %exception</a> section.</p>

 <H4><a name="Ruby_directorin_typemap">35.7.6.14 directorin typemap</a></H4>


 <p>Converts C++ objects in director
 member functions to ruby objects. It is roughly the opposite
 of the "in" typemap, making its typemap rule often similar to the "out"
 typemap.
 </p>

 <div class="code"><pre>
 %typemap(directorin) int {
   $result = INT2NUM($1);
 }
 </pre></div>

 <p> The following special variables are available.</p>

 <div class="diagram">
 <table border="1" cellpadding="2" cellspacing="2" width="100%" summary="Special variables - directorin typemap">
   <tbody>
     <tr>
       <td>$result </td>
       <td> Result object
 returned to target language.</td>
     </tr>
     <tr>
       <td>$symname </td>
       <td> Name of
 function/method being wrapped</td>
     </tr>
     <tr>
       <td>$1...n </td>
       <td> Argument being
 wrapped</td>
     </tr>
     <tr>
       <td>$1_name </td>
       <td> Name of the
 argument (if provided)</td>
     </tr>
     <tr>
       <td>$1_type </td>
       <td> The actual C
 datatype matched by the typemap.</td>
     </tr>
     <tr>
       <td>$1_ltype </td>
       <td> The assignable
 version of the C datatype matched by the typemap.</td>
     </tr>
     <tr>
       <td>this </td>
       <td> C++ this,
 referring to the class itself.</td>
     </tr>
   </tbody>
 </table>
 </div>

 <H4><a name="Ruby_directorout_typemap">35.7.6.15 directorout typemap</a></H4>


 <p>Converts Ruby objects in director
 member functions to C++ objects. It is roughly the opposite
 of the "out" typemap, making its rule often similar to the "in"
 typemap.
 </p>

 <div class="code"><pre>
 %typemap(directorout) int {
   $result = NUM2INT($1);
 }
 </pre>
 </div>

 <p> The following special variables are available:</p>

 <div class="diagram">
 <table border="1" cellpadding="2" cellspacing="2" width="100%" summary="Special variables - directorout typemap">
   <tbody>
     <tr>
       <td>$input</td>
       <td>Ruby object being sent to the function</td>
     </tr>
     <tr>
       <td>$symname </td>
       <td>Name of function/method being wrapped</td>
     </tr>
     <tr>
       <td>$1...n </td>
       <td>Argument being sent to the function</td>
     </tr>
     <tr>
       <td>$1_name </td>
       <td> Name of the
 argument (if provided)</td>
     </tr>
     <tr>
       <td>$1_type </td>
       <td> The actual C
 datatype matched by the typemap.</td>
     </tr>
     <tr>
       <td>$1_ltype </td>
       <td> The assignable
 version of the C datatype matched by the typemap.</td>
     </tr>
     <tr>
       <td>this </td>
       <td> C++ this,
 referring to the class itself.</td>
     </tr>
   </tbody>
 </table>
 </div>

 <p>Currently, the directorout nor the out typemap support the
 option <tt>numoutputs</tt>,
 but the Ruby module provides that functionality through a %feature
 directive. Thus, a function can be made to return "nothing"
 if you do:</p>

 <div class="code"><pre>
 %feature("numoutputs", "0") MyClass::function;
 </pre></div>

 <p>This feature can be useful if a function returns a status
 code, which you want to discard but still use the typemap to raise an
 exception.

 </p>

 <H4><a name="Ruby_directorargout_typemap">35.7.6.16 directorargout typemap</a></H4>


 <p>Output argument processing in director
 member functions.</p>

 <div class="code"><pre>
 %typemap(directorargout,
 fragment="output_helper") int {
   $result = output_helper( $result, NUM2INT($1) );
 }
 </pre></div>

 <p> The following special variables are available:</p>

 <div class="diagram">
 <table style="text-align: left; width: 100%;" border="1" cellpadding="2" cellspacing="2" summary="Special variables - directorargout typemap">
   <tbody>
     <tr>
       <td>$result</td>
       <td>Result that the director function returns</td>
     </tr>
     <tr>
       <td>$input</td>
       <td>Ruby object being sent to the function</td>
     </tr>
     <tr>
       <td>$symname</td>
       <td>name of the function/method being wrapped</td>
     </tr>
     <tr>
       <td>$1...n</td>
       <td>Argument being sent to the function</td>
     </tr>
     <tr>
       <td>$1_name</td>
       <td>Name of the
 argument (if provided)</td>
     </tr>
     <tr>
       <td>$1_type</td>
       <td>The actual C
 datatype matched by the typemap</td>
     </tr>
     <tr>
       <td>$1_ltype</td>
       <td>The assignable
 version of the C datatype matched by the typemap</td>
     </tr>
     <tr>
       <td>this</td>
       <td>C++ this,
 referring to the instance of the class itself</td>
     </tr>
   </tbody>
 </table>
 </div>

 <H4><a name="Ruby_ret_typemap">35.7.6.17 ret typemap</a></H4>


 <p>Cleanup of function return values
 </p>

 <H4><a name="Ruby_globalin_typemap">35.7.6.18 globalin typemap</a></H4>


 <p>Setting of C global variables
 </p>

 <H3><a name="Ruby_nn40">35.7.7 Typemap variables</a></H3>


 <p>
 Within a typemap, a number of special variables prefaced with a <tt>$</tt>
 may appear. A full list of variables can be found in the "<a href="Typemaps.html#Typemaps">Typemaps</a>" chapter.
 This is a list of the most common variables:
 </p>

 <p><tt>$1</tt> </p>

 <div class="indent">A C local variable corresponding to
 the actual type specified in the <tt>%typemap</tt>
 directive. For input values, this is a C local variable that is
 supposed to hold an argument value. For output values, this is the raw
 result that is supposed to be returned to Ruby. </div>

 <p><tt>$input</tt></p>

 <div class="indent">A <tt>VALUE</tt> holding
 a raw Ruby object with an argument or variable value. </div>

 <p><tt>$result</tt></p>

 <div class="indent">A <tt>VALUE</tt> that
 holds the result to be returned to Ruby. </div>

 <p><tt>$1_name</tt></p>

 <div class="indent">The parameter name that was matched. </div>

 <p><tt>$1_type</tt></p>

 <div class="indent">The actual C datatype matched by the
 typemap. </div>

 <p><tt>$1_ltype</tt></p>

 <div class="indent">An assignable version of the datatype
 matched by the typemap (a type that can appear on the left-hand-side of
 a C assignment operation). This type is stripped of qualifiers and may
 be an altered version of <tt>$1_type</tt>. All arguments
 and local variables in wrapper functions are declared using this type
 so that their values can be properly assigned. </div>

 <p><tt>$symname</tt></p>

 <div class="indent">The Ruby name of the wrapper function
 being created. </div>

 <H3><a name="Ruby_nn41">35.7.8 Useful Functions</a></H3>


 <p> When you write a typemap, you usually have to work directly
 with Ruby objects. The following functions may prove to be useful.
 (These functions plus many more can be found in <em>Programming
 Ruby</em> book, by David Thomas and Andrew Hunt.)</p>
 <p>In addition, we list equivalent functions that SWIG defines, which
 provide a language neutral conversion (these functions are defined for
 each swig language supported). If you are trying to create a swig
 file that will work under multiple languages, it is recommended you
 stick to the swig functions instead of the native Ruby functions.
 That should help you avoid having to rewrite a lot of typemaps
 across multiple languages.</p>

 <H4><a name="Ruby_nn42">35.7.8.1 C Datatypes to Ruby Objects</a></H4>


 <div class="diagram">
 <table style="width: 100%;" border="1" cellpadding="2" cellspacing="2" summary="Datatypes">

   <tbody>
     <tr>
       <th><b>RUBY</b></th>
       <th><b>SWIG</b></th>
       <td></td>
     </tr>
     <tr>
       <td>INT2NUM(long or int) </td>
       <td>SWIG_From_int(int x)</td>
       <td> int to Fixnum or Bignum</td>
     </tr>
     <tr>
       <td>INT2FIX(long or int) </td>
       <td></td>
       <td> int to Fixnum (faster than INT2NUM)</td>
     </tr>
     <tr>
       <td>CHR2FIX(char) </td>
       <td>SWIG_From_char(char x)</td>
       <td> char to Fixnum</td>
     </tr>
     <tr>
       <td>rb_str_new2(char*) </td>
       <td>SWIG_FromCharPtrAndSize(char*, size_t)</td>
       <td> char* to String</td>
     </tr>
     <tr>
       <td>rb_float_new(double) </td>
       <td>SWIG_From_double(double), <br>
 SWIG_From_float(float)</td>
       <td>float/double to Float</td>
     </tr>
   </tbody>
 </table>
 </div>

 <H4><a name="Ruby_nn43">35.7.8.2 Ruby Objects to C Datatypes</a></H4>


 <p>Here, while the Ruby versions return the value directly, the SWIG
 versions do not, but return a status value to indicate success (<tt>SWIG_OK</tt>). While more awkward to use, this allows you to write typemaps that report more helpful error messages, like:</p>

 <div class="code">
 <pre>
 %typemap(in) size_t (int ok)
   ok = SWIG_AsVal_size_t($input, &amp;$1);
   if (!SWIG_IsOK(ok)) {
     SWIG_exception_fail(SWIG_ArgError(ok), Ruby_Format_TypeError( "$1_name", "$1_type", "$symname", $argnum, $input));
   }
 }
 </pre>
 </div>

 <div class="diagram">
 <table border="1" cellpadding="2" cellspacing="2" width="100%" summary="Ruby objects">
   <tbody>
     <tr>
       <td>int NUM2INT(Numeric)</td>
       <td>SWIG_AsVal_int(VALUE, int*)</td>
     </tr>
     <tr>
       <td>int FIX2INT(Numeric)</td>
       <td>SWIG_AsVal_int(VALUE, int*)</td>
     </tr>
     <tr>
       <td>unsigned int NUM2UINT(Numeric)</td>
       <td>SWIG_AsVal_unsigned_SS_int(VALUE, int*)</td>
     </tr>
     <tr>
       <td>unsigned int FIX2UINT(Numeric)</td>
       <td>SWIG_AsVal_unsigned_SS_int(VALUE, int*)</td>
     </tr>
     <tr>
       <td>long NUM2LONG(Numeric)</td>
       <td>SWIG_AsVal_long(VALUE, long*)</td>
     </tr>
     <tr>
       <td>long FIX2LONG(Numeric)</td>
       <td>SWIG_AsVal_long(VALUE, long*)</td>
     </tr>
     <tr>
       <td>unsigned long FIX2ULONG(Numeric)</td>
       <td>SWIG_AsVal_unsigned_SS_long(VALUE, unsigned long*)</td>
     </tr>
     <tr>
       <td>char NUM2CHR(Numeric or String)</td>
       <td>SWIG_AsVal_char(VALUE, int*)</td>
     </tr>
     <tr>
       <td>char * StringValuePtr(String)</td>
       <td>SWIG_AsCharPtrAndSize(VALUE, char*, size_t, int* alloc)</td>
     </tr>
     <tr>
       <td>char * rb_str2cstr(String, int*length)</td>
       <td></td>
     </tr>
     <tr>
       <td>double NUM2DBL(Numeric)</td>
       <td>(double) SWIG_AsVal_int(VALUE) or similar</td>
     </tr>
   </tbody>
 </table>
 </div>

 <H4><a name="Ruby_nn44">35.7.8.3 Macros for VALUE</a></H4>


 <p> <tt>RSTRING_LEN(str)</tt> </p>

 <div class="indent">length of the Ruby string</div>

 <p><tt>RSTRING_PTR(str)</tt></p>

 <div class="indent">pointer to string storage</div>

 <p><tt>RARRAY_LEN(arr)</tt></p>

 <div class="indent">length of the Ruby array</div>

 <p><tt>RARRAY(arr)-&gt;capa</tt></p>

 <div class="indent">capacity of the Ruby array</div>

 <p><tt>RARRAY_PTR(arr)</tt></p>

 <div class="indent">pointer to array storage</div>

 <H4><a name="Ruby_nn45">35.7.8.4 Exceptions</a></H4>


 <p> <tt>void rb_raise(VALUE exception, const char *fmt,
 ...)</tt> </p>

 <div class="indent"> Raises an exception. The given format
 string <i>fmt</i> and remaining arguments are interpreted
 as with <tt>printf()</tt>. </div>

 <p><tt>void rb_fatal(const char *fmt, ...)</tt></p>

 <div class="indent"> Raises a fatal exception, terminating
 the process. No rescue blocks are called, but ensure blocks will be
 called. The given format string <i>fmt</i> and remaining
 arguments are interpreted as with <tt>printf()</tt>. </div>

 <p><tt>void rb_bug(const char *fmt, ...)</tt></p>

 <div class="indent"> Terminates the process immediately --
 no handlers of any sort will be called. The given format string <i>fmt</i>
 and remaining arguments are interpreted as with <tt>printf()</tt>.
 You should call this function only if a fatal bug has been exposed. </div>

 <p><tt>void rb_sys_fail(const char *msg)</tt></p>

 <div class="indent"> Raises a platform-specific exception
 corresponding to the last known system error, with the given string <i>msg</i>.
 </div>

 <p><tt>VALUE rb_rescue(VALUE (*body)(VALUE), VALUE args,
 VALUE(*rescue)(VALUE, VALUE), VALUE rargs)</tt></p>

 <div class="indent"> Executes <i>body</i>
 with the given <i>args</i>. If a <tt>StandardError</tt>
 exception is raised, then execute <i>rescue</i> with the
 given <i>rargs</i>. </div>

 <p><tt>VALUE rb_ensure(VALUE(*body)(VALUE), VALUE args,
 VALUE(*ensure)(VALUE), VALUE eargs)</tt></p>

 <div class="indent"> Executes <i>body</i>
 with the given <i>args</i>. Whether or not an exception is
 raised, execute <i>ensure</i> with the given <i>rargs</i>
 after <i>body</i> has completed. </div>

 <p><tt>VALUE rb_protect(VALUE (*body)(VALUE), VALUE args,
 int *result)</tt></p>

 <div class="indent"> Executes <i>body</i>
 with the given <i>args</i> and returns nonzero in result
 if any exception was raised. </div>

 <p><tt>void rb_notimplement()</tt></p>

 <div class="indent"> Raises a <tt>NotImpError</tt>
 exception to indicate that the enclosed function is not implemented
 yet, or not available on this platform. </div>

 <p><tt>void rb_exit(int status)</tt></p>

 <div class="indent"> Exits Ruby with the given <i>status</i>.
 Raises a <tt>SystemExit</tt> exception and calls
 registered exit functions and finalizers. </div>

 <p><tt>void rb_warn(const char *fmt, ...)</tt></p>

 <div class="indent"> Unconditionally issues a warning
 message to standard error. The given format string <i>fmt</i>
 and remaining arguments are interpreted as with <tt>printf()</tt>.
 </div>

 <p><tt>void rb_warning(const char *fmt, ...)</tt></p>

 <div class="indent"> Conditionally issues a warning
 message to standard error if Ruby was invoked with the <tt>-w</tt>
 flag. The given format string <i>fmt</i> and remaining
 arguments are interpreted as with <tt>printf()</tt>. </div>

 <H4><a name="Ruby_nn46">35.7.8.5 Iterators</a></H4>


 <p> <tt>void rb_iter_break()</tt> </p>

 <div class="indent"> Breaks out of the enclosing iterator
 block. </div>

 <p><tt>VALUE rb_each(VALUE obj)</tt></p>

 <div class="indent"> Invokes the <tt>each</tt>
 method of the given <i>obj</i>. </div>

 <p><tt>VALUE rb_yield(VALUE arg)</tt></p>

 <div class="indent"> Transfers execution to the iterator
 block in the current context, passing <i>arg</i> as an
 argument. Multiple values may be passed in an array. </div>

 <p><tt>int rb_block_given_p()</tt></p>

 <div class="indent"> Returns <tt>true</tt> if
 <tt>yield</tt> would execute a block in the current
 context; that is, if a code block was passed to the current method and
 is available to be called. </div>

 <p><tt>VALUE rb_iterate(VALUE (*method)(VALUE), VALUE args,
 VALUE (*block)(VALUE, VALUE), VALUE arg2)</tt></p>

 <div class="indent"> Invokes <i>method</i>
 with argument <i>args</i> and block <i>block</i>.
 A <tt>yield</tt> from that method will invoke <i>block</i>
 with the argument given to <tt>yield</tt>, and a second
 argument <i>arg2</i>. </div>

 <p><tt>VALUE rb_catch(const char *tag, VALUE (*proc)(VALUE,
 VALUE), VALUE value)</tt></p>

 <div class="indent"> Equivalent to Ruby's <tt>catch</tt>.
 </div>

 <p><tt>void rb_throw(const char *tag, VALUE value)</tt></p>

 <div class="indent"> Equivalent to Ruby's <tt>throw</tt>.
 </div>

 <H3><a name="Ruby_nn47">35.7.9 Typemap Examples</a></H3>


 <p> This section includes a few examples of typemaps. For more
 examples, you might look at the examples in the <tt>Example/ruby</tt>
 directory. </p>

 <H3><a name="Ruby_nn48">35.7.10 Converting a Ruby array to a char **</a></H3>


 <p> A common problem in many C programs is the processing of
 command line arguments, which are usually passed in an array of <tt>NULL</tt>
 terminated strings. The following SWIG interface file allows a Ruby
 Array instance to be used as a <tt>char **</tt> object. </p>

 <div class="code">
 <pre>%module argv

 // This tells SWIG to treat char ** as a special case
 %typemap(in) char ** {
   /* Get the length of the array */
   int size = RARRAY($input)-&gt;len;
   int i;
   $1 = (char **) malloc((size+1)*sizeof(char *));
   /* Get the first element in memory */
   VALUE *ptr = RARRAY($input)-&gt;ptr;
   for (i=0; i &lt; size; i++, ptr++) {
     /* Convert Ruby Object String to char* */
     $1[i]= StringValuePtr(*ptr);
   }
   $1[i]=NULL; /* End of list */
 }

 // This cleans up the char ** array created before
 // the function call

 %typemap(freearg) char ** {
   free((char *) $1);
 }

 // Now a test function
 %inline %{
 int print_args(char **argv) {
   int i = 0;
   while (argv[i]) {
     printf("argv[%d] = %s\n", i, argv[i]);
     i++;
   }
   return i;
 }
 %}</pre>
 </div>

 <p> When this module is compiled, the wrapped C function now
 operates as follows : </p>

 <div class="code targetlang">
 <pre>require 'Argv'
 Argv.print_args(["Dave", "Mike", "Mary", "Jane", "John"])
 argv[0] = Dave
 argv[1] = Mike
 argv[2] = Mary
 argv[3] = Jane
 argv[4] = John</pre>
 </div>

 <p> In the example, two different typemaps are used. The "in"
 typemap is used to receive an input argument and convert it to a C
 array. Since dynamic memory allocation is used to allocate memory for
 the array, the "freearg" typemap is used to later release this memory
 after the execution of the C function. </p>

 <H3><a name="Ruby_nn49">35.7.11 Collecting arguments in a hash</a></H3>


 <p> Ruby's solution to the "keyword arguments" capability of some
 other languages is to allow the programmer to pass in one or more
 key-value pairs as arguments to a function. All of those key-value
 pairs are collected in a single <tt>Hash</tt> argument
 that's presented to the function. If it makes sense, you might want to
 provide similar functionality for your Ruby interface. For example,
 suppose you'd like to wrap this C function that collects information
 about people's vital statistics: </p>

 <div class="code">
 <pre>void setVitalStats(const char *person, int nattributes, const char **names, int *values);</pre>
 </div>

 <p> and you'd like to be able to call it from Ruby by passing in
 an arbitrary number of key-value pairs as inputs, e.g. </p>

 <div class="code targetlang">
 <pre>setVitalStats("Fred",
   'weight' =&gt; 270,
   'age' =&gt; 42
 )</pre>
 </div>

 <p> To make this work, you need to write a typemap that expects a
 Ruby <tt>Hash</tt> as its input and somehow extracts the
 last three arguments (<i>nattributes</i>, <i>names</i>
 and <i>values</i>) needed by your C function. Let's start
 with the basics: </p>

 <div class="code">
 <pre>%typemap(in) (int nattributes, const char **names, const int *values)
   (VALUE keys_arr, int i, VALUE key, VALUE val) {
 }
  </pre>
 </div>

 <p> This <tt>%typemap</tt> directive tells SWIG that
 we want to match any function declaration that has the specified types
 and names of arguments somewhere in the argument list. The fact that we
 specified the argument names (<i>nattributes</i>, <i>names</i>
 and <i>values</i>) in our typemap is significant; this
 ensures that SWIG won't try to apply this typemap to <i>other</i>
 functions it sees that happen to have a similar declaration with
 different argument names. The arguments that appear in the second set
 of parentheses (<i>keys_arr</i>, <i>i</i>, <i>key</i>
 and <i>val</i>) define local variables that our typemap
 will need. </p>

 <p>Since we expect the input argument to be a <tt>Hash</tt>,
 let's next add a check for that: </p>

 <div class="code">
 <pre>%typemap(in) (int nattributes, const char **names, const int *values)
   (VALUE keys_arr, int i, VALUE key, VALUE val) {
     <b>Check_Type($input, T_HASH);</b>
 }</pre>
 </div>

 <p> <tt>Check_Type()</tt> is just a macro (defined
 in the Ruby header files) that confirms that the input argument is of
 the correct type; if it isn't, an exception will be raised. </p>

 <p>The next task is to determine how many key-value pairs are
 present in the hash; we'll assign this number to the first typemap
 argument (<tt>$1</tt>). This is a little tricky since the
 Ruby/C API doesn't provide a public function for querying the size of a
 hash, but we can get around that by calling the hash's <i>size</i>
 method directly and converting its result to a C <tt>int</tt>
 value: </p>

 <div class="code">
 <pre>%typemap(in) (int nattributes, const char **names, const int *values)
   (VALUE keys_arr, int i, VALUE key, VALUE val) {
     Check_Type($input, T_HASH);
     <b>$1 = NUM2INT(rb_funcall($input, rb_intern("size"), 0, Qnil));</b>
 }</pre>
 </div>

 <p> So now we know the number of attributes. Next we need to
 initialize the second and third typemap arguments (i.e. the two C
 arrays) to <tt>NULL</tt> and set the stage for extracting
 the keys and values from the hash: </p>

 <div class="code">
 <pre>%typemap(in) (int nattributes, const char **names, const int *values)
   (VALUE keys_arr, int i, VALUE key, VALUE val) {
     Check_Type($input, T_HASH);
     $1 = NUM2INT(rb_funcall($input, rb_intern("size"), 0, Qnil));
     <b>$2 = NULL;
     $3 = NULL;
     if ($1 &gt; 0) {
       $2 = (char **) malloc($1*sizeof(char *));
       $3 = (int *) malloc($1*sizeof(int));
     }</b>
 }</pre>
 </div>

 <p> There are a number of ways we could extract the keys and
 values from the input hash, but the simplest approach is to first call
 the hash's <i>keys</i> method (which returns a Ruby array
 of the keys) and then start looping over the elements in that array: </p>

 <div class="code">
 <pre>%typemap(in) (int nattributes, const char **names, const int *values)
   (VALUE keys_arr, int i, VALUE key, VALUE val) {
     Check_Type($input, T_HASH);
     $1 = NUM2INT(rb_funcall($input, rb_intern("size"), 0, Qnil));
     $2 = NULL;
     $3 = NULL;
     if ($1 &gt; 0) {
       $2 = (char **) malloc($1*sizeof(char *));
       $3 = (int *) malloc($1*sizeof(int));
       <b>keys_arr = rb_funcall($input, rb_intern("keys"), 0, Qnil);
       for (i = 0; i &lt; $1; i++) {
       }</b>
     }
 }</pre>
 </div>

 <p> Recall that <i>keys_arr</i> and <i>i</i>
 are local variables for this typemap. For each element in the <i>keys_arr</i>
 array, we want to get the key itself, as well as the value
 corresponding to that key in the hash: </p>

 <div class="code">
 <pre>%typemap(in) (int nattributes, const char **names, const int *values)
   (VALUE keys_arr, int i, VALUE key, VALUE val) {
     Check_Type($input, T_HASH);
     $1 = NUM2INT(rb_funcall($input, rb_intern("size"), 0, Qnil));
     $2 = NULL;
     $3 = NULL;
     if ($1 &gt; 0) {
       $2 = (char **) malloc($1*sizeof(char *));
       $3 = (int *) malloc($1*sizeof(int));
       keys_arr = rb_funcall($input, rb_intern("keys"), 0, Qnil);
       for (i = 0; i &lt; $1; i++) {
         <b>key = rb_ary_entry(keys_arr, i);
         val = rb_hash_aref($input, key);</b>
       }
     }
 }</pre>
 </div>

 <p> To be safe, we should again use the <tt>Check_Type()</tt>
 macro to confirm that the key is a <tt>String</tt> and the
 value is a <tt>Fixnum</tt>: </p>

 <div class="code">
 <pre>%typemap(in) (int nattributes, const char **names, const int *values)
   (VALUE keys_arr, int i, VALUE key, VALUE val) {
     Check_Type($input, T_HASH);
     $1 = NUM2INT(rb_funcall($input, rb_intern("size"), 0, Qnil));
     $2 = NULL;
     $3 = NULL;
     if ($1 &gt; 0) {
       $2 = (char **) malloc($1*sizeof(char *));
       $3 = (int *) malloc($1*sizeof(int));
       keys_arr = rb_funcall($input, rb_intern("keys"), 0, Qnil);
       for (i = 0; i &lt; $1; i++) {
         key = rb_ary_entry(keys_arr, i);
         val = rb_hash_aref($input, key);
         <b>Check_Type(key, T_STRING);
         Check_Type(val, T_FIXNUM);</b>
       }
     }
 }</pre>
 </div>

 <p> Finally, we can convert these Ruby objects into their C
 equivalents and store them in our local C arrays: </p>

 <div class="code">
 <pre>%typemap(in) (int nattributes, const char **names, const int *values)
   (VALUE keys_arr, int i, VALUE key, VALUE val) {
   Check_Type($input, T_HASH);
   $1 = NUM2INT(rb_funcall($input, rb_intern("size"), 0, Qnil));
   $2 = NULL;
   $3 = NULL;
   if ($1 &gt; 0) {
     $2 = (char **) malloc($1*sizeof(char *));
     $3 = (int *) malloc($1*sizeof(int));
     keys_arr = rb_funcall($input, rb_intern("keys"), 0, Qnil);
     for (i = 0; i &lt; $1; i++) {
       key = rb_ary_entry(keys_arr, i);
       val = rb_hash_aref($input, key);
       Check_Type(key, T_STRING);
       Check_Type(val, T_FIXNUM);
       <b>$2[i] = StringValuePtr(key);
       $3[i] = NUM2INT(val);</b>
     }
   }
 }</pre>
 </div>

 <p> We're not done yet. Since we used <tt>malloc()</tt>
 to dynamically allocate the memory used for the <i>names</i>
 and <i>values</i> arguments, we need to provide a
 corresponding "freearg" typemap to free that memory so that there is no
 memory leak. Fortunately, this typemap is a lot easier to write: </p>

 <div class="code">
 <pre>%typemap(freearg) (int nattributes, const char **names, const int *values) {
   free((void *) $2);
   free((void *) $3);
 }</pre>
 </div>

 <p> All of the code for this example, as well as a sample Ruby
 program that uses the extension, can be found in the <tt>Examples/ruby/hashargs</tt>
 directory of the SWIG distribution. </p>

 <H3><a name="Ruby_nn50">35.7.12 Pointer handling</a></H3>


 <p> Occasionally, it might be necessary to convert pointer values
 that have been stored using the SWIG typed-pointer representation.
 Since there are several ways in which pointers can be represented, the
 following two functions are used to safely perform this conversion: </p>

 <p><tt>int SWIG_ConvertPtr(VALUE obj, void **ptr,
 swig_type_info *ty, int flags)</tt> </p>

 <div class="indent">Converts a Ruby object <i>obj</i>
 to a C pointer whose address is <i>ptr</i> (i.e. <i>ptr</i>
 is a pointer to a pointer). The third argument, <i>ty</i>,
 is a pointer to a SWIG type descriptor structure. If <i>ty</i>
 is not <tt>NULL</tt>, that type information is used to
 validate type compatibility and other aspects of the type conversion.
 If <i>flags</i> is non-zero, any type errors encountered
 during this validation result in a Ruby <tt>TypeError</tt>
 exception being raised; if <i>flags</i> is zero, such type
 errors will cause <tt>SWIG_ConvertPtr()</tt> to return -1
 but not raise an exception. If <i>ty</i> is <tt>NULL</tt>,
 no type-checking is performed. </div>

 <p> <tt>VALUE SWIG_NewPointerObj(void *ptr, swig_type_info
 *ty, int own)</tt> </p>

 <div class="indent">Creates a new Ruby pointer object.
 Here, <i>ptr</i> is the pointer to convert, <i>ty</i>
 is the SWIG type descriptor structure that describes the type, and <i>own</i>
 is a flag that indicates whether or not Ruby should take ownership of
 the pointer (i.e. whether Ruby should free this data when the
 corresponding Ruby instance is garbage-collected). </div>

 <p> Both of these functions require the use of a special SWIG
 type-descriptor structure. This structure contains information about
 the mangled name of the datatype, type-equivalence information, as well
 as information about converting pointer values under C++ inheritance.
 For a type of <tt>Foo *</tt>, the type descriptor
 structure is usually accessed as follows: </p>

 <div class="indent code">
 <pre>Foo *foo;
 SWIG_ConvertPtr($input, (void **) &amp;foo, SWIGTYPE_p_Foo, 1);

 VALUE obj;
 obj = SWIG_NewPointerObj(f, SWIGTYPE_p_Foo, 0);</pre>
 </div>

 <p> In a typemap, the type descriptor should always be accessed
 using the special typemap variable <tt>$1_descriptor</tt>.
 For example: </p>

 <div class="indent code">
 <pre>%typemap(in) Foo * {
   SWIG_ConvertPtr($input, (void **) &amp;$1, $1_descriptor, 1);
 }</pre>
 </div>

 <H4><a name="Ruby_nn51">35.7.12.1 Ruby Datatype Wrapping</a></H4>


 <p> <tt>VALUE Data_Wrap_Struct(VALUE class, void
 (*mark)(void *), void (*free)(void *), void *ptr)</tt> </p>

 <div class="indent">Given a pointer <i>ptr</i>
 to some C data, and the two garbage collection routines for this data (<i>mark</i>
 and <i>free</i>), return a <tt>VALUE</tt> for
 the Ruby object. </div>

 <p><tt>VALUE Data_Make_Struct(VALUE class, <i>c-type</i>,
 void (*mark)(void *), void (*free)(void *), <i>c-type</i>
 *ptr)</tt></p>

 <div class="indent">Allocates a new instance of a C data
 type <i>c-type</i>, assigns it to the pointer <i>ptr</i>,
 then wraps that pointer with <tt>Data_Wrap_Struct()</tt>
 as above. </div>

 <p><tt>Data_Get_Struct(VALUE obj, <i>c-type</i>,
 <i>c-type</i> *ptr)</tt></p>

 <div class="indent">Retrieves the original C pointer of
 type <i>c-type</i> from the data object <i>obj</i>
 and assigns that pointer to <i>ptr</i>. </div>

 <H3><a name="Ruby_nn52">35.7.13 Example: STL Vector to Ruby Array</a></H3>


 <p>Another use for macros and type maps is to create a Ruby array
 from a STL vector of pointers. In essence, copy of all the pointers in
 the vector into a Ruby array. The use of the macro is to make the
 typemap so generic that any vector with pointers can use the type map.
 The following is an example of how to construct this type of
 macro/typemap and should give insight into constructing similar
 typemaps for other STL structures: </p>

 <div class="code">
 <pre>%define PTR_VECTOR_TO_RUBY_ARRAY(vectorclassname, classname)
 %typemap(out) vectorclassname &amp;, const vectorclassname &amp; {
   VALUE arr = rb_ary_new2($1-&gt;size());
   vectorclassname::iterator i = $1-&gt;begin(), iend = $1-&gt;end();
   for ( ; i!=iend; i++ )
     rb_ary_push(arr, Data_Wrap_Struct(c ## classname.klass, 0, 0, *i));
   $result = arr;
 }
 %typemap(out) vectorclassname, const vectorclassname {
   VALUE arr = rb_ary_new2($1.size());
   vectorclassname::iterator i = $1.begin(), iend = $1.end();
   for ( ; i!=iend; i++ )
     rb_ary_push(arr, Data_Wrap_Struct(c ## classname.klass, 0, 0, *i));
   $result = arr;
 }
 %enddef</pre>
 </div>

 <p> Note, that the "<tt>c ## classname.klass"</tt> is
 used in the preprocessor step to determine the actual object from the
 class name. </p>

 <p>To use the macro with a class Foo, the following is used: </p>

 <div class="code">
 <pre>PTR_VECTOR_TO_RUBY_ARRAY(vector&lt;foo *=""&gt;, Foo)</pre>
 </div>

 <p> It is also possible to create a STL vector of Ruby objects: </p>

 <div class="code">
 <pre>%define RUBY_ARRAY_TO_PTR_VECTOR(vectorclassname, classname)
 %typemap(in) vectorclassname &amp;, const vectorclassname &amp; {
   Check_Type($input, T_ARRAY);
   vectorclassname *vec = new vectorclassname;
   int len = RARRAY($input)-&gt;len;
   for (int i=0; i!=len; i++) {
     VALUE inst = rb_ary_entry($input, i);
     //The following _should_ work but doesn't on HPUX
     // Check_Type(inst, T_DATA);
     classname *element = NULL;
     Data_Get_Struct(inst, classname, element);
     vec-&gt;push_back(element);
   }
   $1 = vec;
 }

 %typemap(freearg) vectorclassname &amp;, const vectorclassname &amp; {
   delete $1;
 }
 %enddef</pre>
 </div>

 <p> It is also possible to create a Ruby array from a vector of
 static data types: </p>

 <div class="code">
 <pre>%define VECTOR_TO_RUBY_ARRAY(vectorclassname, classname)
 %typemap(out) vectorclassname &amp;, const vectorclassname &amp; {
   VALUE arr = rb_ary_new2($1-&gt;size());
   vectorclassname::iterator i = $1-&gt;begin(), iend = $1-&gt;end();
   for ( ; i!=iend; i++ )
     rb_ary_push(arr, Data_Wrap_Struct(c ## classname.klass, 0, 0, &amp;(*i)));
   $result = arr;
 }
 %typemap(out) vectorclassname, const vectorclassname {
   VALUE arr = rb_ary_new2($1.size());
   vectorclassname::iterator i = $1.begin(), iend = $1.end();
   for ( ; i!=iend; i++ )
     rb_ary_push(arr, Data_Wrap_Struct(c ## classname.klass, 0, 0, &amp;(*i)));
   $result = arr;
 }
 %enddef</pre>
 </div>

 Note that this is mostly an example of typemaps. If you want to use the
 STL with ruby, you are advised to use the standard swig STL library,
 which does much more than this. Refer to the section called
 the<a href="#Ruby_nn23_1"> C++ Standard Template Library</a>.

 <H2><a name="Ruby_nn65">35.8 Docstring Features</a></H2>


 <p>
 Using ri and rdoc web pages in Ruby libraries is a common practice.
 Given the way that SWIG generates the extensions by default, your users
 will normally not get
 any documentation for it, even if they run 'rdoc' on the resulting .c
 or .cxx file.</p>

 <p>The features described in this section make it easy for you to
 add
 rdoc strings to your modules, functions and methods that can then be
 read by Ruby's rdoc tool to generate html web pages, ri documentation,
 Windows chm file and an .xml description.</p>

 <p>rdoc can then be run from a console or shell window on a swig
 generated file.</p>

 <p>For example, to generate html web pages from a C++ file, you'd
 do:</p>

 <div class="code shell">
 <pre>
 $ rdoc -E cxx=c -f html file_wrap.cxx
 </pre></div>

 <p>To
 generate ri documentation from a c wrap file, you could do:</p>

 <div class="code shell"><pre>
 $ rdoc -r file_wrap.c
 </pre></div>

 <H3><a name="Ruby_nn66">35.8.1 Module docstring</a></H3>


 <p>
 Ruby allows a docstring at the beginning of the file
 before any other statements, and it is typically used to give a
 general description of the entire module. SWIG supports this by
 setting an option of the <tt>%module</tt> directive. For
 example:
 </p>

 <div class="code">
 <pre>%module(docstring="This is the example module's docstring") example</pre>
 </div>

 <p>
 When you have more than just a line or so then you can retain the easy
 readability of the <tt>%module</tt> directive by using a
 macro. For example:
 </p>

 <div class="code">
 <pre>%define DOCSTRING
 "The `XmlResource` class allows program resources defining menus,
 layout of controls on a panel, etc. to be loaded from an XML file."
 %enddef

 %module(docstring=DOCSTRING) xrc</pre>
 </div>

 <H3><a name="Ruby_nn67">35.8.2 %feature("autodoc")</a></H3>


 <p>Since SWIG does know everything about the function it wraps,
 it is possible to generate an rdoc containing the parameter types,
 names
 and default values. Since Ruby ships with one of the best documentation
 systems of any language, it makes sense to take advantage of it.
 </p>

 <p>SWIG's Ruby module provides support for the "autodoc"
 feature,
 which when attached to a node in the parse tree will cause an rdoc
 comment
 to be generated in the wrapper file that includes the name of the
 function, parameter
 names, default values if any, and return type if any. There are also
 several options for autodoc controlled by the value given to the
 feature, described below.
 </p>

 <H4><a name="Ruby_nn68">35.8.2.1 %feature("autodoc", "0")</a></H4>


 <p>
 When the "0" option is given then the types of the parameters will
 <em>not</em> be included in the autodoc string. For
 example, given
 this function prototype:
 </p>

 <div class="code">
 <pre>%feature("autodoc", "0");
 bool function_name(int x, int y, Foo* foo=NULL, Bar* bar=NULL);</pre>
 </div>

 <p>
 Then Ruby code like this will be generated:
 </p>

 <div class="targetlang">
 <pre>function_name(x, y, foo=nil, bar=nil) -&gt; bool
   ...</pre>
 </div>

 <H4><a name="Ruby_autodoc1">35.8.2.2 %feature("autodoc", "1")</a></H4>


 <p>
 When the "1" option is used then the parameter types <em>will</em>
 be used in the rdoc string. In addition, an attempt is made to
 simplify the type name such that it makes more sense to the Ruby
 user. Pointer, reference and const info is removed,
 <tt>%rename</tt>'s are evaluated, etc. (This is not always
 successful, but works most of the time. See the next section for what
 to do when it doesn't.) Given the example above, then turning on the
 parameter types with the "1" option will result in rdoc code like
 this:
 </p>

 <div class="targetlang">
 <pre>function_name(int x, int y, Foo foo=nil, Bar bar=nil) -&gt; bool
   ...</pre>
 </div>

 <H4><a name="Ruby_autodoc2">35.8.2.3 %feature("autodoc", "2")</a></H4>


 <p>
 When the "2" option is used then the parameter types will not
 be
 used in the rdoc string. However, they will be listed in full after the
 function. Given the example above, then turning on the
 parameter types with the "2" option will result in Ruby code like
 this:
 </p>

 <H4><a name="Ruby_feature_autodoc3">35.8.2.4 %feature("autodoc", "3")</a></H4>


 <p>
 When the "3" option is used then the function will be documented using
 a combination of "1" and "2" above. Given the example above,
 then turning on the
 parameter types with the "2" option will result in Ruby code like
 this:
 </p>

 <div class="targetlang">
 <pre>function_name(int x, int y, Foo foo=nil, Bar bar=nil) -&gt; bool

 Parameters:
         x - int
         y - int
         foo - Foo
         bar - Bar</pre>
 </div>

 <H4><a name="Ruby_nn70">35.8.2.5 %feature("autodoc", "docstring")</a></H4>


 <p>
 Finally, there are times when the automatically generated autodoc
 string will make no sense for a Ruby programmer, particularly when a
 typemap is involved. So if you give an explicit value for the autodoc
 feature then that string will be used in place of the automatically
 generated string. For example:
 </p>

 <div class="code">
 <pre>%feature("autodoc", "GetPosition() -&gt; (x, y)") GetPosition;
 void GetPosition(int* OUTPUT, int* OUTPUT);</pre>
 </div>

 <H3><a name="Ruby_nn71">35.8.3 %feature("docstring")</a></H3>


 <p>
 In addition to the autodoc strings described above, you can also
 attach any arbitrary descriptive text to a node in the parse tree with
 the "docstring" feature. When the proxy module is generated then any
 docstring associated with classes, function or methods are output.
 If an item already has an autodoc string then it is combined with the
 docstring and they are output together. </p>

 <H2><a name="Ruby_nn53">35.9 Advanced Topics</a></H2>


 <H3><a name="Ruby_operator_overloading">35.9.1 Operator overloading</a></H3>


 <p> SWIG allows operator overloading with, by using the <tt>%extend</tt>
 or <tt>%rename</tt> commands in SWIG and the following
 operator names (derived from Python): </p>

 <div class="code diagram">
 <table style="width: 100%; font-family: monospace;" border="1" cellpadding="2" cellspacing="2" summary="operator names">
   <tbody>
     <tr>
       <td><b> General</b></td>
     </tr>
     <tr>
       <td>__repr__ </td>
       <td> inspect</td>
     </tr>
     <tr>
       <td>__str__ </td>
       <td> to_s</td>
     </tr>
     <tr>
       <td>__cmp__ </td>
       <td> &lt;=&gt;</td>
     </tr>
     <tr>
       <td>__hash__ </td>
       <td> hash</td>
     </tr>
     <tr>
       <td>__nonzero__ </td>
       <td> nonzero?</td>
     </tr>
     <tr>
       <td></td>
     </tr>
     <tr>
       <td><b> Callable</b></td>
     </tr>
     <tr>
       <td>__call__ </td>
       <td> call</td>
     </tr>
     <tr>
       <td></td>
     </tr>
     <tr>
       <td><b> Collection</b></td>
     </tr>
     <tr>
       <td>__len__ </td>
       <td> length</td>
     </tr>
     <tr>
       <td>__getitem__ </td>
       <td> []</td>
     </tr>
     <tr>
       <td>__setitem__ </td>
       <td> []=</td>
     </tr>
     <tr>
       <td></td>
     </tr>
     <tr>
       <td><b> Numeric</b></td>
     </tr>
     <tr>
       <td>__add__ </td>
       <td> +</td>
     </tr>
     <tr>
       <td>__sub__ </td>
       <td> -</td>
       <td></td>
     </tr>
     <tr>
       <td>__mul__ </td>
       <td> *</td>
     </tr>
     <tr>
       <td>__div__ </td>
       <td> /</td>
     </tr>
     <tr>
       <td>__mod__ </td>
       <td> %</td>
     </tr>
     <tr>
       <td>__divmod__ </td>
       <td> divmod</td>
     </tr>
     <tr>
       <td>__pow__ </td>
       <td> **</td>
     </tr>
     <tr>
       <td>__lshift__ </td>
       <td> &lt;&lt;</td>
     </tr>
     <tr>
       <td>__rshift__ </td>
       <td> &gt;&gt;</td>
     </tr>
     <tr>
       <td>__and__ </td>
       <td> &amp;</td>
     </tr>
     <tr>
       <td>__xor__ </td>
       <td> ^</td>
     </tr>
     <tr>
       <td>__or__ </td>
       <td> |</td>
     </tr>
     <tr>
       <td>__neg__ </td>
       <td> -@</td>
       <td></td>
     </tr>
     <tr>
       <td>__pos__ </td>
       <td> +@</td>
     </tr>
     <tr>
       <td>__abs__ </td>
       <td> abs</td>
     </tr>
     <tr>
       <td>__invert__ </td>
       <td> ~</td>
     </tr>
     <tr>
       <td>__int__ </td>
       <td> to_i</td>
     </tr>
     <tr>
       <td>__float__ </td>
       <td> to_f</td>
     </tr>
     <tr>
       <td>__coerce__ </td>
       <td> coerce</td>
     </tr>
     <tr>
       <td></td>
     </tr>
     <tr>
       <td><b>Additions in 1.3.13 </b></td>
     </tr>
     <tr>
       <td>__lt__ </td>
       <td> &lt;</td>
     </tr>
     <tr>
       <td>__le__ </td>
       <td> &lt;=</td>
     </tr>
     <tr>
       <td>__eq__ </td>
       <td> ==</td>
     </tr>
     <tr>
       <td>__gt__ </td>
       <td> &gt;</td>
     </tr>
     <tr>
       <td>__ge__ </td>
       <td> &gt;=</td>
     </tr>
   </tbody>
 </table>
 </div>

 <p> Note that although SWIG supports the <tt>__eq__</tt>
 magic method name for defining an equivalence operator, there is no
 separate method for handling <i>inequality</i> since Ruby
 parses the expression <i>a != b</i> as <i>!(a == b)</i>.
 </p>

 <H3><a name="Ruby_nn55">35.9.2 Creating Multi-Module Packages</a></H3>


 <p> The chapter on <a href="Modules.html#Modules">Working
 with Modules</a> discusses the basics of creating multi-module
 extensions with SWIG, and in particular the considerations for sharing
 runtime type information among the different modules. </p>

 <p>As an example, consider one module's interface file (<tt>shape.i</tt>)
 that defines our base class: </p>

 <div class="code">
 <pre>%module shape

 %{
 #include "Shape.h"
 %}

 class Shape {
 protected:
   double xpos;
   double ypos;
 protected:
   Shape(double x, double y);
 public:
   double getX() const;
   double getY() const;
 };</pre>
 </div>

 <p> We also have a separate interface file (<tt>circle.i</tt>)
 that defines a derived class: </p>

 <div class="code">
 <pre>%module circle

 %{
 #include "Shape.h"
 #include "Circle.h"
 %}

 // Import the base class definition from Shape module
 %import shape.i

 class Circle : public Shape {
 protected:
   double radius;
 public:
   Circle(double x, double y, double r);
   double getRadius() const;
 };</pre>
 </div>

 <p> We'll start by building the <b>Shape</b>
 extension module: </p>

 <div class="code shell">
 <pre>$ swig -c++ -ruby shape.i
 </pre>
 </div>

 <p> SWIG generates a wrapper file named <tt>shape_wrap.cxx</tt>.
 To compile this into a dynamically loadable extension for Ruby, prepare
 an <tt>extconf.rb</tt> script using this template: </p>

 <div class="code targetlang">
 <pre>require 'mkmf'

 # Since the SWIG runtime support library for Ruby
 # depends on the Ruby library, make sure it's in the list
 # of libraries.
 $libs = append_library($libs, Config::CONFIG['RUBY_INSTALL_NAME'])

 # Create the makefile
 create_makefile('shape')</pre>
 </div>

 <p> Run this script to create a <tt>Makefile</tt>
 and then type <tt>make</tt> to build the shared library: </p>

 <div class="code targetlang">
 <pre>$ <b>ruby extconf.rb</b>
 creating Makefile
 $ <b>make</b>
 g++ -fPIC -g -O2 -I. -I/usr/include/ruby-2.1.0 \
 -I. -c shape_wrap.cxx
 gcc -shared -L/usr/local/lib -o shape.so shape_wrap.o -L. \
 -lruby -lruby -lc</pre>
 </div>

 <p> Note that depending on your installation, the outputs may be
 slightly different; these outputs are those for a Linux-based
 development environment. The end result should be a shared library
 (here, <tt>shape.so</tt>) containing the extension module
 code. Now repeat this process in a separate directory for the <b>Circle</b>
 module: </p>

 <ol>
   <li> Run SWIG to generate the wrapper code (<tt>circle_wrap.cxx</tt>);
   </li>
   <li> Write an <tt>extconf.rb</tt> script that your
 end-users can use to create a platform-specific <tt>Makefile</tt>
 for the extension; </li>
   <li> Build the shared library for this extension by typing <tt>make</tt>.
   </li>
 </ol>

 <p> Once you've built both of these extension modules, you can
 test them interactively in IRB to confirm that the <tt>Shape</tt>
 and <tt>Circle</tt> modules are properly loaded and
 initialized: </p>

 <div class="code targetlang">
 <pre>$ <b>irb</b>
 irb(main):001:0&gt; <b>require 'shape'</b>
 true
 irb(main):002:0&gt; <b>require 'circle'</b>
 true
 irb(main):003:0&gt; <b>c = Circle::Circle.new(5, 5, 20)</b>
 #&lt;Circle::Circle:0xa097208&gt;
 irb(main):004:0&gt; <b>c.kind_of? Shape::Shape</b>
 true
 irb(main):005:0&gt; <b>c.getX()</b>
 5.0</pre>
 </div>

 <H3><a name="Ruby_nn56">35.9.3 Specifying Mixin Modules</a></H3>


 <p> The Ruby language doesn't support multiple inheritance, but
 it does allow you to mix one or more modules into a class using Ruby's <tt>include</tt>
 method. For example, if you have a Ruby class that defines an <em>each</em>
 instance method, e.g. </p>

 <div class="code targetlang">
 <pre>class Set
   def initialize
   @members = []
   end

   def each
   @members.each { |m| yield m }
   end
 end</pre>
 </div>

 <p> then you can mix-in Ruby's <tt>Enumerable</tt>
 module to easily add a lot of functionality to your class: </p>

 <div class="code targetlang">
 <pre>class Set
   <b>include Enumerable</b>
   def initialize
     @members = []
   end
   def each
     @members.each { |m| yield m }
   end
 end</pre>
 </div>

 <p> To get the same benefit for your SWIG-wrapped classes, you
 can use the <tt>%mixin</tt> directive to specify the names
 of one or more modules that should be mixed-in to a class. For the
 above example, the SWIG interface specification might look like this: </p>

 <div class="code">
 <pre>%mixin Set "Enumerable";

 class Set {
 public:
   // Constructor
   Set();

   // Iterates through set members
   void each();
 };</pre>
 </div>

 <p> Multiple modules can be mixed into a class by providing a
 comma-separated list of module names to the <tt>%mixin</tt>
 directive, e.g. </p>

 <div class="code">
 <pre>%mixin Set "Fee, Fi, Fo, Fum";</pre>
 </div>

 <p> Note that the <tt>%mixin</tt> directive is
 implemented using SWIG's "features" mechanism and so the same name
 matching rules used for other kinds of features apply (see the chapter
 on <a href="Customization.html#Customization">"Customization
 Features"</a>) for more details). </p>

 <H2><a name="Ruby_nn57">35.10 Memory Management</a></H2>


 <p>One of the most common issues in generating SWIG bindings for
 Ruby is proper memory management. The key to proper memory management
 is clearly defining whether a wrapper Ruby object owns the underlying C
 struct or C++ class. There are two possibilities:</p>

 <ul>

   <li> The Ruby object is responsible for freeing the C struct or
 C++ object </li>

   <li> The Ruby object should not free the C struct or C++ object
 because it will be freed by the underlying C or C++ code</li>

 </ul>

 <p>To complicate matters, object ownership may transfer from Ruby
 to C++ (or vice versa) depending on what function or methods are
 invoked. Clearly, developing a SWIG wrapper requires a thorough
 understanding of how the underlying library manages memory.</p>

 <H3><a name="Ruby_nn58">35.10.1 Mark and Sweep Garbage Collector </a></H3>


 <p>Ruby uses a mark and sweep garbage collector. When the garbage
 collector runs, it finds all the "root" objects, including local
 variables, global variables, global constants, hardware registers and
 the C stack. For each root object, the garbage collector sets its mark
 flag to true and calls <tt>rb_gc_mark</tt> on the object.
 The job of <tt>rb_gc_mark</tt> is to recursively mark all
 the objects that a Ruby object has a reference to (ignoring those
 objects that have already been marked). Those objects, in turn, may
 reference other objects. This process will continue until all active
 objects have been "marked." After the mark phase comes the sweep phase.
 In the sweep phase, all objects that have not been marked will be
 garbage collected. </p>

 <p>The Ruby C/API provides extension developers two hooks into
 the garbage collector - a "mark" function and a "sweep" function. By
 default these functions are set to NULL.</p>

 <p>If a C struct or C++ class references any other Ruby objects,
 then it must provide a "mark" function. The "mark" function should
 identify any referenced Ruby objects by calling the rb_gc_mark function
 for each one. Unsurprisingly, this function will be called by the Ruby
 garbage during the "mark" phase.</p>

 <p>During the sweep phase, Ruby destroys any unused objects. If
 any memory has been allocated in creating the underlying C struct or
 C++ struct, then a "free" function must be defined that deallocates
 this memory. </p>

 <H3><a name="Ruby_nn59">35.10.2 Object Ownership</a></H3>


 <p>As described above, memory management depends on clearly
 defining who is responsible for freeing the underlying C struct or C++
 class. If the Ruby object is responsible for freeing the C++ object,
 then a "free" function must be registered for the object. If the Ruby
 object is not responsible for freeing the underlying memory, then a
 "free" function must not be registered for the object.</p>

 <p>For the most part, SWIG takes care of memory management
 issues. The rules it uses are:</p>

 <ul>

   <li> When calling a C++ object's constructor from Ruby, SWIG
 will assign a "free" function thereby making the Ruby object
 responsible for freeing the C++ object</li>

   <li> When calling a C++ member function that returns a pointer,
 SWIG will not assign a "free" function thereby making the underlying
 library responsible for freeing the object.</li>

 </ul>

 <p>To make this clearer, let's look at an example. Assume we have
 a Foo and a Bar class. </p>

 <div class="code">
 <pre>/* File "RubyOwernshipExample.h" */

 class Foo
 {
 public:
   Foo() {}
   ~Foo() {}
 };

 class Bar
 {
   Foo *foo_;
 public:
   Bar(): foo_(new Foo) {}
   ~Bar() { delete foo_; }
   Foo* get_foo() { return foo_; }
   Foo* get_new_foo() { return new Foo; }
   void set_foo(Foo *foo) { delete foo_; foo_ = foo; }
 };</pre>
 </div>

 <p>First, consider this Ruby code: </p>

 <div class="code targetlang">
 <pre>foo = Foo.new</pre>
 </div>

 <p>In this case, the Ruby code calls the underlying <tt>Foo</tt>
 C++ constructor, thus creating a new <tt>foo</tt> object.
 By default, SWIG will assign the new Ruby object a "free" function.
 When the Ruby object is garbage collected, the "free" function will be
 called. It in turn will call <tt>Foo</tt>'s destructor.</p>

 <p>Next, consider this code: </p>

 <div class="code targetlang">
 <pre>bar = Bar.new
 foo = bar.get_foo()</pre>
 </div>

 <p>In this case, the Ruby code calls a C++ member function, <tt>get_foo</tt>.
 By default, SWIG will not assign the Ruby object a "free" function.
 Thus, when the Ruby object is garbage collected the underlying C++ <tt>foo</tt>
 object is not affected.</p>

 <p>Unfortunately, the real world is not as simple as the examples
 above. For example:</p>

 <div class="code targetlang">
 <pre>bar = Bar.new
 foo = bar.get_new_foo()</pre>
 </div>

 <p>In this case, the default SWIG behavior for calling member
 functions is incorrect. The Ruby object should assume ownership of the
 returned object. This can be done by using the %newobject directive.
 See <a href="Customization.html#Customization_ownership">
 Object ownership and %newobject</a> for more information. </p>

 <p>The SWIG default mappings are also incorrect in this case:</p>

 <div class="code targetlang">
 <pre>foo = Foo.new
 bar = Bar.new
 bar.set_foo(foo)</pre>
 </div>

 <p>Without modification, this code will cause a segmentation
 fault. When the Ruby <tt>foo</tt> object goes out of
 scope, it will free the underlying C++ <tt>foo</tt>
 object. However, when the Ruby bar object goes out of scope, it will
 call the C++ bar destructor which will also free the C++ <tt>foo</tt>
 object. The problem is that object ownership is transferred from the
 Ruby object to the C++ object when the <tt>set_foo</tt>
 method is called. This can be done by using the special DISOWN type
 map, which was added to the Ruby bindings in SWIG-1.3.26.</p>

 <p>Thus, a correct SWIG interface file correct mapping for these
 classes is:</p>

 <div class="code">
 <pre>/* File RubyOwnershipExample.i */

 %module RubyOwnershipExample

 %{
 #include "RubyOwnershipExample.h"
 %}

 class Foo
 {
 public:
   Foo();
   ~Foo();
 };

 class Bar
 {
   Foo *foo_;
 public:
   Bar();
   ~Bar();
   Foo* get_foo();

 <b>  %newobject get_new_foo;</b>
   Foo* get_new_foo();

 <b>  %apply SWIGTYPE *DISOWN {Foo *foo};</b>
   void set_foo(Foo *foo);
 <b>  %clear Foo *foo;</b>
 };
 </pre>
 </div>

 <p> This code can be seen in swig/examples/ruby/tracking.</p>

 <H3><a name="Ruby_nn60">35.10.3 Object Tracking</a></H3>


 <p>The remaining parts of this section will use the class library
 shown below to illustrate different memory management techniques. The
 class library models a zoo and the animals it contains. </p>

 <div class="code">
 <pre>%module zoo

 %{
 #include &lt;string&gt;
 #include &lt;vector&gt;

 #include "zoo.h"
 %}

 class Animal
 {
 private:
   typedef std::vector&lt;Animal*&gt; AnimalsType;
   typedef AnimalsType::iterator IterType;
 protected:
   AnimalsType animals;
 protected:
   std::string name_;
 public:
   // Construct an animal with this name
   Animal(const char* name) : name_(name) {}

   // Return the animal's name
   const char* get_name() const { return name.c_str(); }
 };

 class Zoo
 {
 protected:
  std::vector&lt;Animal *&gt; animals;

 public:
   // Construct an empty zoo
   Zoo() {}

   /* Create a new animal. */
   static Animal* Zoo::create_animal(const char* name) {
     return new Animal(name);
   }

   // Add a new animal to the zoo
   void add_animal(Animal* animal) {
     animals.push_back(animal);
   }

   Animal* remove_animal(size_t i) {
     Animal* result = this-&gt;animals[i];
     IterType iter = this-&gt;animals.begin();
     std::advance(iter, i);
     this-&gt;animals.erase(iter);

     return result;
   }

   // Return the number of animals in the zoo
   size_t get_num_animals() const {
     return animals.size();
   }

   // Return a pointer to the ith animal
   Animal* get_animal(size_t i) const {
     return animals[i];
   }
 };</pre>
 </div>

 <p>Let's say you SWIG this code and then run IRB:

 </p>

 <div class="code targetlang">
 <pre>$ <b>irb</b>
 irb(main):001:0&gt; <b>require 'example'</b>
 =&gt; true

 irb(main):002:0&gt; <b>tiger1 = Example::Animal.new("tiger1")</b>
 =&gt; #&lt;Example::Animal:0x2be3820&gt;

 irb(main):004:0&gt; <b>tiger1.get_name()</b>
 =&gt; "tiger1"

 irb(main):003:0&gt; <b>zoo = Example::Zoo.new()</b>
 =&gt; #&lt;Example::Zoo:0x2be0a60&gt;

 irb(main):006:0&gt; <b>zoo.add_animal(tiger)</b>
 =&gt; nil

 irb(main):007:0&gt; <b>zoo.get_num_animals()</b>
 =&gt; 1

 irb(main):007:0&gt; <b>tiger2 = zoo.remove_animal(0)</b>
 =&gt; #&lt;Example::Animal:0x2bd4a18&gt;

 irb(main):008:0&gt; <b>tiger2.get_name()</b>
 =&gt; "tiger1"

 irb(main):009:0&gt; <b>tiger1.equal?(tiger2)</b>
 =&gt; false
 </pre>
 </div>

 <p>Pay particular attention to the code <tt>tiger1.equal?(tiger2)</tt>.
 Note that the two Ruby objects are not the same - but they reference
 the same underlying C++ object. This can cause problems. For example:
 </p>

 <div class="code targetlang">
 <pre>irb(main):010:0&gt; <b>tiger1 = nil</b>
 =&gt; nil

 irb(main):011:0&gt; <b>GC.start</b>
 =&gt; nil

 irb(main):012:0&gt; <b>tiger2.get_name()</b>
 (irb):12: [BUG] Segmentation fault
 </pre>
 </div>

 <p>After the garbage collector runs, as a result of our call
 to <tt>GC.start</tt>, calling<tt>tiger2.get_name()</tt>
 causes a segmentation fault. The problem is that when <tt>tiger1</tt>
 is garbage collected, it frees the underlying C++ object. Thus, when <tt>tiger2</tt>
 calls the <tt>get_name()</tt> method it invokes it on a
 destroyed object.</p>

 <p>This problem can be avoided if SWIG enforces a one-to-one
 mapping between Ruby objects and C++ classes. This can be done via the
 use of the <tt>%trackobjects</tt> functionality available
 in SWIG-1.3.26. and later.</p>

 <p>When the <tt>%trackobjects</tt> is turned on,
 SWIG automatically keeps track of mappings between C++ objects and Ruby
 objects. Note that enabling object tracking causes a slight performance
 degradation. Test results show this degradation to be about 3% to 5%
 when creating and destroying 100,000 animals in a row.</p>

 <p>Since <tt>%trackobjects</tt> is implemented as a <tt>%feature</tt>,
 it uses the same name matching rules as other kinds of features (see
 the chapter on <a href="Customization.html#Customization">
 "Customization Features"</a>) . Thus it can be applied on a
 class-by-class basis if needed. To fix the example above:</p>

 <div class="code">
 <pre>%module example

 %{
 #include "example.h"
 %}

 <b>/* Tell SWIG that create_animal creates a new object */</b>
 <b>%newobject Zoo::create_animal;</b>

 <b>/* Tell SWIG to keep track of mappings between C/C++ structs/classes. */</b>
 <b>%trackobjects;</b>

 %include "example.h"</pre>
 </div>

 <p>When this code runs we see:
 </p>

 <div class="code targetlang">
 <pre>$ <b>irb</b>
 irb(main):001:0&gt; <b>require 'example'</b>
 =&gt; true

 irb(main):002:0&gt; <b>tiger1 = Example::Animal.new("tiger1")</b>
 =&gt; #&lt;Example::Animal:0x2be37d8&gt;

 irb(main):003:0&gt; <b>zoo = Example::Zoo.new()</b>
 =&gt; #&lt;Example::Zoo:0x2be0a18&gt;

 irb(main):004:0&gt; <b>zoo.add_animal(tiger1)</b>
 =&gt; nil

 irb(main):006:0&gt; <b>tiger2 = zoo.remove_animal(0)</b>
 =&gt; #&lt;Example::Animal:0x2be37d8&gt;

 irb(main):007:0&gt; <b>tiger1.equal?(tiger2)</b>
 =&gt; true

 irb(main):008:0&gt; <b>tiger1 = nil</b>
 =&gt; nil

 irb(main):009:0&gt; <b>GC.start</b>
 =&gt; nil

 irb(main):010:0&gt; <b>tiger.get_name()</b>
 =&gt; "tiger1"
 irb(main):011:0&gt;</pre>
 </div>

 <p>For those who are interested, object tracking is implemented
 by storing Ruby objects in a hash table and keying them on C++
 pointers. The underlying API is:

 </p>

 <div class="code">
 <pre>static void SWIG_RubyAddTracking(void* ptr, VALUE object);
 static VALUE SWIG_RubyInstanceFor(void* ptr) ;
 static void SWIG_RubyRemoveTracking(void* ptr);
 static void SWIG_RubyUnlinkObjects(void* ptr);</pre>
 </div>

 <p>When an object is created, SWIG will automatically call the <tt>SWIG_RubyAddTracking</tt>
 method. Similarly, when an object is deleted, SWIG will call the <tt>SWIG_RubyRemoveTracking</tt>.
 When an object is returned to Ruby from C++, SWIG will use the <tt>SWIG_RubyInstanceFor</tt>
 method to ensure a one-to-one mapping from Ruby to C++ objects. Last,
 the <tt>RubyUnlinkObjects</tt> method unlinks a Ruby
 object from its underlying C++ object.</p>

 <p>In general, you will only need to use the <tt>SWIG_RubyInstanceFor</tt>,
 which is required for implementing mark functions as shown below.
 However, if you implement your own free functions (see below) you may
 also have to call the <tt>SWIG_RubyRemoveTracking</tt> and <tt>RubyUnlinkObjects</tt>
 methods.</p>

 <H3><a name="Ruby_nn61">35.10.4 Mark Functions</a></H3>


 <p>With a bit more testing, we see that our class library still
 has problems. For example:

 </p>

 <div class="targetlang">
 <pre>$ <b>irb</b>
 irb(main):001:0&gt; <b>require 'example'</b>
 =&gt; true

 irb(main):002:0&gt; tiger1 = <b>Example::Animal.new("tiger1")</b>
 =&gt; #&lt;Example::Animal:0x2bea6a8&gt;

 irb(main):003:0&gt; zoo = <b>Example::Zoo.new()</b>
 =&gt; #&lt;Example::Zoo:0x2be7960&gt;

 irb(main):004:0&gt; <b>zoo.add_animal(tiger1)</b>
 =&gt; nil

 irb(main):007:0&gt; <b>tiger1 = nil</b>
 =&gt; nil

 irb(main):007:0&gt; <b>GC.start</b>
 =&gt; nil

 irb(main):005:0&gt; <b>tiger2 = zoo.get_animal(0)</b>
 (irb):12: [BUG] Segmentation fault</pre>
 </div>

 <p>The problem is that Ruby does not know that the <tt>zoo</tt>
 object contains a reference to a Ruby object. Thus, when Ruby garbage
 collects <tt>tiger1</tt>
 it frees the underlying C++ object.</p>

 <p>This can be fixed by implementing a <tt>mark</tt>
 function as described above in the <a href="Ruby.html#Ruby_nn52">Mark
 and Sweep Garbage Collector</a> section. You can specify a mark
 function by using the <tt>%markfunc</tt> directive. Since
 the <tt>%markfunc</tt> directive is implemented using
 SWIG's' "features" mechanism it uses the same name matching rules as
 other kinds of features (see the chapter on <a href="Customization.html#Customization">"Customization
 Features"</a> for more details). </p>

 <p>A <tt>mark</tt> function takes a single argument,
 which is a pointer to the C++ object being marked; it should, in turn,
 call <tt>rb_gc_mark()</tt> for any instances that are
 reachable from the current object. The mark function for our <tt>
 Zoo</tt> class should therefore loop over all of the C++ animal
 objects in the zoo object, look up their Ruby object equivalent, and
 then call <tt>rb_gc_mark()</tt>. One possible
 implementation is:</p>

 <div class="code">
 <pre>%module example

 %{
 #include "example.h"
 %}

 /* Keep track of mappings between C/C++ structs/classes
  and Ruby objects so we can implement a mark function. */
 <b>%trackobjects;</b>

 /* Specify the mark function */
 <b>%markfunc Zoo "mark_Zoo";</b>

 %include "example.h"

 %header %{

 static void mark_Zoo(void* ptr) {
   Zoo* zoo = (Zoo*) ptr;

   /* Loop over each object and tell the garbage collector
   that we are holding a reference to them. */
   int count = zoo-&gt;get_num_animals();

   for(int i = 0; i &lt; count; ++i) {
     Animal* animal = zoo-&gt;get_animal(i);
     VALUE object = SWIG_RubyInstanceFor(animal);

     if (object != Qnil) {
       rb_gc_mark(object);
     }
   }
 }
 %}</pre>
 </div>

 <p> Note the <tt>mark</tt> function is dependent on
 the <tt>SWIG_RUBY_InstanceFor</tt> method, and thus
 requires that <tt>%trackobjects</tt> is enabled. For more
 information, please refer to the ruby_track_objects.i test case in the SWIG
 test suite.</p>

 <p>When this code is compiled we now see:</p>

 <div class="targetlang">
 <pre>$ <b>irb
 </b>irb(main):002:0&gt; <b>tiger1=Example::Animal.new("tiger1")</b>
 =&gt; #&lt;Example::Animal:0x2be3bf8&gt;

 irb(main):003:0&gt; <b>Example::Zoo.new()</b>
 =&gt; #&lt;Example::Zoo:0x2be1780&gt;

 irb(main):004:0&gt; <b>zoo = Example::Zoo.new()</b>
 =&gt; #&lt;Example::Zoo:0x2bde9c0&gt;

 irb(main):005:0&gt; <b>zoo.add_animal(tiger1)</b>
 =&gt; nil

 irb(main):009:0&gt; <b>tiger1 = nil</b>
 =&gt; nil

 irb(main):010:0&gt; <b>GC.start</b>
 =&gt; nil
 irb(main):014:0&gt; <b>tiger2 = zoo.get_animal(0)</b>
 =&gt; #&lt;Example::Animal:0x2be3bf8&gt;

 irb(main):015:0&gt; <b>tiger2.get_name()</b>
 =&gt; "tiger1"
 irb(main):016:0&gt;</pre>
 </div>

 <p>This code can be seen in swig/examples/ruby/mark_function.</p>

 <H3><a name="Ruby_nn62">35.10.5 Free Functions</a></H3>


 <p>By default, SWIG creates a "free" function that is called when
 a Ruby object is garbage collected. The free function simply calls the
 C++ object's destructor.</p>

 <p>However, sometimes an appropriate destructor does not exist or
 special processing needs to be performed before the destructor is
 called. Therefore, SWIG allows you to manually specify a "free"
 function via the use of the <tt>%freefunc</tt> directive.
 The <tt>%freefunc</tt> directive is implemented using
 SWIG's' "features" mechanism and so the same name matching rules used
 for other kinds of features apply (see the chapter on <a href="Customization.html#Customization">"Customization
 Features"</a>) for more details).</p>

 <p>IMPORTANT ! - If you define your own free function, then you
 must ensure that you call the underlying C++ object's destructor. In
 addition, if object tracking is activated for the object's class, you
 must also call the <tt>SWIG_RubyRemoveTracking</tt>
 function (of course call this before you destroy the C++ object). Note
 that it is harmless to call this method if object tracking if off so it
 is advised to always call it.</p>

 <p>Note there is a subtle interaction between object ownership
 and free functions. A custom defined free function will only be called
 if the Ruby object owns the underlying C++ object. This also to Ruby
 objects which are created, but then transfer ownership to C++ objects
 via the use of the <tt>disown</tt> typemap described
 above. </p>

 <p>To show how to use the <tt>%freefunc</tt>
 directive, let's slightly change our example. Assume that the zoo
 object is responsible for freeing any animal that it contains. This means
 that the <tt>Zoo::add_animal</tt>
 function should be marked with a <tt>DISOWN</tt> typemap
 and the destructor should be updated as below:</p>

 <div class="code">
 <pre>
 Zoo::~Zoo() {
   IterType iter = this-&gt;animals.begin();
   IterType end = this-&gt;animals.end();

   for(iter; iter != end; ++iter) {
     Animal* animal = *iter;
     delete animal;
   }
 }
 </pre>
 </div>

 <p>When we use these objects in IRB we see:</p>

 <div class="code targetlang">
 <pre class="targetlang"><b>$irb</b>
 irb(main):002:0&gt; <b>require 'example'</b>
 =&gt; true

 irb(main):003:0&gt; <b>zoo = Example::Zoo.new()</b>
 =&gt; #&lt;Example::Zoo:0x2be0fe8&gt;

 irb(main):005:0&gt; <b>tiger1 = Example::Animal.new("tiger1")</b>
 =&gt; #&lt;Example::Animal:0x2bda760&gt;

 irb(main):006:0&gt; <b>zoo.add_animal(tiger1)</b>
 =&gt; nil

 irb(main):007:0&gt; <b>zoo = nil</b>
 =&gt; nil

 irb(main):008:0&gt; <b>GC.start</b>
 =&gt; nil

 irb(main):009:0&gt; <b>tiger1.get_name()</b>
 (irb):12: [BUG] Segmentation fault
 </pre>
 </div>

 <p>The error happens because the C++ <tt>animal</tt>
 object is freed when the <tt>zoo</tt> object is freed.
 Although this error is unavoidable, we can at least prevent the
 segmentation fault. To do this requires enabling object tracking and
 implementing a custom free function that calls the <tt>SWIG_RubyUnlinkObjects</tt>
 function for each animal object that is destroyed. The <tt>SWIG_RubyUnlinkObjects</tt>
 function notifies SWIG that a Ruby object's underlying C++ object is no
 longer valid. Once notified, SWIG will intercept any calls from the
 existing Ruby object to the destroyed C++ object and raise an exception.
 </p>

 <div class="code">
 <pre>%module example

 %{
 #include "example.h"
 %}

 /* Specify that ownership is transferred to the zoo when calling add_animal */
 %apply SWIGTYPE *DISOWN { Animal* animal };

 /* Track objects */
 %trackobjects;

 /* Specify the mark function */
 %freefunc Zoo "free_Zoo";

 %include "example.h"

 %header %{
   static void free_Zoo(void* ptr) {
     Zoo* zoo = (Zoo*) ptr;

     /* Loop over each animal */
     int count = zoo-&gt;get_num_animals();

     for(int i = 0; i &lt; count; ++i) {
       /* Get an animal */
       Animal* animal = zoo-&gt;get_animal(i);

       /* Unlink the Ruby object from the C++ object */
       SWIG_RubyUnlinkObjects(animal);

       /* Now remove the tracking for this animal */
       SWIG_RubyRemoveTracking(animal);
     }

     /* Now call SWIG_RubyRemoveTracking for the zoo */
     SWIG_RubyRemoveTracking(ptr);
     /* Now free the zoo which will free the animals it contains */
     delete zoo;
   }
 %} </pre>
 </div>

 <p>Now when we use these objects in IRB we see:</p>

 <div class="code targetlang">
 <pre><b>$irb</b>
 irb(main):002:0&gt; <b>require 'example'</b>
 =&gt; true

 irb(main):003:0&gt; <b>zoo = Example::Zoo.new()</b>
 =&gt; #&lt;Example::Zoo:0x2be0fe8&gt;

 irb(main):005:0&gt; <b>tiger1 = Example::Animal.new("tiger1")</b>
 =&gt; #&lt;Example::Animal:0x2bda760&gt;

 irb(main):006:0&gt; <b>zoo.add_animal(tiger1)</b>
 =&gt; nil

 irb(main):007:0&gt; <b>zoo = nil</b>
 =&gt; nil

 irb(main):008:0&gt; <b>GC.start</b>
 =&gt; nil

 irb(main):009:0&gt; <b>tiger1.get_name()</b>
 RuntimeError: This Animal * already released
  from (irb):10:in `get_name'
  from (irb):10
 irb(main):011:0&gt;</pre>
 </div>

 <p>Notice that SWIG can now detect the underlying C++ object has
 been freed, and thus raises a runtime exception.</p>

 <p>This code can be seen in swig/examples/ruby/free_function.</p>

 <H3><a name="Ruby_nn63">35.10.6 Embedded Ruby and the C++ Stack</a></H3>


 <p>As has been said, the Ruby GC runs and marks objects before
 its
 sweep phase. When the garbage collector is called, it will
 also
 try to mark any Ruby objects (VALUE) it finds in the machine registers
 and in the C++ stack.</p>

 <p>The stack is basically the history of the functions that have
 been
 called and also contains local variables, such as the ones you define
 whenever you do inside a function:</p>

 <div class="diagram">VALUE obj; </div>

 <p>For ruby to determine where its stack space begins, during
 initialization a normal Ruby interpreter will call the ruby_init()
 function which in turn will call a function called Init_stack or
 similar. This function will store a pointer to the location
 where
 the stack points at that point in time.</p>

 <p>ruby_init() is presumed to always be called within the main()
 function of your program and whenever the GC is called, ruby will
 assume that the memory between the current location in memory and the
 pointer that was stored previously represents the stack, which may
 contain local (and temporary) VALUE ruby objects.  Ruby will
 then be careful not to remove any of those objects in that location.</p>

 <p>So far so good. For a normal Ruby session, all the
 above is
 completely transparent and magic to the extensions developer.
 </p>

 <p>However, with an embedded Ruby, it may not always be possible
 to
 modify main() to make sure ruby_init() is called there.  As
 such,
 ruby_init() will likely end up being called from within some other
 function. This can lead Ruby to measure incorrectly where the
 stack begins and can result in Ruby incorrectly collecting
 those
 temporary VALUE objects that are created once another function
 is
 called. The end result: random crashes and segmentation
 faults.</p>

 <p>This problem will often be seen in director functions that are
 used for callbacks, for example. </p>

 <p>To solve the problem, SWIG can now generate code with director
 functions containing the optional macros SWIG_INIT_STACK and
 SWIG_RELEASE_STACK. These macros will try to force Ruby to
 reinitialize the beginning of the stack the first time a
 director
 function is called. This will lead Ruby to measure and not
 collect any VALUE objects defined from that point on. </p>

 <p>To mark functions to either reset the ruby stack or not, you
 can use:</p>

 <div class="code"><pre>
 %initstack Class::memberfunction;   // only re-init the stack in this director method
 %ignorestack Class::memberfunction; // do not re-init the stack in this director method
 %initstack Class;                   // init the stack on all the methods of this class
 %initstack;                         // all director functions will re-init the stack
 </pre></div>

 </body>
 </html>