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c++classc++11operator-overloadingfriend-function

Why can not the operator<< for test2 + test3 be overloaded?

I try to overload the operator<<, but there is a warning that I cannot overload it. I can overload this operator as follows:

std::cout << test4 << std::endl;

But I can not overload it as follows:

std::cout << test2 + test3 << std::endl;

My main code is:

Stonewt test2(2, 8, Stonewt::STONE);
std::cout << "test2: " << test2 << std::endl;

Stonewt test3(2, 3, Stonewt::POUNDS);

std::cout << "test3: " << test3 << std::endl;

Stonewt test4 = test2 + test3;
std::cout << test4 << std::endl;         // operator << can overload
std::cout << test2 + test3 << std::endl; // operator << cannot overload

Below is the friend function

std::ostream& operator <<(std::ostream& os, Stonewt& a)
{
    if (a.format == Stonewt::STONE)
    {
        os << "stone format" << '\n';
        os << a.stone << " stones " << a.pound << " pounds" << '\n';
    }
    else if (a.format == Stonewt::POUNDS)
    {
        os << "pounds format" << '\n';
        os << a.pounds << " pounds" << '\n';
    }
    else
        os << "not valdid" << '\n';
    return os;
}
Solution

  • The test2+test3 results a temporary Stonewt object(rvalue) which can not be bound to a non-const reference(lvalue: i.e. Stonewt &a),rather with a const qualified lvalue reference. Therefore change the non-member function to:

    std::ostream & operator <<(std::ostream &os, const Stonewt &a)
//                                           ^^^^^^^^^^^^^^^^
{
      // ....
      return os;
}

    Further readings: