How do I call SetWindowLong() in the 64-bit versions of Windows?

Question

In the header file WinUser.h, there is a part in which the constants of the second parameter of SetWindowLong() are defined.

// ...

#define GWL_WNDP         


        

Answer 1

Some of the window data values (the ones that refer to "pointer sized" objects like the window procedure, for example) need to be 64 bit in an x64 build. The old SetWindowLong() and GetWindowLong() functions are limited to DWORD sized (32 bit) values for backwards compatibility, and Microsoft have introduced new versions, SetWindowLongPtr() and GetWindowLongPtr() that allow you to work with pointer-sized values (32 bit in a 32 bit build, and 64 bit in a 64 bit build).

These days it is recommended that you always use SetWindowLongPtr() and the GWLP_xxx constants, whether you are building for 32 or 64 bit, but in a 64 bit build you need to use the new functions and so the defines are #undefined to cause build errors that force you to fix your code.