Why do I get a compiler error when I try to use a member function as a callback function?

This is a pretty common problem that arises when you try to use a member function (one that's a non-static member of a class) as some sort of callback, like a thread or a window procedure. The problem is really a language issue as it arises from the fact that in C++, pointers to member functions are different from pointers to global functions. The reason: each non-static member function has to receive one extra parameter you don't get to see, but that is very important: the this pointer. Without it, the function wouldn't be able to access class members.

There are several workaround, the most common being declaring the function static. That means that to actually get access to other class members, you need to figure out a way to give the this pointer to the function. When it comes to Win32, there's usually an easy way, as most API's that require a callback provide an extra parameter that they pass directly when they call the callback function. Another way comes up when you're creating windows, as you could use the lParam parameter of the CreateWindow() api to pass the value, and then retrieve it when processing WM_CREATE. Here, a third way pops up: use the Window Extra bytes to store the pointer, which you can use by allocating enough space in the cbWndExtra member of the WNDCLASS[EX] structure, SetWindowLong() and GetWindowLong().

References and samples:

• Tomas Restrepo's web page: "Class Members as Callbacks"
• Knowledge Base Article Q102352:  "Passing a Pointer to a Member function to the Win32 API"