c++ - << 模板类对象的运算符重载

Question

我写了一个像bitset类的C++ STL:

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;
}

我正在尝试这样使用它:

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

错误信息是——

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

究竟是什么问题？

Answer 1

你 friend 的声明也必须是模板，就像定义一样:

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;
}

---

或者，您可以声明 operator<<直接在类范围内:

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