Windows 64 bit or 32 bit preprocessor directives

[OReubens]

but in case you have an actual code path that depends on 64 vs 32bit...

_M_X64 is defined for 64bit builds
_M_IX86 is defined for 32bit builds (the actual value will depend on what processor you're targetting)
_WIN64 is defined for Win64 builds
_WIN32 is defined for Win32 builds

the above is for Visual C++. Other compiler will have different macro's.