Re: Reference to a C++ object in GTK+ callbacks

[Chris Vine <chris cvine freeserve co uk>]

On Wednesday 14 December 2005 20:38, Wallace Owen wrote:

[snip]

> Warning!
>
> It may work for you.  It's not portable.  C++ implementations are free
> to use a calling convention different from the C calling convention: In
> C, parameters are pushed in reverse order to support varargs.  C++
> functions are specified in the standard as not supporting varargs so
> that compiler vendors are free to use certain optimizations.  For
> example, in C, it's the caller's responsibility to clean up the stack
> after a call because the called function can't know at compile time how
> many parms were passed.  C++ compilers are free to optimize this such
> that the called function does the stack cleanup, so that a function
> that's called from many different places only has one stack cleanup
> chunk generated.
>
> It's for this reason that a static function should be used to catch
> callbacks, so it can cast the parameter to a pointer to the object, then
> invoke the member function.  Static member functions don't have an
> implied first parm that's it's object.  See below:
>
> class Foo
> {
> public:
> 	static redrawCB(Event *event, void *parm)
> 	{
> 		Foo *obj = reinterpret_cast<Foo*>(parm);
> 		obj->redraw(event);
> 	}
>
> 	redraw(Event *event)
> 	{
> 	  ...
> 	}
> };

Your version isn't portable either.  GTK+ callbacks required C linkage, so to 
be portable your code should provide C linkage, and static member functions 
cannot have C linkage.  This means that to be portable the callback has to be 
declared extern "C" in file scope (and if it needs access to the class's 
internals, needs to be declared a friend in the class definition).

Paragraph 7.5.1 of the standard says this:

"All function types, function names, and variable names have a language
linkage. The default language linkage of all function types, function names,
and variable names is C++ language linkage. Two function types with
different language linkages are distinct types even if they are otherwise
identical."

Static member functions happen to work with g++.  They may not work with other 
compilers.

Chris