Compilation error SetWindowPos on 64-bit Windows

Question

I am currently modifying my code base to compile it under 64-bit architecture. Most of the changes I need to make are obvious, but it puzzled me. SetWindowPos has a second argument, hWndInsertAfter , which can be either a window handle or one of the predefined values HWND_TOP , HWND_BOTTOM , HWND_TOPMOST and HWND_NOTOPMOST (see here for MSDN information). These values ​​are defined in WinUser.h .

In a 32-bit architecture, using one of them in a call to SetWindowPos works fine, but in a 64-bit compiler it complains in this way:

warning C4306: 'type cast': conversion from 'int' to 'HWND' of larger size

This is because #defines distinguishes integers [32 bits] as HWNDs, for example:

#define HWND_TOPMOST ((HWND)-1)

What do I need to change to make this compilation in 64-bit architecture without a compiler throwing a warning? I can turn off warnings with #pragma warning( disable: 4306 ) or make my own definition using a 64-bit int in #define, but of course, is there a “right” way for Microsoft to do this?

Answer 1

Well, after testing MUCH, the problem was that my file was a .c file. I renamed it to .cpp and SetWindowPos, and then compiled without errors (and, conversely, in the new test application that I created to try bare bones, when I renamed the default .cpp file to a .c file, it started to complain).

It seems that the .c files do not want to use 32-bit int values ​​for 64-bit pointers. This makes sense, but does not explain why it works in .cpp files. If anyone has any idea why this is so, note this in the comments ...

Answer 2

The warning is triggered because you pass the 32-bit value of int -1 to the 64-bit pointer type void* without any intermediate cast to 64-bit integer type, for example intptr_t . MSVC should have suppressed the warning in this case, because (A) it was only caused by the extension of the macro provided by the system HWND_TOPMOST and (B), the decimal literal was insulting to the int , but MSVC developers apparently did not think of heuristics.

There is nothing to do in the code to turn off the warning if you are not happy

 #undef HWND_TOPMOST #define HWND_TOPMOST ((HWND)(intptr_t)-1) 

Alternatively, you can try to suppress it in the IDE. This thread offers

Project Settings | C / C ++ | General and disable "Detecting 64-bit portability problems"

or pass /wd4306 on the command line.