C ++模板和朋友，链接器错误 [英] C++ templates and friends, linker error

问题描述

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运算符<<（ class std :: basic_ostream< char，struct std :: char_traits< char>& ;,类cClass< int> const&）（?? 6 @ YAAEAV？$ basic_ostream @ DU？$ char_traits @ D @ std @ @@ std @@ AEAV01 @ AEBV？$ cClass @ H @@@ Z））

Why does the following code not compile (linker error, unresolved external symbol, 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);
}

推荐答案

此行：

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);

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