CODEWITHSUNDEEP
c++windowswinapi
Alexander Bily
Alexander Bily

Reputation: 975

WinAPI SetWindowLongPtr - Changing windowProc

On MSDN website, I found following:

Calling SetWindowLongPtr with the GWLP_WNDPROC index creates a subclass of the window class used to create the window. An application can subclass a system class, but should not subclass a window class created by another process. The SetWindowLongPtr function creates the window subclass by changing the window procedure associated with a particular window class, causing the system to call the new window procedure instead of the previous one. An application must pass any messages not processed by the new window procedure to the previous window procedure by calling CallWindowProc. This allows the application to create a chain of window procedures.

Does this mean, that every time I call SetWindowLongPtr with GWLP_WNDPROC, new subclass will be created, or is Windows smart enough to create subclass only once if same procedure is passed multiple times as parameter?

Upvotes: 3

Views: 1226

Answers (1)

Hans Passant
Hans Passant

Reputation: 942207

The term "subclass" is conceptual in the winapi. It was originally designed back in 1983 to target the C language. Which does not have any support for classes of course.

1983 was an important year in software development, that's when objected-oriented programming started to take flight. That's when Bjarne Stroustrup renamed his "C with Classes" to C++. And when Xerox released their Smalltalk-80 implementation to anybody who wanted a copy. Given Smalltalk's focus on message passing, and Xerox Parc's focus on creating a graphical user interface, it is likely to have been a strong inspiration to the Windows designers.

So while the winapi could not be purely objected-oriented, it certainly got a whiff of the technique. RegisterClass() function is most obvious, acts like a base class that gives any window "object" created from that class the same behavior. The window procedure specified with WNDCLASS.lpfnWndProc acts like a virtual method that can be overridden.

So a "subclass" of the class registered with RegisterClass() merely overrides the lpfnWndProc function. You do that with SetWindowLongPtr(), you have to bring the new function yourself. You do have to call the "base" function like you would commonly do when you override a virtual function in a C++ program.

Not the only way to do it. The SubWindowSubclass() function is a helper function to get it right. The DefSubclassProc() function helps you call the "base class". Recommended.

Upvotes: 9

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