Why can't I use sizeof in a preprocessor condition?

I understand that sizeof is an oper开发者_如何学运维ator, 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 sizeof is 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 what sizeof means.

The sizeof is a C operator. You can't use C code in preprocessor directives. Preprocessor directives are evaluated before the compilation takes place.