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| <title>SWIG:Examples:python:funcptr</title> |
| </head> |
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| <body bgcolor="#ffffff"> |
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| <tt>SWIG/Examples/python/funcptr/</tt> |
| <hr> |
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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 script, |
| 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 script, you would do this: |
| |
| <blockquote> |
| <pre> |
| r = do_op(x,y, ADD) |
| </pre> |
| </blockquote> |
| |
| <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="example.py">example.py</a> (Sample script) |
| </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 Python function object in as a substitute for a C |
| function pointer. |
| |
| <p> |
| <li>A python 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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| <hr> |
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