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<H1><a name="Go"></a>23 SWIG and Go</H1>
<!-- INDEX -->
<div class="sectiontoc">
<ul>
<li><a href="#Go_overview">Overview</a>
<li><a href="#Go_examples">Examples</a>
<li><a href="#Go_running_swig">Running SWIG with Go</a>
<ul>
<li><a href="#Go_commandline">Additional Commandline Options</a>
<li><a href="#Go_outputs">Go Output Files</a>
</ul>
<li><a href="#Go_basic_tour">A tour of basic C/C++ wrapping</a>
<ul>
<li><a href="#Go_package">Go Package Name</a>
<li><a href="#Go_names">Go Names</a>
<li><a href="#Go_constants">Go Constants</a>
<li><a href="#Go_enumerations">Go Enumerations</a>
<li><a href="#Go_classes">Go Classes</a>
<ul>
<li><a href="#Go_class_memory">Go Class Memory Management</a>
<li><a href="#Go_class_inheritance">Go Class Inheritance</a>
</ul>
<li><a href="#Go_templates">Go Templates</a>
<li><a href="#Go_director_classes">Go Director Classes</a>
<li><a href="#Go_primitive_type_mappings">Default Go primitive type mappings</a>
<li><a href="#Go_output_arguments">Output arguments</a>
<li><a href="#Go_adding_additional_code">Adding additional go code</a>
<li><a href="#Go_typemaps">Go typemaps</a>
</ul>
</ul>
</div>
<!-- INDEX -->
<p>
This chapter describes SWIG's support of Go. For more information on
the Go programming language
see <a href="http://golang.org/">golang.org</a>.
</p>
<H2><a name="Go_overview"></a>23.1 Overview</H2>
<p>
Go is a compiled language, not a scripting language. However, it does
not support direct calling of functions written in C/C++. The cgo
program may be used to generate wrappers to call C code from Go, but
there is no convenient way to call C++ code. SWIG fills this gap.
</p>
<p>
There are (at least) two different Go compilers. One is the gc
compiler, normally invoked via the go tool. The other
is the gccgo compiler, which is a frontend to the gcc compiler suite.
The interface to C/C++ code is completely different for the two Go
compilers. SWIG supports both, selected by a command line option.
</p>
<p>
Because Go is a type-safe compiled language, SWIG's runtime type
checking and runtime library are not used with Go. This should be
borne in mind when reading the rest of the SWIG documentation.
</p>
<H2><a name="Go_examples"></a>23.2 Examples</H2>
<p>
Working examples can be found here:
</p>
<ul>
<li><a href="https://golang.org/misc/swig">Examples from the Go source tree</a>
<li><a href="https://github.com/swig/swig/tree/master/Examples/go">Examples from the SWIG source tree</a>
</ul>
<p>
The examples in the 2nd link are shipped with the SWIG distribution under the Examples/go directory.
</p>
<H2><a name="Go_running_swig"></a>23.3 Running SWIG with Go</H2>
<p>
To generate Go code, use the <tt>-go</tt> option with SWIG. By
default SWIG will generate code for the gc compilers. To generate
code for gccgo, you should also use the <tt>-gccgo</tt> option.
</p>
<H3><a name="Go_commandline"></a>23.3.1 Additional Commandline Options</H3>
<p>
These are the command line options for SWIG's Go module. They can
also be seen by using:
</p>
<div class="code"><pre>
swig -go -help
</pre></div>
<table summary="Go specific options">
<tr>
<th>Go specific options</th>
</tr>
<tr>
<td>-cgo</td>
<td>Generate files to be used as input for the Go cgo tool. This
option is required for Go 1.5 and later, and works for Go 1.2 and
later. In the future this option will likely become the
default.</td>
</tr>
<tr>
<td>-intgosize &lt;s&gt;</td>
<td>Set the size for the Go type <tt>int</tt>. This controls the size
that the C/C++ code expects to see. The &lt;s&gt; argument should
be 32 or 64. This option is currently required during the
transition from Go 1.0 to Go 1.1, as the size of <tt>int</tt> on
64-bit x86 systems changes between those releases (from 32 bits to
64 bits). In the future the option may become optional, and SWIG
will assume that the size of <tt>int</tt> is the size of a C
pointer.</td>
</tr>
<tr>
<td>-gccgo</td>
<td>Generate code for gccgo. The default is to generate code for
the gc compiler.</td>
</tr>
<tr>
<td>-package &lt;name&gt;</td>
<td>Set the name of the Go package to &lt;name&gt;. The default
package name is the SWIG module name.</td>
</tr>
<tr>
<td>-use-shlib</td>
<td>Tell SWIG to emit code that uses a shared library. This is only
meaningful for the gc compiler, which needs to know at compile time
whether a shared library will be used.</td>
</tr>
<tr>
<td>-soname &lt;name&gt;</td>
<td>Set the runtime name of the shared library that the dynamic linker
should include at runtime. The default is the package name with
".so" appended. This is only used when generating code for
the gc compiler; when using gccgo, the equivalent name will be taken from
the <code>-soname</code> option passed to the linker. Using this
option implies the -use-shlib option.</td>
</tr>
<tr>
<td>-go-pkgpath &lt;pkgpath&gt;</td>
<td>When generating code for gccgo, set the pkgpath to use. This
corresponds to the <tt>-fgo-pkgpath</tt> option to gccgo.</td>
</tr>
<tr>
<td>-go-prefix &lt;prefix&gt;</td>
<td>When generating code for gccgo, set the prefix to use. This
corresponds to the <tt>-fgo-prefix</tt> option to gccgo.
If <tt>-go-pkgpath</tt> is used, <tt>-go-prefix</tt> will be
ignored.</td>
</tr>
</table>
<H3><a name="Go_outputs"></a>23.3.2 Go Output Files</H3>
<p>There are two different approaches to generating output files,
controlled by SWIG's <tt>-cgo</tt> option. The <tt>-cgo</tt> option
works with Go version 1.2 or later. It is required when using Go
version 1.5 or later.</p>
<p>With or without the <tt>-cgo</tt> option, SWIG will generate the
following files when generating Go code:</p>
<ul>
<li>
MODULE.go will contain the Go functions that your Go code will call.
These functions will be wrappers for the C++ functions defined by your
module. This file should, of course, be compiled with the Go
compiler.
</li>
<li>
MODULE_wrap.c or MODULE_wrap.cxx will contain C/C++ functions will be
invoked by the Go wrapper code. This file should be compiled with the
usual C or C++ compiler.
</li>
<li>
MODULE_wrap.h will be generated if you use the directors feature. It
provides a definition of the generated C++ director classes. It is
generally not necessary to use this file, but in some special cases it
may be helpful to include it in your code, compiled with the usual C
or C++ compiler.
</li>
</ul>
<p>When neither the <tt>-cgo</tt> nor the <tt>-gccgo</tt> option is
used, SWIG will also generate an additional file:</p>
<ul>
<li>
MODULE_gc.c will contain C code which should be compiled with the C
compiler distributed as part of the gc compiler. It should then be
combined with the compiled MODULE.go using go tool pack.
</li>
</ul>
<p>
Most Go programs are built using the go tool. The go tool has limited
support for SWIG. To use it, put your SWIG interface into a file with
the extension .swig, or, if you are wrapping C++ code, .swigcxx. Put
that file in a GOPATH/src directory as usual for Go sources. Put
other interface code in the same directory with extensions of .c and
.cxx. The <tt>go build</tt> and <tt>go install</tt> commands will
automatically run SWIG for you and will build the interface code.
</p>
<p>
You can also use SWIG directly yourself. When using
the <tt>-cgo</tt> option, SWIG will generate files that can be used
directly by <tt>go build</tt>. Put your SWIG input file in a
directory under GOPATH/src, and give it a name that does not end in
.swig or .swigcxx.
</p>
<div class="code"><pre>
% swig -go -cgo example.i
% go install
</pre></div>
<p>
You will now have a Go package that you can import from other Go
packages as usual.
</p>
<p>
To use SWIG without the <tt>-cgo</tt> option, more steps are required.
Recall that this only works with Go versions before 1.5. When using
Go version 1.2 or later, or when using gccgo, the code generated by
SWIG can be linked directly into the Go program. A typical command
sequence when using the gc compiler would look like this:
</p>
<div class="code"><pre>
% swig -go example.i
% gcc -c code.c # The C library being wrapped.
% gcc -c example_wrap.c
% go tool 6g example.go
% go tool 6c example_gc.c
% go tool pack grc example.a example.6 example_gc.6 code.o example_wrap.o
% go tool 6g main.go
% go tool 6l main.6
</pre></div>
<p>
You can also put the wrapped code into a shared library, and when
using the Go versions before 1.2 this is the only supported option. A
typical command sequence for this approach would look like this:
</p>
<div class="code"><pre>
% swig -go -use-shlib example.i
% gcc -c -fpic example.c
% gcc -c -fpic example_wrap.c
% gcc -shared example.o example_wrap.o -o example.so
% go tool 6g example.go
% go tool 6c example_gc.c
% go tool pack grc example.a example.6 example_gc.6
% go tool 6g main.go # your code, not generated by SWIG
% go tool 6l main.6
</pre></div>
<H2><a name="Go_basic_tour"></a>23.4 A tour of basic C/C++ wrapping</H2>
<p>
By default, SWIG attempts to build a natural Go interface to your
C/C++ code. However, the languages are somewhat different, so some
modifications have to occur. This section briefly covers the
essential aspects of this wrapping.
</p>
<H3><a name="Go_package"></a>23.4.1 Go Package Name</H3>
<p>
All Go source code lives in a package. The name of this package will
default to the name of the module from SWIG's <tt>%module</tt>
directive. You may override this by using SWIG's <tt>-package</tt>
command line option.
</p>
<H3><a name="Go_names"></a>23.4.2 Go Names</H3>
<p>
In Go, a function is only visible outside the current package if the
first letter of the name is uppercase. This is quite different from
C/C++. Because of this, C/C++ names are modified when generating the
Go interface: the first letter is forced to be uppercase if it is not
already. This affects the names of functions, methods, variables,
constants, enums, and classes.
</p>
<p>
C/C++ variables are wrapped with setter and getter functions in Go.
First the first letter of the variable name will be forced to
uppercase, and then <tt>Get</tt> or <tt>Set</tt> will be prepended.
For example, if the C/C++ variable is called <tt>var</tt>, then SWIG
will define the functions <tt>GetVar</tt> and <tt>SetVar</tt>. If a
variable is declared as <tt>const</tt>, or if
SWIG's <a href="SWIG.html#SWIG_readonly_variables">
<tt>%immutable</tt> directive</a> is used for the variable, then only
the getter will be defined.
</p>
<p>
C++ classes will be discussed further below. Here we'll note that the
first letter of the class name will be forced to uppercase to give the
name of a type in Go. A constructor will be named <tt>New</tt>
followed by that name, and the destructor will be
named <tt>Delete</tt> followed by that name.
</p>
<H3><a name="Go_constants"></a>23.4.3 Go Constants</H3>
<p>
C/C++ constants created via <tt>#define</tt> or the <tt>%constant</tt>
directive become Go constants, declared with a <tt>const</tt>
declaration.
<H3><a name="Go_enumerations"></a>23.4.4 Go Enumerations</H3>
<p>
C/C++ enumeration types will cause SWIG to define an integer type with
the name of the enumeration (with first letter forced to uppercase as
usual). The values of the enumeration will become variables in Go;
code should avoid modifying those variables.
</p>
<H3><a name="Go_classes"></a>23.4.5 Go Classes</H3>
<p>
Go has interfaces, methods and inheritance, but it does not have
classes in the same sense as C++. This sections describes how SWIG
represents C++ classes represented in Go.
</p>
<p>
For a C++ class <tt>ClassName</tt>, SWIG will define two types in Go:
an underlying type, which will just hold a pointer to the C++ type,
and an interface type. The interface type will be
named <tt>ClassName</tt>. SWIG will define a
function <tt>NewClassName</tt> which will take any constructor
arguments and return a value of the interface
type <tt>ClassName</tt>. SWIG will also define a
destructor <tt>DeleteClassName</tt>.
</p>
<p>
SWIG will represent any methods of the C++ class as methods on the
underlying type, and also as methods of the interface type. Thus C++
methods may be invoked directly using the
usual <tt>val.MethodName</tt> syntax. Public members of the C++ class
will be given getter and setter functions defined as methods of the
class.
</p>
<p>
SWIG will represent static methods of C++ classes as ordinary Go
functions. SWIG will use names like <tt>ClassNameMethodName</tt>.
SWIG will give static members getter and setter functions with names
like <tt>GetClassName_VarName</tt>.
</p>
<p>
Given a value of the interface type, Go code can retrieve the pointer
to the C++ type by calling the <tt>Swigcptr</tt> method. This will
return a value of type <tt>SwigcptrClassName</tt>, which is just a
name for <tt>uintptr</tt>. A Go type conversion can be used to
convert this value to a different C++ type, but note that this
conversion will not be type checked and is essentially equivalent
to <tt>reinterpret_cast</tt>. This should only be used for very
special cases, such as where C++ would use a <tt>dynamic_cast</tt>.
</p>
<p>Note that C++ pointers to compound objects are represented in go as objects
themselves, not as go pointers. So, for example, if you wrap the following
function:</p>
<div class="code">
<pre>
class MyClass {
int MyMethod();
static MyClass *MyFactoryFunction();
};
</pre>
</div>
<p>You will get go code that looks like this:</p>
<div class="code">
<pre>
type MyClass interface {
Swigcptr() uintptr
SwigIsMyClass()
MyMethod() int
}
MyClassMyFactoryFunction() MyClass {
// swig magic here
}
</pre>
</div>
<p>Note that the factory function does not return a go pointer; it actually
returns a go interface. If the returned pointer can be null, you can check
for this by calling the Swigcptr() method.
</p>
<H4><a name="Go_class_memory"></a>23.4.5.1 Go Class Memory Management</H4>
<p>
Calling <tt>NewClassName</tt> for a C++ class <tt>ClassName</tt> will allocate
memory using the C++ memory allocator. This memory will not be automatically
freed by Go's garbage collector as the object ownership is not tracked. When
you are done with the C++ object you must free it using
<tt>DeleteClassName</tt>.<br>
<br>
The most Go idiomatic way to manage the memory for some C++ class is to call
<tt>NewClassName</tt> followed by a
<tt><a href="https://golang.org/doc/effective_go.html#defer">defer</a></tt> of
the <tt>DeleteClassName</tt> call. Using <tt>defer</tt> ensures that the memory
of the C++ object is freed as soon as the function containing the <tt>defer</tt>
statement returns. Furthemore <tt>defer</tt> works great for short-lived
objects and fits nicely C++'s RAII idiom. Example:
</p>
<div class="code">
<pre>
func UseClassName(...) ... {
o := NewClassName(...)
defer DeleteClassName(o)
// Use the ClassName object
return ...
}
</pre>
</div>
<p>
With increasing complexity, especially complex C++ object hierarchies, the
correct placement of <tt>defer</tt> statements becomes harder and harder as C++
objects need to be freed in the correct order. This problem can be eased by
keeping a C++ object function local so that it is only available to the function
that creates a C++ object and functions called by this function. Example:
</p>
<div class="code">
<pre>
func WithClassName(constructor args, f func(ClassName, ...interface{}) error, data ...interface{}) error {
o := NewClassName(constructor args)
defer DeleteClassName(o)
return f(o, data...)
}
func UseClassName(o ClassName, data ...interface{}) (err error) {
// Use the ClassName object and additional data and return error.
}
func main() {
WithClassName(constructor args, UseClassName, additional data)
}
</pre>
</div>
<p>
Using <tt>defer</tt> has limitations though, especially when it comes to
long-lived C++ objects whichs lifetimes are hard to predict. For such C++
objects a common technique is to store the C++ object into a Go object, and to
use the Go function <tt>runtime.SetFinalizer</tt> to add a finalizer which frees
the C++ object when the Go object is freed. It is strongly recommended to read
the <a href="https://golang.org/pkg/runtime/#SetFinalizer">runtime.SetFinalizer
</a> documentation before using this technique to understand the
<tt>runtime.SetFinalizer</tt> limitations.<br>
</p>
<p>
Common pitfalls with <tt>runtime.SetFinalizer</tt> are:
</p>
<ul>
<li>
If a hierarchy of C++ objects will be automatically freed by Go finalizers then
the Go objects that store the C++ objects need to replicate the hierarchy of the
C++ objects to prevent that C++ objects are freed prematurely while other C++
objects still rely on them.
</li>
<li>
The usage of Go finalizers is problematic with C++'s RAII idiom as it isn't
predictable when the finalizer will run and this might require a Close or Delete
method to be added the Go object that stores a C++ object to mitigate.
</li>
<li>
The Go finalizer function typically runs in a different OS thread which can be
problematic with C++ code that uses thread-local storage.
</li>
</ul>
<p>
<tt>runtime.SetFinalizer</tt> Example:
</p>
<div class="code">
<pre>
import (
"runtime"
"wrap" // SWIG generated wrapper code
)
type GoClassName struct {
wcn wrap.ClassName
}
func NewGoClassName() *GoClassName {
o := &amp;GoClassName{wcn: wrap.NewClassName()}
runtime.SetFinalizer(o, deleteGoClassName)
return o
}
func deleteGoClassName(o *GoClassName) {
// Runs typically in a different OS thread!
wrap.DeleteClassName(o.wcn)
o.wcn = nil
}
func (o *GoClassName) Close() {
// If the C++ object has a Close method.
o.wcn.Close()
// If the GoClassName object is no longer in an usable state.
runtime.SetFinalizer(o, nil) // Remove finalizer.
deleteGoClassName() // Free the C++ object.
}
</pre>
</div>
<H4><a name="Go_class_inheritance"></a>23.4.5.2 Go Class Inheritance</H4>
<p>
C++ class inheritance is automatically represented in Go due to its
use of interfaces. The interface for a child class will be a superset
of the interface of its parent class. Thus a value of the child class
type in Go may be passed to a function which expects the parent class.
Doing the reverse will require an explicit type assertion, which will
be checked dynamically.
</p>
<H3><a name="Go_templates"></a>23.4.6 Go Templates</H3>
<p>
In order to use C++ templates in Go, you must tell SWIG to create
wrappers for a particular template instantation. To do this, use
the <tt>%template</tt> directive.
<H3><a name="Go_director_classes"></a>23.4.7 Go Director Classes</H3>
<p>
SWIG's director feature permits a Go type to act as the subclass of a
C++ class with virtual methods. This is complicated by the fact that
C++ and Go define inheritance differently. In Go, structs can inherit
methods via anonymous field embedding. However, when a method is
called for an embedded struct, if that method calls any other methods,
they are called for the embedded struct, not for the original type.
Therefore, SWIG must use Go interfaces to represent C++ inheritance.
</p>
<p>
In order to use the director feature in Go, you must define a type in
your Go code. You must then add methods for the type. Define a
method in Go for each C++ virtual function that you want to override.
You must then create a value of your new type, and pass a pointer to
it to the function <tt>NewDirectorClassName</tt>,
where <tt>ClassName</tt> is the name of the C++ class. That will
return a value of type <tt>ClassName</tt>.
</p>
<p>
For example:
</p>
<div class="code">
<pre>
type GoClass struct { }
func (p *GoClass) VirtualFunction() { }
func MakeClass() ClassName {
return NewDirectorClassName(&amp;GoClass{})
}
</pre>
</div>
<p>
Any call in C++ code to the virtual function will wind up calling the
method defined in Go. The Go code may of course call other methods on
itself, and those methods may be defined either in Go or in C++.
</p>
<H3><a name="Go_primitive_type_mappings"></a>23.4.8 Default Go primitive type mappings</H3>
<p>
The following table lists the default type mapping from C/C++ to Go.
This table will tell you which Go type to expect for a function which
uses a given C/C++ type.
</p>
<table BORDER summary="Go primitive type mappings">
<tr>
<td><b>C/C++ type</b></td>
<td><b>Go type</b></td>
</tr>
<tr>
<td>bool</td>
<td>bool</td>
</tr>
<tr>
<td>char</td>
<td>byte</td>
</tr>
<tr>
<td>signed char</td>
<td>int8</td>
</tr>
<tr>
<td>unsigned char</td>
<td>byte</td>
</tr>
<tr>
<td>short</td>
<td>int16</td>
</tr>
<tr>
<td>unsigned short</td>
<td>uint16</td>
</tr>
<tr>
<td>int</td>
<td>int</td>
</tr>
<tr>
<td>unsigned int</td>
<td>uint</td>
</tr>
<tr>
<td>long</td>
<td>int64</td>
</tr>
<tr>
<td>unsigned long</td>
<td>uint64</td>
</tr>
<tr>
<td>long long</td>
<td>int64</td>
</tr>
<tr>
<td>unsigned long long</td>
<td>uint64</td>
</tr>
<tr>
<td>float</td>
<td>float32</td>
</tr>
<tr>
<td>double</td>
<td>float64</td>
</tr>
<tr>
<td>char *<br>char []</td>
<td>string</td>
</tr>
</table>
<p>
Note that SWIG wraps the C <tt>char</tt> type as a character. Pointers
and arrays of this type are wrapped as strings. The <tt>signed
char</tt> type can be used if you want to treat <tt>char</tt> as a
signed number rather than a character. Also note that all const
references to primitive types are treated as if they are passed by
value.
</p>
<p>
These type mappings are defined by the "gotype" typemap. You may change
that typemap, or add new values, to control how C/C++ types are mapped
into Go types.
</p>
<H3><a name="Go_output_arguments"></a>23.4.9 Output arguments</H3>
<p>Because of limitations in the way output arguments are processed in swig,
a function with output arguments will not have multiple return values.
Instead, you must pass a pointer into the C++ function to tell it where to
store the output value. In go, you supply a slice in the place of the output
argument.</p>
<p>For example, suppose you were trying to wrap the modf() function in the
C math library which splits x into integral and fractional parts (and
returns the integer part in one of its parameters):</p>
<div class="code">
<pre>
double modf(double x, double *ip);
</pre>
</div>
<p>You could wrap it with SWIG as follows:</p>
<div class="code">
<pre>
%include &lt;typemaps.i&gt;
double modf(double x, double *OUTPUT);
</pre>
</div>
<p>or you can use the <code>%apply</code> directive:</p>
<div class="code">
<pre>
%include &lt;typemaps.i&gt;
%apply double *OUTPUT { double *ip };
double modf(double x, double *ip);
</pre>
</div>
<p>In Go you would use it like this:</p>
<div class="code">
<pre>
ptr := []float64{0.0}
fraction := modulename.Modf(5.0, ptr)
</pre>
</div>
<p>Since this is ugly, you may want to wrap the swig-generated API with
some <a href="#Embedded_go_code">additional functions written in go</a> that
hide the ugly details.</p>
<p>There are no <code>char&nbsp;*OUTPUT</code> typemaps. However you can
apply the <code>signed&nbsp;char&nbsp;*</code> typemaps instead:</p>
<div class="code">
<pre>
%include &lt;typemaps.i&gt;
%apply signed char *OUTPUT {char *output};
void f(char *output);
</pre>
</div>
<H3><a name="Go_adding_additional_code"></a>23.4.10 Adding additional go code</H3>
<p>Often the APIs generated by swig are not very natural in go, especially if
there are output arguments. You can
insert additional go wrapping code to add new APIs
with <code>%insert(go_wrapper)</code>, like this:</p>
<div class="code">
<pre>
%include &lt;typemaps.i&gt;
// Change name of what swig generates to Wrapped_modf. This function will
// have the following signature in go:
// func Wrapped_modf(float64, []float64) float64
%rename(wrapped_modf) modf(double x, double *ip);
%apply double *OUTPUT { double *ip };
double modf(double x, double *ip);
%insert(go_wrapper) %{
// The improved go interface to this function, which has two return values,
// in the more natural go idiom:
func Modf(x float64) (fracPart float64, intPart float64) {
ip := []float64{0.0}
fracPart = Wrapped_modf(x, ip)
intPart = ip[0]
return
}
%}
</pre>
</div>
<p>For classes, since swig generates an interface, you can add additional
methods by defining another interface that includes the swig-generated
interface. For example,</p>
<div class="code">
<pre>
%rename(Wrapped_MyClass) MyClass;
%rename(Wrapped_GetAValue) MyClass::GetAValue(int *x);
%apply int *OUTPUT { int *x };
class MyClass {
public:
MyClass();
int AFineMethod(const char *arg); // Swig's wrapping is fine for this one.
bool GetAValue(int *x);
};
%insert(go_wrapper) %{
type MyClass interface {
Wrapped_MyClass
GetAValue() (int, bool)
}
func (arg SwigcptrWrapped_MyClass) GetAValue() (int, bool) {
ip := []int{0}
ok := arg.Wrapped_GetAValue(ip)
return ip[0], ok
}
%}
</pre>
</div>
<p>Of course, if you have to rewrite most of the methods, instead of just a
few, then you might as well define your own struct that includes the
swig-wrapped object, instead of adding methods to the swig-generated object.</p>
<p>If you need to import other go packages, you can do this with
<code>%go_import</code>. For example,</p>
<div class="code">
<pre>
%go_import("fmt", _ "unusedPackage", rp "renamed/package")
%insert(go_wrapper) %{
func foo() {
fmt.Println("Some string:", rp.GetString())
}
// Importing the same package twice is permitted,
// Go code will be generated with only the first instance of the import.
%go_import("fmt")
%insert(go_wrapper) %{
func bar() {
fmt.Println("Hello world!")
}
%}
</pre>
</div>
<H3><a name="Go_typemaps"></a>23.4.11 Go typemaps</H3>
<p>
You can use the <tt>%typemap</tt> directive to modify SWIG's default
wrapping behavior for specific C/C++ types. You need to be familiar
with the material in the general
"<a href="Typemaps.html#Typemaps">Typemaps</a>" chapter. That chapter
explains how to define a typemap. This section describes some
specific typemaps used for Go.
</p>
<p>
In general type conversion code may be written either in C/C++ or in
Go. The choice to make normally depends on where memory should be
allocated. To allocate memory controlled by the Go garbage collector,
write Go code. To allocate memory in the C/C++ heap, write C code.
</p>
<table BORDER summary="Go Typemaps">
<tr>
<td><b>Typemap</b></td>
<td><b>Description</b></td>
</tr>
<tr>
<td>gotype</td>
<td>
The Go type to use for a C++ type. This type will appear in the
generated Go wrapper function. If this is not defined SWIG will use a
default as <a href="#Go_primitive_type_mappings">described above</a>.
</td>
</tr>
<tr>
<td>imtype</td>
<td>
An intermediate Go type used by the "goin", "goout", "godirectorin",
and "godirectorout" typemaps. If this typemap is not defined for a
C/C++ type, the gotype typemape will be used. This is useful when
gotype is best converted to C/C++ using Go code.
</td>
</tr>
<tr>
<td>goin</td>
<td>
Go code to convert from gotype to imtype when calling a C/C++
function. SWIG will then internally convert imtype to a C/C++ type
and pass it down. If this is not defined, or is the empty string, no
conversion is done.
</td>
</tr>
<tr>
<td>in</td>
<td>
C/C++ code to convert the internally generated C/C++ type, based on
imtype, into the C/C++ type that a function call expects. If this is
not defined the value will simply be cast to the desired type.
</td>
</tr>
<tr>
<td>out</td>
<td>
C/C++ code to convert the C/C++ type that a function call returns into
the internally generated C/C++ type, based on imtype, that will be
returned to Go. If this is not defined the value will simply be cast
to the desired type.
</td>
</tr>
<tr>
<td>goout</td>
<td>
Go code to convert a value returned from a C/C++ function from imtype
to gotype. If this is not defined, or is the empty string, no
conversion is done.
</td>
</tr>
<tr>
<td>argout</td>
<td>
C/C++ code to adjust an argument value when returning from a function.
This is called after the real C/C++ function has run. This uses the
internally generated C/C++ type, based on imtype. This is only useful
for a pointer type of some sort. If this is not defined nothing will
be done.
</td>
</tr>
<tr>
<td>goargout</td>
<td>
Go code to adjust an argument value when returning from a function.
This is called after the real C/C++ function has run. The value will
be in imtype. This is only useful for a pointer type of some sort.
If this is not defined, or is the empty string, nothing will be done.
</td>
</tr>
<tr>
<td>directorin</td>
<td>
C/C++ code to convert the C/C++ type used to call a director method
into the internally generated C/C++ type, based on imtype, that will
be passed to Go. If this is not defined the value will simply be cast
to the desired type.
</td>
</tr>
<tr>
<td>godirectorin</td>
<td>
Go code to convert a value used to call a director method from imtype
to gotype. If this is not defined, or is the empty string, no
conversion is done.
</td>
</tr>
<tr>
<td>godirectorout</td>
<td>
Go code to convert a value returned from a director method from gotype
to imtype. If this is not defined, or is the empty string, no
conversion is done.
</td>
</tr>
<tr>
<td>directorout</td>
<td>
C/C++ code to convert a value returned from a director method from the
internally generated C/C++ type, based on imtype, into the type that
the method should return If this is not defined the value will simply
be cast to the desired type.
</td>
</tr>
</table>
</body>
</html>