Re: gdk_input_add() with class member as callback problem

[Eelco Schijf <eelco schijf org>]

Thank you! That solved the problem and everything works like a charm. And 
you're right I'm not using the user data.

Also thanks for the link. I've found it a nice to read into understanding how 
this stuff works.

On Wednesday 16 March 2005 13:21, Melvin Hadasht wrote:
> Eelco Schijf a écrit :
> > Hi,
> >
> > I'm stuck with the gdk_input_add() function and could use a hint to solve
> > the problem. So here goes;
> >
> > The callback function I want to call is a member of a class, but g++
> > won't compile this. Placing the function outside the class works, but
> > this is not the solution I'm looking for.
> >
> > The code:
> >
> > InputLirc :: InputLirc()
> > {
> >  ...
> >
> >  tag = gdk_input_add( sock,
> >      GDK_INPUT_READ,
> >      &InputLirc::ProcessLircInput,
> >      NULL );
> >
> >  ...
> > }
> >
> > void InputLirc :: ProcessLircInput( gpointer data, gint source,
> > GdkInputCondition condition )
> > {
> >  ...
> > }
>
> In C++, pointer to (class) methods are not equal to pointer to functions.
>
> Class methods has an invisible additional argument which is the pointer
> to the instance. Thus, the "signature" of a method is not the same as
> the "signature" of a function (i.e: void (InputLirc::*)(...) is not
> equal to void(*)(...)). So, when calling a method of an object, the
> compiler needs two informations: the object and the method.
>
> gdk_input_add expects a pointer to a function and a user data.
> So you must create a dispatch function that calls the object method. It
> looks from your code that you don't need the user data, so you can use
> it to pass the instance of your object. The dispatch function can also
> be a static method of your class, as static methods are handled as
> normal functions (they do not apply to instances of the class, so they
> do not use the invisible argument described previously). So one solution
> would be:
>
> class InputLirc
> {
> public:
> static void DispatchProcessLircInput (gpointer aInputLirc, gint source,
> GdkInputCondition condition)
>   {
>  InputLirc *obj = static_cast<InputLirc*>(aInputLirc);
>  return obj->ProcessLircInput (NULL, source, condition);
> }
>
> InputLirc :: InputLirc()
> {
>   ...
>
>   tag = gdk_input_add( sock,
>       GDK_INPUT_READ,
>       DispatchProcessLircInput,
>       this );
>
>   ...
> }
>
>
> See http://www.newty.de/fpt/functor.html for more information.