C ++ ostream <<运算符

Question

I have a Class that deals with Music Albums. The artists and albums are strings . It also has a collection ( vector ) of tracks called contents . Each track has a title and a duration .

This is my ostream << :

    ostream& operator<<(ostream& ostr, const Album& a){
        ostr << "Album: "    << a.getAlbumTitle() << ", ";
        ostr << "Artist: "   << a.getArtistName() << ", ";
        ostr << "Contents: " << a.getContents()   << ". "; //error thrown here
        return ostr;
    }

The << next to the a.getContents() is underlined and says: "Error: no operator "<<" matches these operands.

What am I missing out or doing wrong? Can you not use vectors in this way? or maybe its something I'm missing from my Track class?

Answer 1

Assuming Album::getContents() returns std::vector<Track> , you need to provide

std::ostream& operator<<(std::ostream& o, const Track& t);

and

std::ostream& operator<<(std::ostream& o, const std::vector<Track>& v);

where the latter can use the former. For example:

struct Track
{
  int duration;
  std::string title;
};

std::ostream& operator<<(std::ostream& o, const Track& t)
{
  return o <<"Track[ " << t.title << ", " << t.duration << "]";
}

std::ostream& operator<<(std::ostream& o, const std::vector<Track>& v)
{
  for (const auto& t : v) {
    o << t << " ";
  }
  return o;
}

There's a C++03 demo here .

Answer 2

If Album::getContents() is about your vector and you just return the vector than ostream does not know how to write it, because there is no '<<' operator .

Just overload the '<<' operator for a vector and you are happy.