Examples/go/reference/index.html

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<tt>SWIG/Examples/go/reference/</tt>
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<H2>C++ Reference Handling</H2>

<p>
This example tests SWIG's handling of C++ references.  A reference in
C++ is much like a pointer.  Go represents C++ classes as pointers
which are stored in interface values.  Therefore, a reference to a
class in C++ simply becomes an object of the class type in Go.  For
types which are not classes, a reference in C++ is represented as a
pointer in Go.

<h2>Some examples</h2>

References are most commonly used as function parameters.  For
example, you might have a function like this:

<blockquote>
<pre>
Vector addv(const Vector &amp;a, const Vector &amp;b) {
   Vector result;
   result.x = a.x + b.x;
   result.y = a.y + b.y;
   result.z = a.z + b.z;
   return result;
}
</pre>
</blockquote>

In these cases, SWIG transforms everything into a pointer and creates
a wrapper that looks like this in C++.

<blockquote>
<pre>
Vector wrap_addv(Vector *a, Vector *b);
</pre>
</blockquote>

or like this in Go:

<blockquote>
<pre>
func Addv(arg1 Vector, arg2 Vector) Vector
</pre>
</blockquote>

Occasionally, a reference is used as a return value of a function
when the return result is to be used as an lvalue in an expression.
The prototypical example is an operator like this:

<blockquote>
<pre>
Vector &amp;operator[](int index);
</pre>
</blockquote>

or a method:

<blockquote>
<pre>
Vector &amp;get(int index);
</pre>
</blockquote>

For functions returning references, a wrapper like this is created:

<blockquote>
<pre>
Vector *wrap_Object_get(Object *self, int index) {
    Vector &amp;result = self-&gt;get(index);
    return &amp;result;
}
</pre>
</blockquote>

The following <a href="example.h">header file</a> contains some class
definitions with some operators and use of references.

<h2>SWIG Interface</h2>

SWIG does NOT support overloaded operators so it can not directly
build an interface to the classes in the above file.  However, a
number of workarounds can be made.  For example, an overloaded
operator can be stuck behind a function call such as the <tt>addv</tt>
function above.  Array access can be handled with a pair of set/get
functions like this:

<blockquote>
<pre>
class VectorArray {
public:
 ...
   %extend {
    Vector &amp;get(int index) {
      return (*self)[index];
    }
    void set(int index, Vector &amp;a) {
      (*self)[index] = a;
    }
   }
   ...
}
</pre>
</blockquote>

Click <a href="example.i">here</a> to see a SWIG interface file with
these additions.

<h2>Sample Go program</h2>

Click <a href="runme.go">here</a> to see a Go program that manipulates
some C++ references.

<h2>Notes:</h2>

<ul>
<li>C++ references primarily provide notational convenience for C++
source code.  However, Go only supports the 'x.a' notation so it
doesn't much matter.

<p>
<li>When a program returns a reference, a pointer is returned.  Unlike
return by value, memory is not allocated to hold the return result.

<p>
<li>SWIG has particular trouble handling various combinations of
references and pointers.  This is side effect of an old parsing scheme
and type representation that will be replaced in future versions.

</ul>

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