Question

Possible Duplicate:
overloading friend operator<< for template class

I’m trying to overload operator<< for a template class but I’m getting errors…

---

Final(fixed) code:

template<class T>
class mytype
{
    T atr;
public:
    mytype();
    mytype(T);
    mytype(mytype&);
    T getAtr() const;
    T& operator=(const T&);
    template<class U> friend ostream& operator<<(ostream&,const mytype<U>&);
};

template<class T>
mytype<T>::mytype()
{
    atr=0;
}

template<class T>
mytype<T>::mytype(T value)
{
    atr=value;
}

template<class T>
mytype<T>::mytype(mytype& obj)
{
    atr=obj.getAtr();
}


template<class T>
T mytype<T>::getAtr() const
{
    return atr;
}

template<class T>
T& mytype<T>::operator=(const T &other)
{
    atr=other.getAtr();
    return *this;
}

template<class U>
ostream& operator<<(ostream& out,const mytype<U> &obj)
{
    out<<obj.getAtr();
    return out;
}

(All in the header file)

---

VS2012 errors:

1)

Error 1 error LNK2019: unresolved external symbol “public: __thiscall mytype::mytype(int)” (??0?$mytype@H@@QAE@H@Z) referenced in function _wmain

2)

Error 2 error LNK2019: unresolved external symbol “class std::basic_ostream > & __cdecl operator<<(class std::basic_ostream > &,class mytype const &)” (??6@YAAAV?$basic_ostream@DU?$char_traits@D@std@@@std@@AAV01@ABV?$mytype@H@@@Z) referenced in function _wmain

3)

Error 3 error LNK1120: 2 unresolved externals

---

What’s wrong with my code?

Answer 1

You told the compiler to expect a free non-template function:

friend ostream& operator<<(ostream&,const mytype<T>&);

…but then you defined a function template instead:

template<class T>
ostream& operator<<(ostream& out,const mytype<T> &obj)
{
    out<<obj.getAtr();
    return out;
}

Tell the compiler instead to expect a function template:

template<class T> friend ostream& operator<<(ostream&,const mytype<T>&);