Calling a C++ function pointer on a specific object instance

Question

I have a function pointer defined by:

typedef void (*EventFunction)(int nEvent);

Is there a way to handle that function with a specific ins

Answer 1

You may find C++ FAQ by Marshall Cline helpful to what you're trying to accomplish.

Answer 2

I have a set of classes for this exact thing that I use in my c++ framework.

http://code.google.com/p/kgui/source/browse/trunk/kgui.h

How I handle it is each class function that can be used as a callback needs a static function that binds the object type to it. I have a set of macros that do it automatically. It makes a static function with the same name except with a "CB_" prefix and an extra first parameter which is the class object pointer.

Checkout the Class types kGUICallBack and various template versions thereof for handling different parameters combinations.

#define CALLBACKGLUE(classname , func) static void CB_ ## func(void *obj) {static_cast< classname *>(obj)->func();}
#define CALLBACKGLUEPTR(classname , func, type) static void CB_ ## func(void *obj,type *name) {static_cast< classname *>(obj)->func(name);}
#define CALLBACKGLUEPTRPTR(classname , func, type,type2) static void CB_ ## func(void *obj,type *name,type2 *name2) {static_cast< classname *>(obj)->func(name,name2);}
#define CALLBACKGLUEPTRPTRPTR(classname , func, type,type2,type3) static void CB_ ## func(void *obj,type *name,type2 *name2,type3 *name3) {static_cast< classname *>(obj)->func(name,name2,name3);}
#define CALLBACKGLUEVAL(classname , func, type) static void CB_ ## func(void *obj,type val) {static_cast< classname *>(obj)->func(val);}

Answer 3

Unfortunately, the EventFunction type cannot point to a function of B, because it is not the correct type. You could make it the correct type, but that probably isn't really the solution you want:

typedef void (*B::EventFunction)(int nEvent);

... and then everything works once you call the callback with an obhect of B. But you probably want to be able to call functions outside of B, in other classes that do other things. That is sort of the point of a callback. But now this type points to something definitely in B. More attractive solutions are:

• Make B a base class, then override a virtual function for each other class that might be called. A then stores a pointer to B instead of a function pointer. Much cleaner.
• If you don't want to bind the function to a specific class type, even a base class (and I wouldn't blame you), then I suggest you make the function that gets called a static function: "static void EventFrom A(int nEvent);". Then you can call it directly, without an object of B. But you probably want it to call a specific instance of B (unless B is a singleton).
• So if you want to be able to call a specific instance of B, but be able to call non-B's, too, then you need to pass something else to your callback function so that the callback function can call the right object. Make your function a static, as above, and add a void* parameter which you will make a pointer to B.

In practice you see two solutions to this problem: ad hoc systems where you pass a void* and the event, and hierarchies with virtual functions in a base class, like windowing systems

Answer 4

I highly recommend Don Clugston's excellent FastDelegate library. It provides all the things you'd expect of a real delegate and compiles down to a few ASM instructions in most cases. The accompanying article is a good read on member function pointers as well.

http://www.codeproject.com/KB/cpp/FastDelegate.aspx