Why return const Rational rather than Rational

Question

I saw the following implementation of the operator* as follows:

class Rational {
public: 
       Rational(int numerator=0, int denominator=1);
       ...
private:
       int n, d; // numerator and denominator
       friend const Rational operator*(const Rational& lhs, const Rational& rhs)
      { 
          return Rational(lhs.n * rhs.n, lhs.d * rhs.d); 
      }    
};

I have two questions here:

• Q1> why the operator* has to return const Rational rather than simply Rational
• Q2> when we define a friend function, should we care about the access modifier?

Answer 1

1. So that you can't do something like Rational a, b, c; (a * b) = c;.

2. No.

Answer 2

Note that returning const Rational instead of Rational not only prevents nonsensical assignments but also move semantics (because Rational&& does not bind to const Rational) and is thus not recommended practice anymore in C++0x.

Scott Meyers wrote a note on this matter:

Declaring by-value function return values const will prevent their being bound to rvalue references in C++0x. Because rvalue references are designed to help improve the efficiency of C++ code, it's important to take into account the interaction of const return values and the initialization of rvalue references when specifying function signatures.