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I understand that sizeof is an operator, which is evaluated at compile time to an integer constant.
But it seem it can not be used in the #if preprocessor directive like:
#if 4 == sizeof(int)
typedef int Int32;
#endif
(cygwin-gcc 3.4.4 as well as Visual C++ 6.0 report compile errors)
Why is such usage not allowed?
Because
sizeofis evaluated at compilation time while directives are evaluated before compilation, and the part that does that is not the compiler, so it won’t understand whatsizeofmeans.