Path: chuka.playstation.co.uk!news
From: Andrew Partington <andrew.partington90@ntlworld.com>
Newsgroups: scee.yaroze.freetalk.english
Subject: Using C++ with C function pointers
Date: Tue, 19 Sep 2006 22:13:55 +0000
Organization: PlayStation Net Yaroze (SCEE)
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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)'

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'


Any C++ gurus there who can help?

Cheers,

Andy P







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;
	...
}