Should `operator

"Operator<<: Friend or Member Function? Considering the Type of Operation"

The selection between implementing operator<< as a friend or member function hinges on the type of operation being performed.

For comparison operators (==, !=, etc.), it's advisable to treat them as member functions within the class. As the class is inherently a friend of itself, it can effortlessly access and compare private members of its instances. Additionally, enabling auto conversion for one operand through free-standing functions might lead to unintended consequences; hence, the preference for member functions.

Conversely, when dealing with stream operators (<<, >>), they demand external functions as they necessitate access to a stream object outside the class's control. These functions can be either friends of the class or utilize a public method that handles streaming.

It's customary for stream operators to return references to stream objects, enabling seamless chaining of operations. Here's an example demonstrating this approach:

#include <iostream>

class Paragraph {
public:
    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
    {
        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;
}

By understanding the nature of each operation and adhering to appropriate implementation choices, you can effectively leverage the operator<< to manipulate objects as desired.

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