我决定亲自尝试一下Substitution Failure Is Not A Error (SFINAE) 代码,以测试是否operator<<
为自定义类型定义了全局。
Stack Overflow 问题SFINAE + sizeof = detect if expression compiles already address testing for operator <<
through SFINAE,但我的代码略有不同,产生了令人费解的结果。
具体来说,如果我尝试在SFINAE 模板代码之后operator<<
为我的自定义类型(结构 A)定义,我下面的测试代码甚至不会编译——但是,据我了解,它应该可以正常工作,因为它是在 test_ostr 的任何实际实例化之前定义的班级。test_ostr
OTOH,如果我为一个甚至没有实例化或定义的不同类定义 a ,它将编译。但是,然后代码无法正确找到.operator<<
test_ostr
operator<<
此代码在 GCC 4.4.3 中编译和运行:
//#define BUG 1 // Uncomment and the program will not compile in GCC 4.4.3
//#define BUG 2 // Uncomment and the program will compile, but produces an incorrect result, claiming operator<< is not defined for A.
#include <iostream>
struct A{};
struct B{};
// If BUG is #defined, the operator<< for struct A will be defined AFTER the test_ostr code
// and if BUG <=1, then GCC 4.4.3 will not compile with the error:
// sfinae_bug.cpp:28: error: template argument 2 is invalid
#ifdef BUG
// if BUG > 1, defining the opertor << for *C*, an un-defined type, will make GCC magically compile!?
# if BUG > 1
struct C;
std::ostream& operator<<(std::ostream&, const C&);
# endif
#endif
#ifndef BUG
std::ostream& operator<<(std::ostream& ostr, const A&) { return ostr; };
#endif
template<class T>
struct test_ostr
{
template <class U, std::ostream& (*)(std::ostream&, const U&) >
struct ostrfn;
template<class U>
static short sfinae(ostrfn<U, &operator<< >*);
template<class U>
static char sfinae(...);
enum { VALUE = sizeof(sfinae<T>(0)) - 1 };
};
#ifdef BUG
std::ostream& operator<<(std::ostream& ostr, const A&) { return ostr; };
#endif
int main(void)
{
std::cout << "std::ostream defined for A: " << int(test_ostr<A>::VALUE) << std::endl;
std::cout << "std::ostream defined for B: " << int(test_ostr<B>::VALUE) << std::endl;
return 0;
}
显示错误的输出:
>c++ sfinae_bug.cpp && ./a.out
std::ostream defined for A: 1
std::ostream defined for B: 0
>c++ -DBUG sfinae_bug.cpp && ./a.out
sfinae_bug.cpp:28: error: template argument 2 is invalid
>c++ -DBUG=2 sfinae_bug.cpp && ./a.out
std::ostream defined for A: 0
std::ostream defined for B: 0
这些是编译器错误吗?我错过了什么吗?不同编译器的结果是否不同?