c++ - 如何为内部类声明运算符<<

Question

//cannot declare operator<<(...) here:
//forward declarations:
class External;

template<class T, class Y>
class External::Internal;

template<class T, class Y>
std::ostream& operator<<(std::ostream& out, const External::Internal<T,Y>&);

class External
{
    template<class T, class Y>
    class Internal
    {};

    Internal data_;

    void print() {
        /*out is a std::ostream*/
        out << data_;
    }
};

template<class T, class Y>
std::ostream& operator<<(std::ostream& out, const External::Internal<T,Y>&)
{ }

我确实想实现 operator<<对于 Internal但是当我尝试使用来自 External 的运算符(operator)调用时出现问题: 在此类定义下声明此运算符时看不到此运算符，并且似乎无法在此类定义之上声明此运算符。

Answer 1

如果你问如何定义 Internal<>::operator<<作为 friend ，那么:

class External
{
  template<class T, class Y>
  class Internal
  {
    friend std::ostream& operator <<(std::ostream& out, const Internal&)
    {
      // impl
      return out;
    }
  };

  Internal<Foo, Bar> data_;

public:
  void print() const
  {
    /*out is a std::ostream*/
    out << data_;
  }
};