C++ #if#elif#endif 似乎不起作用

Question

有人可以告诉我我做错了什么吗？

#include <iostream>
using namespace std;

int main() {

#define myvar B

#if myvar == A
        cout << "A" << endl;
#elif myvar == B
        cout << "B" << endl;
#else
        cout << "Neither" << endl;
#endif
}

输出是 A 但显然我期待 B

Answer 1

这个:

#if myvar == A

扩展为:

#if B == A

引用C++标准:

After all replacements due to macro expansion and the defined unary operator have been performed, all remaining identifiers and keywords, except for true and false, are replaced with the pp-number 0, and then each preprocessing token is converted into a token.

所以这等同于:

#if 0 == 0

这当然是真的。

预处理器表达式中的 == 运算符不比较字符串或标识符，它只比较整数表达式。您可以通过为 A、B 和 myvar 定义整数值来完成您想做的事情，例如:

#define A 1
#define B 2
#define myvar B

#if myvar == A
    cout << "A" << endl;
#elif myvar == B
    cout << "B" << endl;
#else
    cout << "Neither" << endl;
#endif