Why not calling window procedure instead of calling CallWindowProc?

Question

Why is there such function CallWindowProc? We are supplying the address of the window procedure, so isn't it better to call the function instead of calling another function which calls the function?

Answer 1

Because GetWindowLong (or GetWindowLongPtr) might return a value that is not a function pointer, which CallWindowProc can recognise and translate into a proper call. [1]

Answer 2

The CallWindowProc function handles Unicode-to-ANSI conversion. You cannot take advantage of this conversion if you call the window procedure directly.

http://msdn.microsoft.com/en-us/library/windows/desktop/ms633571(v=vs.85).aspx