c++ - 可以为嵌套的私有(private)类重载 operator<< 吗？

Question

如何为像这样的嵌套私有(private)类重载运算符<<？

class outer {
  private:
    class nested {
       friend ostream& operator<<(ostream& os, const nested& a);
    };
  // ...
};

在外部类编译器之外尝试时会提示隐私问题:

error: ‘class outer::nested’ is private

Answer 1

你可以制作 operator<< outer的 friend 以及。或者您 可以完全实现它inline在nested ，例如:

class Outer
{
    class Inner
    {
        friend std::ostream& 
        operator<<( std::ostream& dest, Inner const& obj )
        {
            obj.print( dest );
            return dest;
        }
        //  ...
        //  don't forget to define print (which needn't be inline)
    };
    //  ...
};