Is #define X defined(Y) a valid C/C++ expression?

I read somewhere that

#define X defined(Y)

was an invalid expression, but it works (?)

Here an example

#define WIN_PLAT defined(_WIN32)

#if WIN_PLAT
#    undef  WIN_PLAT
#    define WIN_PLAT 1
#else
#    undef  WIN_PLAT
#    define WIN_PLAT 0
#endif

Thanks y’all.

Edit: fixed ‘duplicate definitions’ warning in the else branch.

>Solution :

Macros are text replacements, as long as you replace defined(Y) back into a preprocessor instruction it will work. It will however not work if you try to use it in C++ code:

cout << X; // error: undefined symbol defined