<< operator overloading for template class object

Question

I have written a C++ STL like bitset class:

template<size_t N>
class bitset {
public:
 ...........
 friend std::ostream& operator << (std::ostream &, bitset<N> const&);
private:
 ..........
};
// end of class

template<size_t N>
std::ostream& operator << (std::ostream &os, bitset<N> const& rhs) {
    ............
    .........
    return os;
}

And I am trying to use it like this:

bitset<5> foo; // success
std::cout << foo << std::endl; // fail

And the error message is -

undefined reference to `operator<<(std::ostream&, bitSet<5u> const&)

What's the problem actually?

Answer 1

Your friend's declaration must be a template as well, just like the definition is:

template <size_t N>
class bitset {
public:
    template <size_t M>
    friend std::ostream& operator << (std::ostream &, bitset<M> const&);
};

template <size_t M>
std::ostream& operator << (std::ostream &os, bitset<M> const& rhs) {
    return os;
}

---

Alternatively, you could declare the operator<< directly within the class scope:

template<size_t N>
class bitset {
public:
    friend std::ostream& operator << (std::ostream & os, bitset const&) {
        return os;
    }
};