Re: [Plib-users] Callback

[Steve B. <sjb...@ai...>]

Arne Kreutzmann wrote:
> 
> Hi,
> 
> I'm lerning plib right now, and have one question.
> 
> button->setCallback ( widget_cb );
> //     "void widget_cb ( puObject *ob )"
> 
> I would like classes to handel the callback.
> I know I can do that if I use *static*, is there an another way ?

No - there is a real problem with the C++ language - this is not something
you can solve.

The bottom line is that C++ does not allow you to take the
address of a non-static member function...and there are good reasons
for this.

You end up doing this:

  * Write a short 'wrapper' function:

      void my_callback ( puObject *ob )
      {
        MyClass *my_class = (MyClass *)(ob->getUserData()) ;
        my_class -> memberFunction ( ob ) ;
      }

    (This could be a static member function of the class)

  * Put a pointer to the class to which the button belongs
    into the userdata field of the button.

      button -> setUserData ( this ) ;

  * Make the wrapper function be the callback:

      button -> setCallback ( my_callback ) ;

This is a pain - but it's the best you can do in C++.

----------------------------- Steve Baker -------------------------------
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