C++ 奇怪的行为 Visual Studio

Question

此代码为 Visual Studio 2012 和 2008 输出 T2、T4，为 gcc 输出 T1、T2、T3、T4。这是什么原因？

#include <iostream>

#define ABC

#define T1 defined(ABC)

#define T2 defined( ABC )

#define T3 defined(ABC )

#define T4 defined( ABC)


int main(int argc, char* argv[])
{

#if T1

    std::cout<<"T1"<<std::endl;
#endif


#if T2

    std::cout<<"T2"<<std::endl;
#endif


#if T3

    std::cout<<"T3"<<std::endl;
#endif


#if T4

    std::cout<<"T4"<<std::endl;
#endif

    return 0;
}

Answer 1

查看conditional directives页。我发现:

The defined directive can be used in an #if and an #elif directive, but nowhere else.

将您的代码更改为:

#include <iostream>

#define ABC

int main(int argc, char* argv[])
{

#if defined(ABC)
    std::cout << "T1" << std::endl;
#endif


#if defined( ABC )
    std::cout << "T2" << std::endl;
#endif


#if defined(ABC )
    std::cout << "T3" << std::endl;
#endif


#if defined( ABC)
    std::cout << "T4" << std::endl;
#endif

    return 0;
}

将在 VS 2013 中生成 T1,T2,T3,T4 输出