如果指令宏比较 [英] If-directive macro comparison

问题描述

为什么满足以下代码中的 #if 条件:

Why is the #if condition in the following code fulfilled:

#include <iostream>
#define VALUE foo    

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

推荐答案

第页面上cppreference.com 状态:

所有宏扩展后，对defined和__has_include进行评估(自C ++ 17起)表达式，不是布尔值的任何标识符文字被替换为数字``0''(包括标识符是词汇上的关键字，但不是替代标记，如and).

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).

因此，先将 VALUE 替换为 foo ，然后将 foo 和 bar 都替换为0.

So VALUE is first replaced with foo, and then both foo and bar are replaced with 0.

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