C++ 模板和 friend ，链接器错误

Question

为什么以下代码无法编译(链接器错误，未解析的外部符号，class std::basic_ostream<char,struct std::char_traits<char> > & __cdecl operator<<(class std::basic_ostream<char,struct std::char_traits<char> > &,class cClass<int> const &)" (??6@YAAEAV?$basic_ostream@DU?$char_traits@D@std@@@std@@AEAV01@AEBV?$cClass@H@@@Z))

#include <iostream>

template <class Type> class Class{
    public:
        friend std::ostream& operator<<(std::ostream& Stream, const Class& Op);
};

template <class Type> std::ostream& operator<<(std::ostream& Stream, const Class<Type>& Op){
    return(Stream);
}

int main(){
    Class<int> A;

    std::cout << A;

    return(0);
}

Answer 1

这一行:

friend std::ostream& operator<<(std::ostream& Stream, const Class& Op);

应该是:

template <class T> friend std::ostream& operator<<(std::ostream& Stream, const Class<T>& Op);