what's the "#define XXX" 's value?

Question

what's the "#define XXX" 's value?

it has no value but it seems no compile error .normally the define is define type replace, but it is define type wiouht replace str.

Answer 1

It'll replace the replacement text by nothing:

#define FOO

int FOO main() FOO
{
}

Moreover, #ifdef FOO will succeed.

---

Empty defines can be quite useful, for example in this (naive) functional form:

#ifndef NDEBUG
#  include <cstdlib>
#  define MakeSureThat(X) if (!(X)) { std::abort(); }
#else
#  define MakeSureThat(X)
#endif

Usage:

void do_stuff(Foo * p)
{
    MakeSureThat(p != nullptr);       // won't generate any code if NDEBUG
}