C++ overload operator+, problem with copy constructor

Question

I'm learning C++ and I came across this problem while learning operator overloading:

I defined a complex class:

class Complex {
private:
    double real, imag;

public:
    Complex() = default;

    Complex(double real,double imag): real(real), imag(imag) {
        std::cout << "Double constructor" << std::endl;
    }

//    Complex(Complex &other) {
//        this->real = other.real;
//        this->imag = other.imag;
//        std::cout << "Copy constructor" << std::endl;
//    }

    Complex operator+(Complex &other) {
            return Complex(real+other.real, imag+other.imag);
    }

    ~Complex() {
        std::cout << "Desctuctor called: " << real << std::endl;
    }
};

With the copy constructor commented out, this code will work, but it won't otherwise. The error it gives is, on constructing a Complex object in the operator+ function, there's no appropriate constructor to call.

I wonder, why the compiler gives such error? Also, when I comment out the copy constructor, I guess the default copy constructor is called when I do something like

C3 = C1 + C2;

Is that correct?

I couldn't find anything useful to this on SO (or maybe I'm too dumb to see it through), any help is greatly appreciated!

what error are you getting? PLease paste in the exact error message — pm100, Oct 18 '18 at 23:50
`Complex operator+(Complex &other) {` should be `Complex operator+(const Complex &other) {` same for the copy constructor parameter. — πάντα ῥεῖ, Oct 18 '18 at 23:51
πάντα ῥεῖ Thanks for pointing out. And adding the `const` actually made the thing working, could you explain that a bit? — user3723199, Oct 19 '18 at 00:02

score 1 · Accepted Answer · answered Oct 19 '18 at 00:03

Your operator+ function returns a Complex by value, so it has to call the copy constructor.

Without your custom constructor, the compiler generates a default copy constructor, which works just fine (it performs a memberwise copy).

With your custom constructor, the compiler will not generate a default copy constructor. However, your custom copy constructor has an unusual type:

Complex(Complex &other)

requires an lvalue as input. It cannot be used to copy temporaries.

The compiler-generated copy constructor takes const Complex &other instead, which can bind to temporary values.

C++ overload operator+, problem with copy constructor

1 Answers1