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
definedunary operator),just as in normal text. If the tokendefinedis generated as a result of this replacement process or use of thedefinedunary operator does not match one of the two specified forms prior to macro replacement,the behavior is undefined.
