Examples/python/reference/index.html

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

<p>
This example tests SWIG's handling of C++ references.  Since C++
references are closely related to pointers (as both refer to a
location in memory), SWIG simply collapses all references into
pointers when creating wrappers.

<h2>Some examples</h2>

References are most commonly used as function parameter.  For example,
you might have an operator like this:

<blockquote>
<pre>
Vector operator+(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>

or a function:

<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:

<blockquote>
<pre>
Vector wrap_addv(Vector *a, Vector *b) {
    return addv(*a,*b);
}
</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:
 ...
   %addmethods {
    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 Python script</h2>

Click <a href="runme.py">here</a> to see a script that manipulates some C++ references.

<h2>Notes:</h2>

<ul>
<li>C++ references primarily provide notational convenience for C++
source code.  However, Python 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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