Why not calling window procedure instead of calling CallWindowProc?

Why is ther开发者_运维问答e 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?

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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]

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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