Examples/java/funcptr/index.html

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<tt>SWIG/Examples/java/funcptr/</tt>
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<H2>Pointers to Functions</H2>

<p>
Okay, just what in the heck does SWIG do with a declaration like this?

<blockquote>
<pre>
int do_op(int a, int b, int (*op)(int, int));
</pre>
</blockquote>

Well, it creates a wrapper as usual.  Of course, that does raise some
questions about the third argument (the pointer to a function). 

<p>
In this case, SWIG will wrap the function pointer as it does for all other
pointers.  However, in order to actually call this function from a Java program,
you will need to pass some kind of C function pointer object.  In C,
this is easy, you just supply a function name as an argument like this:

<blockquote>
<pre>
/* Some callback function */
int add(int a, int b) {
   return a+b;
} 
...
int r = do_op(x,y,add);
</pre>
</blockquote>

To make this work with SWIG, you will need to do a little extra work.  Specifically,
you need to create some function pointer objects using the %constant directive like this:

<blockquote>
<pre>
%constant(int (*)(int,int)) ADD = add;
</pre>
</blockquote>

Now, in a Java program, you would do this:

<blockquote>
<pre>
int r = do_op(x,y, example.ADD)
</pre>
</blockquote>
where <tt>example</tt> is the module name.

<h2>An Example</h2>

Here are some files that illustrate this with a simple example:

<ul>
<li><a href="example.c">example.c</a>
<li><a href="example.h">example.h</a>
<li><a href="example.i">example.i</a> (SWIG interface)
<li><a href="runme.java">runme.java</a> (Sample program)
</ul>

<h2>Notes</h2>

<ul>
<li>The value of a function pointer must correspond to a function written in C or C++.
It is not possible to pass an arbitrary Java function in as a substitute for a C 
function pointer.

<p>
<li>A Java function can be used as a C/C++ callback if you write some
clever typemaps and are very careful about how you create your extension.
This is an advanced topic not covered here.
</ul>

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