`SWIG/Examples/java/funcptr/`

## Pointers to Functions

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

>     int do_op(int a, int b, int (*op)(int, int));

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

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:

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

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:

>     %constant(int (*)(int,int)) ADD = add;

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

>     int r = do_op(x,y, example.ADD)

where `example` is the module name.

## An Example

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

- example.c
- example.h
- example.i (SWIG interface)
- main.java (Sample program)

## Notes

- 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.

- 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.