Why the << operator is friend

#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;
}

Source: stackoverflow.com

Similar codes

1 vote203 views1 answer
What is time complexity of min_element()
C++
0 vote241 views1 answer
C++ program to convert fahrenheit to celsius
C++
4 votes200 views2 answers
Check uppercase c++
C++
0 vote224 views1 answer
Parenthesis Checker using stack in c++
C++
1 vote271 views1 answer
How to compile opencv c++ in ubuntu
C++
-1 vote233 views1 answer
Arrays left rotation hackerrank solution
C++
1 vote220 views2 answers
C++ function as paramter
C++
0 vote195 views1 answer
Can you use rand to read in from an external file inc++
C++
3 votes215 views2 answers
Search in vector of pairs c++
C++
2 votes251 views2 answers
Resize 2d vector c++
C++