Define friend template method outside namespace

Question

I have code with the following basic structure:

namespace A{

    template<class T,unsigned DIM>
    class CMyTable{
        ...
            public:
            template<class T,unsigned DIM>
            friend std::ostream& operator<<(std::ostream& s, const CMyTable<T,DIM>& vec);
        }    

    };

}

The initial problem was to get my operator<< outside the namespace A.

I tried this solution : How do I define friends in global namespace within another C++ namespace?

namespace A{

    template<class T,unsigned DIM>
    class CMyTable;
}

template<class T,unsigned DIM>
std::ostream& operator<<(std::ostream& s, const CMyTable<T,DIM>& vec);

namespace A{

    template<class T,unsigned DIM>
    class CMyTable{
        ...
            public:
            template<class T,unsigned DIM>
            friend std::ostream& ::operator<<(std::ostream& s, const CMyTable<T,DIM>& vec);
        }    

    };

}

template<class T,unsigned DIM>
std::ostream& operator<<(std::ostream& s, const CMyTable<T,DIM>& vec){
// [...]
}

I got this error : error C2063: 'operator <<' : not a function inside the class declaration.

public:
template<class T,unsigned DIM>
friend std::ostream& ::operator<<(std::ostream& s, const CMyTable<T,DIM>& 

Does anyone have any ideas ?

Thanks.

Answer 1

If your ostream overload has to be a friend (needs access to protected members) then define it inline so that you can use the template arguments passed to the class.

namespace A{

    template<class T,unsigned DIM>
    class CMyTable{
        ...
        public:
        // template<class T,unsigned DIM>  // This will shadow otherwise
        friend std::ostream& operator<<(std::ostream& s, const CMyTable<T,DIM>& vec) {
        // [...]
        }
    };
}

---

Otherwise completely remove its declaration from the class and namesapce and just define it outside as a templated overload.

Remember it doesn't have to be friend if it doesn't need access to private or protected elements.

namespace A{
   template<class T,unsigned DIM>
   class CMyTable{
    ...
        public:
    };
}



template<class T,unsigned DIM>
std::ostream& operator<<(std::ostream& s, const A::CMyTable<T,DIM>& vec){
// [...]
}

---

The third option is to keep it friend but define it inside namespace A

namespace A{

    template<class T,unsigned DIM>
    class CMyTable{
        public:
            int i;

        template<class V,unsigned E>
        friend std::ostream& operator<<(std::ostream& s, const CMyTable<V,E>& vec);
    };  

};
//
//

namespace A{

    template<class T,unsigned DIM>
    std::ostream& operator<<(std::ostream& s, const CMyTable<T,DIM>& vec) {
        s << vec.i;
    }

}