c++ - Should operator<< be implemented as a friend or as a member function?

Question

That's basically the question, is there a "right" way to implement operator<< ? Reading this I can see that something like:

friend bool operator<<(obj const& lhs, obj const& rhs);

is preferred to something like

ostream& operator<<(obj const& rhs);

But I can't quite see why should I use one or the other.

My personal case is:

friend ostream & operator<<(ostream &os, const Paragraph& p) {
    return os << p.to_str();
}

But I could probably do:

ostream & operator<<(ostream &os) {
    return os << paragraph;
}

What rationale should I base this decision on?

Note:

 Paragraph::to_str = (return paragraph) 

where paragraph's a string.

Answer 1

The problem here is in your interpretation of the article you link.

Equality

This article is about somebody that is having problems correctly defining the bool relationship operators.

The operator:

• Equality == and !=
• Relationship < > <= >=

These operators should return a bool as they are comparing two objects of the same type. It is usually easiest to define these operators as part of the class. This is because a class is automatically a friend of itself so objects of type Paragraph can examine each other (even each others private members).

There is an argument for making these free standing functions as this lets auto conversion convert both sides if they are not the same type, while member functions only allow the rhs to be auto converted. I find this a paper man argument as you don't really want auto conversion happening in the first place (usually). But if this is something you want (I don't recommend it) then making the comparators free standing can be advantageous.

Streaming

The stream operators:

• operator << output
• operator >> input

When you use these as stream operators (rather than binary shift) the first parameter is a stream. Since you do not have access to the stream object (its not yours to modify) these can not be member operators they have to be external to the class. Thus they must either be friends of the class or have access to a public method that will do the streaming for you.

It is also traditional for these objects to return a reference to a stream object so you can chain stream operations together.

#include <iostream>

class Paragraph
{
    public:
        explicit Paragraph(std::string const& init)
            :m_para(init)
        {}

        std::string const&  to_str() const
        {
            return m_para;
        }

        bool operator==(Paragraph const& rhs) const
        {
            return m_para == rhs.m_para;
        }
        bool operator!=(Paragraph const& rhs) const
        {
            // Define != operator in terms of the == operator
            return !(this->operator==(rhs));
        }
        bool operator<(Paragraph const& rhs) const
        {
            return  m_para < rhs.m_para;
        }
    private:
        friend std::ostream & operator<<(std::ostream &os, const Paragraph& p);
        std::string     m_para;
};

std::ostream & operator<<(std::ostream &os, const Paragraph& p)
{
    return os << p.to_str();
}


int main()
{
    Paragraph   p("Plop");
    Paragraph   q(p);

    std::cout << p << std::endl << (p == q) << std::endl;
}