问题
I have come across some example code that goes like this:
#ifdef WIN32
...
#elif WIN64
...
#endif
In an #ifdef block, is it actually legal to use #elif to mean #elif defined?
回答1:
No, it shouldn't be. That's not to say that some obscure C compiler wouldn't accept it as such, but it isn't part of the C standard.
Normally, for something like this you would use either #elifdef FOO (which I've never actually seen in production code) or #elif defined(FOO) (like you mentioned).
This code appears to be working in a odd way; it's rather first checking if WIN32 is defined, then checking if WIN64 is nonzero.
来源：https://stackoverflow.com/questions/25451376/ifdef-win32-elif-win64-endif