SetWindowPos compile error in 64-bit Windows

Question

I'm currently changing our codebase to make it compile under 64-bit architecture. Most of the changes I'm having to make are obvious, but this one has got me stumped. SetWindowPos has a second argument, hWndInsertAfter, that 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 info). These values are defined in WinUser.h.

In 32-bit architecture, using one of those in a call to SetWindowPos works fine, but in 64-bit, the compiler complains thus:

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

This is because the #defines are casting [32-bit] integers as HWNDs, e.g.:

#define HWND_TOPMOST ((HWND)-1)

What do I need to change to make this compile in 64-bit architecture without the compiler throwing a warning? I can disable the warnings using #pragma warning( disable: 4306 ), or make my own definition using a 64-bit int in the #define, but surely there's a "proper" Microsoft way of doing this?

Answer 1

The warning is triggered because you're casting the 32-bit int value -1 to a 64-bit pointer type void* without any intervening cast to a 64-bit integer type such as intptr_t. MSVC should have suppressed the warning in this case since (A) it's triggered only by the expansion of the system-provided macro HWND_TOPMOST and (B) the offending int is a decimal literal, but apparently MSVC's developers didn't think of those heuristics.

There's nothing you can do in your code to silence the warning, unless you're happy with

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

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

Project Settings | C/C++ | General and turn off "Detect 64-bit portability issues"

or pass /wd4306 on the command line.

Answer 2

Ok, after MUCH testing, the problem was that my file was a .c file. I renamed it to .cpp and SetWindowPos then compiled without error (and conversely, in the new test app I created to try a 'bare bones' solution, when I renamed the default .cpp file to a .c file, it started complaining).

Looks like .c files don't want to be able to cast 32-bit int values to 64-bit pointers. Which makes sense, but doesn't explain why it works in .cpp files. If anyone has any ideas on why this is, do note it in the comments...