Hi all,

I'm trying to use C-style callback functions from within C++, but i'm not having much luck.

Specifically, i'm using SDL_mixer, and I want to be able to pass my callback function to Mix_ChannelFinished so I can tell when its OK to play another sample on a channel once it's finished.

I looked about on the web, and read that you can cast the function pointer to the type that the C function is expecting like so:

Mix_ChannelFinished((void (*)(int)) myCallbackFunction);

Still can't get it to compile though, I get the following error message instead:

Audio.cpp:106: error: argument of type `void (Audio::)(int)' does not match ` void (*)(int)'

Strange, since the cast should sort that out I would have thought. Fine, says I, i'll make the callback a static method. Now, this works if I comment out anything that touches the flag, but if I try to use the flag inside the (now static) callback, I get this error:

Audio.cpp:162: error: invalid use of member `Audio::chDone' in static member function

Making the boolean flag static produces this error at every point it is used in the code (doesn't matter if I use the scope resolution operator or not):

/home/apartington/source/c++/gg-sdl/gravsdl/src/Audio.cpp:108: undefined reference to `Audio::chDone'


Can anyone help?

Cheers,

Hexstatic







P.S.: Code looks something like this...

//Class definition in Audio.h
class Audio
{
...
...
void someMemberFunction();
void myCallbackFunction(int i);
bool flag;
...
...
};


//Implementation in Audio.cpp
Audio::someMemberFunction()
{
...
...
Mix_ChannelFinished((void (*)(int)) myCallbackFunction);
...
...
}

Audio::myCallbackFunction(int i)
{
...
flag = true;
...
}

1. Use callback functions, it looks like ye be trying ta use callback methods.
2. Don't use casts, just write th' function prototype th' way it likes.
3. Use [code] tags ta preserve indentation.

There are two ways to resolve:

1. Modify callback function of Audio to static
2. Add addition C-style callback function to call audio's callback

Cheers,
GH

You should precede the function name with '&'; I don't think conversion to pointer is automatic when casting. You should do that anyway, actually, because it's good form, and you can also use function references in C++ (meaning you should make your code explicit.)
ta0kira