Olivier Langlois's blog

Suppose we have

class A
{
  private:
  class B
  {
  };
};

and we would like to define operator<< for class B. I had this situation yesterday and it took me few tries to make it work. I first tried:

class A
{
...
};
std::ostream operator<<( ostream &, const A::B & );

It did not work because the compiler complained that B was private. My second attempt:

class A
{
  private:
  class B
  {
  };
  static std::ostream &operator<<( ostream &, const B & );
};

I was getting closer to the solution but this was still not quite right. Apparently, you do not have the right to declare operator<< as a static member of another class or as soon as an operator is defined as a class member, automatically, the compiler expect the left operand to be of this class type.

Then I tried this:

class A
{
  private:
  class B
  {
  };
  friend std::ostream &operator<<( ostream &, const B & );
};
std::ostream &operator<<( ostream &o, const A::B &b )
{
}

This time, I am not sure why but the linker now was complaining about duplicate symbols for my operator<< function. Not sure if it is the code that has a problem. It looks good to me. I suspect that maybe it is parameter type mangling problem and the compiler does not recognize 'const B &' as the same as 'const A::B &' but anyhow, I did not pursue the investigation I have finally made the code work like this:

class A
{
  private:
  class B
  {
  };
  friend std::ostream &operator<<( ostream &o, const B &b )
  {
  }
};