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

Question

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?

Answer 1

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.