Doc/Manual/Customization.html - third_party/swig - Git at Google

 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
 <html>
 <head>
 <title>Customization Features</title>
 <link rel="stylesheet" type="text/css" href="style.css">
 </head>

 <body bgcolor="#ffffff">
 <H1><a name="Customization"></a>11 Customization Features</H1>
 <!-- INDEX -->
 <div class="sectiontoc">
 <ul>
 <li><a href="#exception">Exception handling with %exception</a>
 <ul>
 <li><a href="#Customization_nn3">Handling exceptions in C code</a>
 <li><a href="#Customization_nn4">Exception handling with longjmp()</a>
 <li><a href="#Customization_nn5">Handling C++ exceptions</a>
 <li><a href="#Customization_allowexcept">Exception handlers for variables</a>
 <li><a href="#Customization_nn6">Defining different exception handlers</a>
 <li><a href="#Customization_exception_special_variables">Special variables for %exception</a>
 <li><a href="#Customization_nn7">Using The SWIG exception library</a>
 </ul>
 <li><a href="#ownership">Object ownership and %newobject</a>
 <li><a href="#features">Features and the %feature directive</a>
 <ul>
 <li><a href="#Customization_feature_attributes">Feature attributes</a>
 <li><a href="#Customization_feature_flags">Feature flags</a>
 <li><a href="#Customization_clearing_features">Clearing features</a>
 <li><a href="#Customization_features_default_args">Features and default arguments</a>
 <li><a href="#features_example">Feature example</a>
 </ul>
 </ul>
 </div>
 <!-- INDEX -->


 <p>
 In many cases, it is desirable to change the default wrapping of
 particular declarations in an interface.  For example, you might want
 to provide hooks for catching C++ exceptions, add assertions, or
 provide hints to the underlying code generator.  This chapter
 describes some of these customization techniques.  First, a discussion
 of exception handling is presented.  Then, a more general-purpose
 customization mechanism known as "features" is described.
 </p>

 <H2><a name="exception"></a>11.1 Exception handling with %exception</H2>


 <p>
 The <tt>%exception</tt> directive allows you to define a general purpose exception
 handler. For example, you can specify the following:
 </p>

 <div class="code"><pre>
 %exception {
     try {
         $action
     }
     catch (RangeError) {
         PyErr_SetString(PyExc_IndexError,"index out-of-bounds");
         return NULL;
     }
 }
 </pre></div>

 <p>
 When defined, the code enclosed in braces is inserted directly into the low-level wrapper
 functions.  The special variable <tt>$action</tt> is one of a few
 <a href="Customization.html#Customization_exception_special_variables">%exception special variable</a>
 supported and gets replaced with the actual operation
 to be performed (a function call, method invocation, attribute access, etc.).  An exception handler
 remains in effect until it is explicitly deleted.  This is done by using either <tt>%exception</tt>
 or <tt>%noexception</tt> with no code. For example:
 </p>

 <div class="code"><pre>
 %exception;   // Deletes any previously defined handler
 </pre></div>

 <p>
 <b>Compatibility note:</b>  Previous versions of SWIG used a special directive <tt>%except</tt>
 for exception handling.   That directive is deprecated--<tt>%exception</tt>
 provides the same functionality, but is substantially more flexible.
 </p>

 <H3><a name="Customization_nn3"></a>11.1.1 Handling exceptions in C code</H3>


 <p>
 C has no formal exception handling mechanism so there are several approaches that might be
 used.  A somewhat common technique is to simply set a special error code.  For example:
 </p>

 <div class="code"><pre>
 /* File : except.c */

 static char error_message[256];
 static int error_status = 0;

 void throw_exception(char *msg) {
 	strncpy(error_message,msg,256);
 	error_status = 1;
 }

 void clear_exception() {
 	error_status = 0;
 }
 char *check_exception() {
 	if (error_status) return error_message;
 	else return NULL;
 }

 </pre></div>

 <p>
 To use these functions, functions simply call
 <tt>throw_exception()</tt> to indicate an error occurred. For example
 :</p>

 <div class="code"><pre>
 double inv(double x) {
 	if (x != 0) return 1.0/x;
 	else {
 		throw_exception("Division by zero");
 		return 0;
 	}
 }

 </pre></div>

 <p>
 To catch the exception, you can write a simple exception handler such
 as the following (shown for Perl5) :</p>

 <div class="code"><pre>
 %exception {
     char *err;
     clear_exception();
     $action
     if ((err = check_exception())) {
        croak(err);
     }
 }
 </pre></div>

 <p>
 In this case, when an error occurs, it is translated into a Perl error.
 Each target language has its own approach to creating a runtime error/exception in
 and for Perl it is the <tt>croak</tt> method shown above.
 </p>

 <H3><a name="Customization_nn4"></a>11.1.2 Exception handling with longjmp()</H3>


 <p>
 Exception handling can also be added to C code using the
 <tt>&lt;setjmp.h&gt;</tt> library.  Here is a minimalistic implementation that
 relies on the C preprocessor :
 </p>

 <div class="code"><pre>
 /* File : except.c
    Just the declaration of a few global variables we're going to use */

 #include &lt;setjmp.h&gt;
 jmp_buf exception_buffer;
 int exception_status;

 /* File : except.h */
 #include &lt;setjmp.h&gt;
 extern jmp_buf exception_buffer;
 extern int exception_status;

 #define try if ((exception_status = setjmp(exception_buffer)) == 0)
 #define catch(val) else if (exception_status == val)
 #define throw(val) longjmp(exception_buffer,val)
 #define finally else

 /* Exception codes */

 #define RangeError     1
 #define DivisionByZero 2
 #define OutOfMemory    3

 </pre></div>

 <p>
 Now, within a C program, you can do the following :</p>

 <div class="code"><pre>
 double inv(double x) {
 	if (x) return 1.0/x;
 	else throw(DivisionByZero);
 }

 </pre></div>

 <p>
 Finally, to create a SWIG exception handler, write the following :</p>

 <div class="code"><pre>
 %{
 #include "except.h"
 %}

 %exception {
 	try {
 		$action
 	} catch(RangeError) {
 		croak("Range Error");
 	} catch(DivisionByZero) {
 		croak("Division by zero");
 	} catch(OutOfMemory) {
 		croak("Out of memory");
 	} finally {
 		croak("Unknown exception");
 	}
 }
 </pre></div>

 <p>
 Note: This implementation is only intended to illustrate the general idea.  To make it work better, you'll need to
 modify it to handle nested <tt>try</tt> declarations.
 </p>

 <H3><a name="Customization_nn5"></a>11.1.3 Handling C++ exceptions</H3>


 <p>
 Handling C++ exceptions is also straightforward.  For example:
 </p>

 <div class="code"><pre>
 %exception {
 	try {
 		$action
 	} catch(RangeError) {
 		croak("Range Error");
 	} catch(DivisionByZero) {
 		croak("Division by zero");
 	} catch(OutOfMemory) {
 		croak("Out of memory");
 	} catch(...) {
 		croak("Unknown exception");
 	}
 }

 </pre></div>

 <p>
 The exception types need to be declared as classes elsewhere, possibly
 in a header file :</p>

 <div class="code"><pre>
 class RangeError {};
 class DivisionByZero {};
 class OutOfMemory {};
 </pre>
 </div>

 <H3><a name="Customization_allowexcept"></a>11.1.4 Exception handlers for variables</H3>


 <p>
 By default all variables will ignore <tt>%exception</tt>, so it is effectively turned off for all variables wrappers.
 This applies to global variables, member variables and static member variables.
 The approach is certainly a logical one when wrapping variables in C.
 However, in C++, it is quite possible for an exception to be thrown while the variable is being assigned.
 To ensure <tt>%exception</tt> is used when wrapping variables, it needs to be 'turned on' using the <tt>%allowexception</tt> feature.
 Note that <tt>%allowexception</tt> is just a macro for <tt>%feature("allowexcept")</tt>, that is, it is a feature called "allowexcept".
 Any variable which has this feature attached to it, will then use the <tt>%exception</tt> feature, but of course,
 only if there is a <tt>%exception</tt> attached to the variable in the first place.
 The <tt>%allowexception</tt> feature works like any other feature and so can be used globally or for selective variables.
 </p>

 <div class="code">
 <pre>
 %allowexception;                // turn on globally
 %allowexception Klass::MyVar;   // turn on for a specific variable

 %noallowexception Klass::MyVar; // turn off for a specific variable
 %noallowexception;              // turn off globally
 </pre>
 </div>

 <H3><a name="Customization_nn6"></a>11.1.5 Defining different exception handlers</H3>


 <p>
 By default, the <tt>%exception</tt> directive creates an exception
 handler that is used for all wrapper functions that follow it.  Unless
 there is a well-defined (and simple) error handling mechanism in place,
 defining one universal exception handler may be unwieldy and result
 in excessive code bloat since the handler is inlined into each wrapper function.
 </p>

 <p>
 To fix this, you can be more selective about how you use the
 <tt>%exception</tt> directive.  One approach is to only place it around
 critical pieces of code.  For example:
 </p>

 <div class="code"><pre>
 %exception {
 	... your exception handler ...
 }
 /* Define critical operations that can throw exceptions here */

 %exception;

 /* Define non-critical operations that don't throw exceptions */
 </pre></div>

 <p>
 More precise control over exception handling can be obtained by attaching an exception handler
 to specific declaration name. For example:
 </p>

 <div class="code">
 <pre>
 %exception allocate {
     try {
         $action
     }
     catch (MemoryError) {
         croak("Out of memory");
     }
 }
 </pre>
 </div>

 <p>
 In this case, the exception handler is only attached to declarations
 named "allocate".  This would include both global and member
 functions.  The names supplied to <tt>%exception</tt> follow the same
 rules as for <tt>%rename</tt> described in the section on
 <a href="SWIGPlus.html#ambiguity_resolution_renaming">Ambiguity resolution and renaming</a>.
 For example, if you wanted to define
 an exception handler for a specific class, you might write this:
 </p>

 <div class="code">
 <pre>
 %exception Object::allocate {
     try {
         $action
     }
     catch (MemoryError) {
         croak("Out of memory");
     }
 }
 </pre>
 </div>

 <p>
 When a class prefix is supplied, the exception handler is applied to the corresponding declaration
 in the specified class as well as for identically named functions appearing in derived classes.
 </p>

 <p>
 <tt>%exception</tt> can even be used to pinpoint a precise declaration when overloading is used. For example:
 </p>

 <div class="code">
 <pre>
 %exception Object::allocate(int) {
     try {
         $action
     }
     catch (MemoryError) {
         croak("Out of memory");
     }
 }
 </pre>
 </div>

 <p>
 Attaching exceptions to specific declarations is a good way to reduce code bloat.  It can also be a useful way
 to attach exceptions to specific parts of a header file. For example:
 </p>

 <div class="code">
 <pre>
 %module example
 %{
 #include "someheader.h"
 %}

 // Define a few exception handlers for specific declarations
 %exception Object::allocate(int) {
     try {
         $action
     }
     catch (MemoryError) {
         croak("Out of memory");
     }
 }

 %exception Object::getitem {
     try {
        $action
     }
     catch (RangeError) {
        croak("Index out of range");
     }
 }
 ...
 // Read a raw header file
 %include "someheader.h"
 </pre>
 </div>

 <p>
 <b>Compatibility note:</b> The <tt>%exception</tt> directive replaces
 the functionality provided by the deprecated "except" typemap.
 The typemap would allow exceptions to be thrown in the target
 language based on the return type of a function and
 was intended to be a mechanism for pinpointing specific
 declarations.  However, it never really worked that well and the new
 %exception directive is much better.
 </p>

 <H3><a name="Customization_exception_special_variables"></a>11.1.6 Special variables for %exception</H3>


 <p>
 The %exception directive supports a few special variables which are placeholders for
 code substitution.
 The following table shows the available special variables and details what the special
 variables are replaced with.
 </p>

 <table summary="Special variables for %exception">

 <tr>
 <td>$action</td>
 <td>The actual operation to be performed (a function call, method invocation, variable access, etc.)</td>
 </tr>

 <tr>
 <td>$symname</td>
 <td>The symbol name used internally by SWIG</td>
 </tr>

 <tr>
 <td>$overname</td>
 <td>The extra mangling used in the symbol name for overloaded method. Expands to nothing if the wrapped method is not overloaded.</td>
 </tr>

 <tr>
 <td>$wrapname</td>
 <td>The language specific wrapper name (usually a C function name exported from the shared object/dll)</td>
 </tr>

 <tr>
 <td>$decl</td>
 <td>The fully qualified C/C++ declaration of the method being wrapped without the return type</td>
 </tr>

 <tr>
 <td>$fulldecl</td>
 <td>The fully qualified C/C++ declaration of the method being wrapped including the return type</td>
 </tr>

 </table>

 <p>
 The special variables are often used in situations where method calls are logged. Exactly which form of the method call needs logging is up to individual requirements, but the example code below shows all the possible expansions, plus how an exception message could be tailored to show the C++ method declaration:
 </p>

 <div class="code"><pre>
 %exception Special::something {
   log("symname: $symname");
   log("overname: $overname");
   log("wrapname: $wrapname");
   log("decl: $decl");
   log("fulldecl: $fulldecl");
   try {
     $action
   }
   catch (MemoryError) {
       croak("Out of memory in $decl");
   }
 }
 void log(const char *message);
 struct Special {
   void something(const char *c);
   void something(int i);
 };
 </pre></div>

 <p>
 Below shows the expansions for the 1st of the overloaded <tt>something</tt> wrapper methods for Perl:
 </p>

 <div class="code"><pre>
   log("symname: Special_something");
   log("overname: __SWIG_0");
   log("wrapname: _wrap_Special_something__SWIG_0");
   log("decl: Special::something(char const *)");
   log("fulldecl: void Special::something(char const *)");
   try {
     (arg1)-&gt;something((char const *)arg2);
   }
   catch (MemoryError) {
     croak("Out of memory in Special::something(char const *)");
   }
 </pre></div>


 <H3><a name="Customization_nn7"></a>11.1.7 Using The SWIG exception library</H3>


 <p>
 The <tt>exception.i</tt> library file provides support for creating
 language independent exceptions in your interfaces.  To use it, simply
 put an "<tt>%include exception.i</tt>" in your interface file.  This
 creates a function<tt> SWIG_exception()</tt> that can be used to raise
 common scripting language exceptions in a portable manner.  For example :</p>

 <div class="code"><pre>
 // Language independent exception handler
 %include exception.i

 %exception {
     try {
         $action
     } catch(RangeError) {
         SWIG_exception(SWIG_ValueError, "Range Error");
     } catch(DivisionByZero) {
         SWIG_exception(SWIG_DivisionByZero, "Division by zero");
     } catch(OutOfMemory) {
         SWIG_exception(SWIG_MemoryError, "Out of memory");
     } catch(...) {
         SWIG_exception(SWIG_RuntimeError,"Unknown exception");
     }
 }

 </pre></div>

 <p>
 As arguments, <tt>SWIG_exception()</tt> takes an error type code (an
 integer) and an error message string.  The currently supported error
 types are :</p>

 <div class="diagram"><pre>
 SWIG_UnknownError
 SWIG_IOError
 SWIG_RuntimeError
 SWIG_IndexError
 SWIG_TypeError
 SWIG_DivisionByZero
 SWIG_OverflowError
 SWIG_SyntaxError
 SWIG_ValueError
 SWIG_SystemError
 SWIG_AttributeError
 SWIG_MemoryError
 SWIG_NullReferenceError
 </pre></div>

 <p>
 Since the <tt>SWIG_exception()</tt> function is defined at the C-level
 it can be used elsewhere in SWIG. This includes typemaps and helper
 functions.
 </p>

 <H2><a name="ownership"></a>11.2 Object ownership and %newobject</H2>


 <p>
 A common problem in some applications is managing proper ownership of objects.  For
 example, consider a function like this:
 </p>

 <div class="code">
 <pre>
 Foo *blah() {
    Foo *f = new Foo();
    return f;
 }
 </pre>
 </div>

 <p>
 If you wrap the function <tt>blah()</tt>, SWIG has no idea that the
 return value is a newly allocated object.  As a result, the resulting
 extension module may produce a memory leak (SWIG is conservative and
 will never delete objects unless it knows for certain that the
 returned object was newly created).
 </p>

 <p>
 To fix this, you can provide an extra hint to the code generator using
 the <tt>%newobject</tt> directive.  For example:
 </p>

 <div class="code">
 <pre>
 %newobject blah;
 Foo *blah();
 </pre>
 </div>

 <p>
 <tt>%newobject</tt> works exactly like <tt>%rename</tt> and <tt>%exception</tt>.  In other words,
 you can attach it to class members and parameterized declarations as before.  For example:
 </p>

 <div class="code">
 <pre>
 %newobject ::blah();                   // Only applies to global blah
 %newobject Object::blah(int,double);   // Only blah(int,double) in Object
 %newobject *::copy;                    // Copy method in all classes
 ...
 </pre>
 </div>

 <p>
 When <tt>%newobject</tt> is supplied, many language modules will
 arrange to take ownership of the return value.  This allows the value
 to be automatically garbage-collected when it is no longer in use.  However,
 this depends entirely on the target language (a language module may also choose to ignore
 the <tt>%newobject</tt> directive).
 </p>

 <p>
 Closely related to <tt>%newobject</tt> is a special typemap.  The "newfree" typemap
 can be used to deallocate a newly allocated return value.  It is only available on
 methods for which <tt>%newobject</tt> has been applied and is commonly used to clean-up string
 results.  For example:
 </p>

 <div class="code">
 <pre>
 %typemap(newfree) char * "free($1);";
 ...
 %newobject strdup;
 ...
 char *strdup(const char *s);
 </pre>
 </div>

 <p>
 In this case, the result of the function is a string in the target language.  Since this string
 is a copy of the original result, the data returned by <tt>strdup()</tt> is no longer needed.
 The "newfree" typemap in the example simply releases this memory.
 </p>

 <p>
 As a complement to the <tt>%newobject</tt>, from SWIG 1.3.28, you can
 use the <tt>%delobject</tt> directive. For example, if you have two
 methods, one to create objects and one to destroy them, you can use:
 </p>

 <div class="code">
 <pre>
 %newobject create_foo;
 %delobject destroy_foo;
 ...
 Foo *create_foo();
 void destroy_foo(Foo *foo);
 </pre>
 </div>

 <p> or in a member method as: </p>
 <div class="code">
 <pre>
 %delobject Foo::destroy;

 class Foo {
 public:
   void destroy() { delete this;}

 private:
   ~Foo();
 };
 </pre>
 </div>

 <p>
 <tt>%delobject</tt> instructs SWIG that the first argument passed to
 the method will be destroyed, and therefore, the target language
 should not attempt to deallocate it twice. This is similar to use the
 DISOWN typemap in the first method argument, and in fact, it also
 depends on the target language on implementing the 'disown' mechanism
 properly.
 </p>

 <p>
 <b>Compatibility note:</b>  Previous versions of SWIG had a special <tt>%new</tt> directive.  However, unlike <tt>%newobject</tt>,
 it only applied to the next declaration.  For example:
 </p>

 <div class="code">
 <pre>
 %new char *strdup(const char *s);
 </pre>
 </div>

 <p>
 For now this is still supported but is deprecated.
 </p>

 <p>
 <b>How to shoot yourself in the foot:</b>  The <tt>%newobject</tt> directive is not a declaration modifier like the old
 <tt>%new</tt> directive.   Don't write code like this:
 </p>

 <div class="code">
 <pre>
 %newobject
 char *strdup(const char *s);
 </pre>
 </div>
 <p>
 The results might not be what you expect.
 </p>

 <H2><a name="features"></a>11.3 Features and the %feature directive</H2>


 <p>
 Both <tt>%exception</tt> and <tt>%newobject</tt> are examples of a
 more general purpose customization mechanism known as "features."  A
 feature is simply a user-definable property that is attached to
 specific declarations.  Features are attached
 using the <tt>%feature</tt> directive. For example:
 </p>

 <div class="code">
 <pre>
 %feature("except") Object::allocate {
     try {
         $action
     }
     catch (MemoryError) {
         croak("Out of memory");
     }
 }

 %feature("new","1") *::copy;
 </pre>
 </div>

 <p>
 In fact, the <tt>%exception</tt> and <tt>%newobject</tt> directives are really nothing more than macros
 involving <tt>%feature</tt>:
 </p>

 <div class="code">
 <pre>
 #define %exception %feature("except")
 #define %newobject %feature("new","1")
 </pre>
 </div>

 <p>
 The name matching rules outlined in the <a href="SWIGPlus.html#ambiguity_resolution_renaming">Ambiguity resolution and renaming</a>
 section applies to all <tt>%feature</tt> directives.
 In fact the the <tt>%rename</tt> directive is just a special form of <tt>%feature</tt>.
 The matching rules mean that features are very flexible and can be applied with
 pinpoint accuracy to specific declarations if needed.
 Additionally, if no declaration name is given, a global feature is said to be defined.
 This feature is then
 attached to <em>every</em> declaration that follows.  This is how global exception handlers
 are defined.  For example:
 </p>

 <div class="code">
 <pre>
 /* Define a global exception handler */
 %feature("except") {
    try {
      $action
    }
    ...
 }

 ... bunch of declarations ...
 </pre>
 </div>

 <p>
 The <tt>%feature</tt> directive can be used with different syntax.
 The following are all equivalent:
 </p>

 <div class="code">
 <pre>
 %feature("except") Object::method { $action };
 %feature("except") Object::method %{ $action %};
 %feature("except") Object::method " $action ";
 %feature("except","$action") Object::method;
 </pre>
 </div>

 <p>
 The syntax in the first variation will generate the <tt>{ }</tt> delimiters used whereas the other variations will not.
 </p>

 <H3><a name="Customization_feature_attributes"></a>11.3.1 Feature attributes</H3>


 <p>
 The <tt>%feature</tt> directive also accepts XML style attributes in the same way that typemaps do.
 Any number of attributes can be specified.
 The following is the generic syntax for features:
 </p>

 <div class="code">
 <pre>
 %feature("name","value", attribute1="AttributeValue1") symbol;
 %feature("name", attribute1="AttributeValue1") symbol {value};
 %feature("name", attribute1="AttributeValue1") symbol %{value%};
 %feature("name", attribute1="AttributeValue1") symbol "value";
 </pre>
 </div>

 <p>
 More than one attribute can be specified using a comma separated list.
 The Java module is an example that uses attributes in <tt>%feature("except")</tt>.
 The <tt>throws</tt> attribute specifies the name of a Java class to add to a proxy method's throws clause.
 In the following example, <tt>MyExceptionClass</tt> is the name of the Java class for adding to the throws clause.
 </p>

 <div class="code">
 <pre>
 %feature("except", throws="MyExceptionClass") Object::method {
    try {
      $action
    } catch (...) {
      ... code to throw a MyExceptionClass Java exception ...
    }
 };
 </pre>
 </div>

 <p>
 Further details can be obtained from the <a href="Java.html#exception_handling">Java exception handling</a> section.
 </p>

 <H3><a name="Customization_feature_flags"></a>11.3.2 Feature flags</H3>


 <p>
 Feature flags are used to enable or disable a particular feature. Feature flags are a common but simple usage of <tt>%feature</tt>
 and the feature value should be either <tt>1</tt> to enable or <tt>0</tt> to disable the feature.
 </p>

 <div class="code">
 <pre>
 %feature("featurename")          // enables feature
 %feature("featurename", "1")     // enables feature
 %feature("featurename", "x")     // enables feature
 %feature("featurename", "0")     // disables feature
 %feature("featurename", "")      // clears feature
 </pre>
 </div>

 <p>
 Actually any value other than zero will enable the feature.
 Note that if the value is omitted completely, the default value becomes <tt>1</tt>, thereby enabling the feature.
 A feature is cleared by specifying no value, see <a href="#Customization_clearing_features">Clearing features</a>.
 The <tt>%immutable</tt> directive described in the <a href="SWIG.html#SWIG_readonly_variables">Creating read-only variables</a> section,
 is just a macro for <tt>%feature("immutable")</tt>, and can be used to demonstrates feature flags:
 </p>

 <div class="code">
 <pre>
                                 // features are disabled by default
 int red;                        // mutable

 %feature("immutable");          // global enable
 int orange;                     // immutable

 %feature("immutable","0");      // global disable
 int yellow;                     // mutable

 %feature("immutable","1");      // another form of global enable
 int green;                      // immutable

 %feature("immutable","");       // clears the global feature
 int blue;                       // mutable
 </pre>
 </div>

 <p>
 Note that features are disabled by default and must be explicitly enabled either globally or by specifying a targeted declaration.
 The above intersperses SWIG directives with C code. Of course you can target features explicitly, so the above could also be rewritten as:
 </p>

 <div class="code">
 <pre>
 %feature("immutable","1") orange;
 %feature("immutable","1") green;
 int red;                        // mutable
 int orange;                     // immutable
 int yellow;                     // mutable
 int green;                      // immutable
 int blue;                       // mutable
 </pre>
 </div>

 <p>
 The above approach allows for the C declarations to be separated from the SWIG directives for when the C declarations are parsed from a C header file.
 The logic above can of course be inverted and rewritten as:
 </p>

 <div class="code">
 <pre>
 %feature("immutable","1");
 %feature("immutable","0") red;
 %feature("immutable","0") yellow;
 %feature("immutable","0") blue;
 int red;                        // mutable
 int orange;                     // immutable
 int yellow;                     // mutable
 int green;                      // immutable
 int blue;                       // mutable
 </pre>
 </div>

 <p>
 As hinted above for <tt>%immutable</tt>, most feature flags can also be specified via alternative syntax. The alternative syntax is just a macro
 in the <tt>swig.swg</tt> Library file. The following shows the alternative syntax for the imaginary <tt>featurename</tt> feature:
 </p>

 <div class="code">
 <pre>
 %featurename       // equivalent to %feature("featurename", "1") ie enables feature
 %nofeaturename     // equivalent to %feature("featurename", "0") ie disables feature
 %clearfeaturename  // equivalent to %feature("featurename", "")  ie clears feature
 </pre>
 </div>

 <p>
 The concept of clearing features is discussed next.
 </p>

 <H3><a name="Customization_clearing_features"></a>11.3.3 Clearing features</H3>


 <p>
 A feature stays in effect until it is explicitly cleared.  A feature is cleared by
 supplying a <tt>%feature</tt> directive with no value.  For example <tt>%feature("name","")</tt>.
 A cleared feature means that any feature exactly matching any previously defined feature is no longer used in the name matching rules.
 So if a feature is cleared, it might mean that another name matching rule will apply.
 To clarify, let's consider the <tt>except</tt> feature again (<tt>%exception</tt>):
 </p>

 <div class="code">
 <pre>
 // Define global exception handler
 %feature("except") {
     try {
         $action
     } catch (...) {
         croak("Unknown C++ exception");
     }
 }

 // Define exception handler for all clone methods to log the method calls
 %feature("except") *::clone() {
     try {
         logger.info("$action");
         $action
     } catch (...) {
         croak("Unknown C++ exception");
     }
 }

 ... initial set of class declarations with clone methods ...

 // clear the previously defined feature
 %feature("except","") *::clone();

 ... final set of class declarations with clone methods ...
 </pre>
 </div>

 <p>
 In the above scenario, the initial set of clone methods will log all method invocations from the target language.
 This specific feature is cleared for the final set of clone methods.
 However, these clone methods will still have an exception handler (without logging) as the next best feature match for them is the global exception handler.
 </p>

 <p>
 Note that clearing a feature is not always the same as disabling it.
 Clearing the feature above with <tt>%feature("except","") *::clone()</tt> is not the same as specifying
 <tt>%feature("except","0") *::clone()</tt>. The former will disable the feature for clone methods -
 the feature is still a better match than the global feature.
 If on the other hand, no global exception handler had been defined at all,
 then clearing the feature would be the same as disabling it as no other feature would have matched.
 </p>

 <p>
 Note that the feature must match exactly for it to be cleared by any previously defined feature.
 For example the following attempt to clear the initial feature will not work:
 </p>

 <div class="code">
 <pre>
 %feature("except") clone() { logger.info("$action"); $action }
 %feature("except","") *::clone();
 </pre>
 </div>

 <p>
 but this will:
 </p>

 <div class="code">
 <pre>
 %feature("except") clone() { logger.info("$action"); $action }
 %feature("except","") clone();
 </pre>
 </div>

 <p>
 SWIG provides macros for disabling and clearing features. Many of these can be found in the <tt>swig.swg</tt> library file.
 The typical pattern is to define three macros; one to define the feature itself, one to disable the feature and one to clear the feature.
 The three macros below show this for the "except" feature:
 </p>

 <div class="code">
 <pre>
 #define %exception      %feature("except")
 #define %noexception    %feature("except","0")
 #define %clearexception %feature("except","")
 </pre>
 </div>

 <H3><a name="Customization_features_default_args"></a>11.3.4 Features and default arguments</H3>


 <p>
 SWIG treats methods with default arguments as separate overloaded methods as detailed
 in the <a href="SWIGPlus.html#SWIGPlus_default_args">default arguments</a> section.
 Any <tt>%feature</tt> targeting a method with default arguments
 will apply to all the extra overloaded methods that SWIG generates if the
 default arguments are specified in the feature. If the default arguments are
 not specified in the feature, then the feature will match that exact
 wrapper method only and not the extra overloaded methods that SWIG generates.
 For example:
 </p>

 <div class="code">
 <pre>
 %feature("except") void hello(int i=0, double d=0.0) { ... }
 void hello(int i=0, double d=0.0);
 </pre>
 </div>

 <p>
 will apply the feature to all three wrapper methods, that is:
 </p>

 <div class="code">
 <pre>
 void hello(int i, double d);
 void hello(int i);
 void hello();
 </pre>
 </div>

 <p>
 If the default arguments are not specified in the feature:
 </p>

 <div class="code">
 <pre>
 %feature("except") void hello(int i, double d) { ... }
 void hello(int i=0, double d=0.0);
 </pre>
 </div>

 <p>
 then the feature will only apply to this wrapper method:
 </p>

 <div class="code">
 <pre>
 void hello(int i, double d);
 </pre>
 </div>

 <p>
 and not these wrapper methods:
 </p>

 <div class="code">
 <pre>
 void hello(int i);
 void hello();
 </pre>
 </div>

 <p>
 If <a href="SWIGPlus.html#SWIGPlus_default_args">compactdefaultargs</a> are being used, then the difference between
 specifying or not specifying default arguments in a feature is not applicable as just one wrapper is generated.
 </p>

 <p>
 <b>Compatibility note:</b> The different behaviour of features specified with or without default arguments was introduced
 in SWIG-1.3.23 when the approach to wrapping methods with default arguments was changed.
 </p>

 <H3><a name="features_example"></a>11.3.5 Feature example</H3>


 <p>
 As has been shown earlier, the intended use for the <tt>%feature</tt> directive is as a highly flexible customization mechanism that can be used to annotate
 declarations with additional information for use by specific target language modules.  Another example is
 in the Python module. You might use <tt>%feature</tt> to rewrite proxy/shadow class code as follows:
 </p>

 <div class="code">
 <pre>
 %module example
 %rename(bar_id) bar(int,double);

 // Rewrite bar() to allow some nice overloading

 %feature("shadow") Foo::bar(int) %{
 def bar(*args):
     if len(args) == 3:
          return apply(examplec.Foo_bar_id,args)
     return apply(examplec.Foo_bar,args)
 %}

 class Foo {
 public:
     int bar(int x);
     int bar(int x, double y);
 }
 </pre>
 </div>

 <p>
 Further details of <tt>%feature</tt> usage is described in the documentation for specific language modules.
 </p>

 </body>
 </html>