cout a class object using conversion operator

Question

#include <iostream>
#include <cmath>

using namespace std;

class Complex
{
private:
    double real;
    double imag;

public:
    // Default constructor
    Complex(double r = 0.0, double i = 0.0) : real(r), imag(i)
    {}

    // magnitude : usual function style
    double mag()
    {
        return getMag();
    }

    // magnitude : conversion operator
    operator int ()
    {
        return getMag();
    }

private:
    // class helper to get magnitude
    double getMag()
    {
        return sqrt(real * real + imag * imag);
    }
};

int main()
{
    // a Complex object
    Complex com(3.0, 4.0);

    // print magnitude
    cout << com.mag() << endl;
    // same can be done like this
    cout << com << endl;
}

I don't understand how the compiler is resolving to call the conversion operator for cout << com << endl;.

I can also have more than one conversion operator in the same class. How will the resolution be done in that case?

Answer 1

You have declared a conversion operator to int. Since this operator is not explicit, the compiler considers it when finding the best overload of ostream::operator<<. Keep in mind that C++ compiler always attempts to automatically convert types to find a matching call, including conversion constructors, conversion operators and implicit type conversions.

If you do not want this behavior, then starting from C++11 you can mark the operator as explicit:

explicit operator int() {
     return getMag();
}

Regarding the second part of your question, if there are multiple conversions that are equally good, a compile error is invoked. If you added, say, operator double, then as there exists ostream::operator(double), the call would be ambiguous and you would need to cast com to Your desired type.