<<运算符重载模板类对象

Question

我写过类似C++的STL bitset:

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;
}

我试图像这样使用它:

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

错误信息是 -

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

实际上有什么问题？

Answer 1

您朋友的声明也必须是模板,就像定义是:

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operator<<直接在类范围内声明:

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