问题
In the header file WinUser.h, there is a part in which the constants of the second parameter of SetWindowLong() are defined.
// ...
#define GWL_WNDPROC (-4)
#define GWL_HINSTANCE (-6)
#define GWL_HWNDPARENT (-8)
#define GWL_STYLE (-16)
#define GWL_EXSTYLE (-20)
#define GWL_USERDATA (-21)
#define GWL_ID (-12)
#ifdef _WIN64
#undef GWL_WNDPROC
#undef GWL_HINSTANCE
#undef GWL_HWNDPARENT
#undef GWL_USERDATA
#endif /* _WIN64 */
#define GWLP_WNDPROC (-4)
#define GWLP_HINSTANCE (-6)
#define GWLP_HWNDPARENT (-8)
#define GWLP_USERDATA (-21)
#define GWLP_ID (-12)
// ...
But they are right after undefined if _WIN64 is defined; and it is defined in my 64-bit system.
As you see, there is also a GWLP_*** set of constants, but they are not documented in the page of SetWindowLong().
Why are these constants undefined in x64 systems?
What is the alternative way of calling SetWindowLong() in x64 systems?
My system:
OS: Windows 7 Ultimate x64 SP1
IDE: Visual Studio 2012 Ultimate Update 3
回答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.
来源：https://stackoverflow.com/questions/18178628/how-do-i-call-setwindowlong-in-the-64-bit-versions-of-windows