13. Assignment operators

• Should return *this (by reference), to allow for assignment chaining.

• Almost always needs to check for self assignment condition:

  
      Rational& Rational::operator=( Rational const& other )
      {
          if ( this == &other ) // beware of self assignment!
              return *this;
  
          m_n = other.m_n;
          m_d = other.m_d;
  
          return *this;
      }