c – MSVC未正确评估定义(VARIABLE)？

发布时间：2020-12-16 04:58:26 所属栏目：百科来源：网络整理

导读：运行以下代码 #include iostream#define FOO#define BAR defined(FOO)int main() {#if BAR std::cout "BAR enabled!" std::endl;#else std::cout "BAR disabled!" std::endl;#endif return 0;} 在Visual Studio中显示Bar disabled！,在gcc或clang中运行相同

运行以下代码

#include <iostream>

#define FOO
#define BAR defined(FOO)

int main() {
#if BAR
    std::cout << "BAR enabled!" << std::endl;
#else
    std::cout << "BAR disabled!" << std::endl;
#endif
    return 0;
}

在Visual Studio中显示Bar disabled！,在gcc或clang中运行相同的代码时显示Bar enabled！.

这是Microsoft编译器中的错误吗？根据标准,什么是正确的？

解决方法

根据标准,这是未定义的行为.

[cpp.cond],强调我的

Prior to evaluation,macro invocations in the list of preprocessing tokens that will become the controlling constant expression are replaced (except for those macro names modified by the defined unary operator),just as in normal text. If the token defined is generated as a result of this replacement process or use of the defined unary operator does not match one of the two specified forms prior to macro replacement,the behavior is undefined.

（编辑：李大同）

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