c++ - if-指令宏比较

Question

为什么是#if满足以下代码中的条件:

#include <iostream>
#define VALUE foo    

int main() {    
#if VALUE == bar
    std::cout << "WORKS!" << std::endl;
#endif // VALUE
}

Answer 1

page on cppreference.com状态:

After all macro expansion and evaluation of defined and __has_include (since C++17) expressions, any identifier which is not a boolean literal is replaced with the number ​0​ (this includes identifiers that are lexically keywords, but not alternative tokens like and).

所以 VALUE首先替换为 foo ，然后都是 foo和 bar替换为 0。