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<title>SWIG:Examples:go:pointer</title>
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<tt>SWIG/Examples/go/pointer/</tt>
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<H2>Simple Pointer Handling</H2>
<p>
This example illustrates a couple of techniques for handling simple
pointers in SWIG. The prototypical example is a C function that
operates on pointers such as this:
<blockquote>
<pre>
void add(int *x, int *y, int *r) {
*r = *x + *y;
}
</pre>
</blockquote>
By default, SWIG wraps this function exactly as specified and creates
an interface that expects pointer objects for arguments. This only
works when there is a precise correspondence between the C type and
some Go type.
<p>
<h2>Other approaches</h2>
<p>
<li>The SWIG pointer library provides a different, safer, way to
handle pointers. For example, in the interface file you would do
this:
<blockquote>
<pre>
%include cpointer.i
%pointer_functions(int, intp);
</pre>
</blockquote>
and from Go you would use pointers like this:
<blockquote>
<pre>
a := example.New_intp()
b := example.New_intp()
c := example.New_intp()
Intp_Assign(a, 37)
Intp_Assign(b, 42)
fmt.Println(" a =", a)
fmt.Println(" b =", b)
fmt.Println(" c =", c)
// Call the add() function with some pointers
example.Add(a,b,c)
// Now get the result
res := example.Intp_value(c)
fmt.Println(" 37 + 42 =", res)
// Clean up the pointers
example.Delete_intp(a)
example.Delete_intp(b)
example.Delete_intp(c)
</pre>
</blockquote>
<p>
<li>Use the SWIG typemap library. This library allows you to
completely change the way arguments are processed by SWIG. For
example:
<blockquote>
<pre>
%include "typemaps.i"
void add(int *INPUT, int *INPUT, int *OUTPUT);
</pre>
</blockquote>
And in a Go program:
<blockquote>
<pre>
r := []int{0}
example.Sub(37,42,r)
fmt.Println("Result =", r[0])
</pre>
</blockquote>
Needless to say, this is substantially easier although a bit unusual.
<p>
<li>A final alternative is to use the typemaps library in combination
with the %apply directive. This allows you to change the names of parameters
that behave as input or output parameters. For example:
<blockquote>
<pre>
%include "typemaps.i"
%apply int *INPUT {int *x, int *y};
%apply int *OUTPUT {int *r};
void add(int *x, int *y, int *r);
void sub(int *x, int *y, int *r);
void mul(int *x, int *y, int *r);
... etc ...
</pre>
</blockquote>
</ul>
<h2>Example</h2>
The following example illustrates the use of these features for pointer
extraction.
<ul>
<li> <a href="example.c">example.c</a> (C Source)
<li> <a href="example.i">example.i</a> (SWIG interface)
<li> <a href="runme.go">runme.go</a> (Go program)
</ul>
<h2>Notes</h2>
<ul>
<li>Since pointers are used for so many different things (arrays, output values,
etc...) the complexity of pointer handling can be as complicated as you want to
make it.
<p>
<li>More documentation on the typemaps.i and cpointer.i library files can be
found in the SWIG user manual. The files also contain documentation.
</ul>
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