No operator found which takes a right-hand operand of type 'B' (or there is no acceptable conversion)

Question

I have the following piece of code. I'm trying to print the sum of two objects ints but the compiler gives me the following error:

binary '<<' : No operator found which takes a right-hand operand of type 'B' (or there is no acceptable conversion)

I don't really understand what the error means. Why does it say the operator << needs a type 'B'. Isn't there a sum of two ints?

#include <iostream>
using namespace std;

class A
{
    protected:
        int x;

    public:
        A(int i) :x(i) {}
        int get_x() {
            return x;
        }
};

class B : public A
{
    public:
        B(int i) :A(i) {}
        B operator+(const B& b) const {
            return x + b.x;
        }
};

int main()
{
    const B a(22), b(-12);
    cout << a + b;
    system("Pause");
}

Answer 1

The a + b expression is using your custom operator - so it's as if you'd written (module constness - I'm just trying to get the flavour of what's going on):

B c = a + b;
cout << c;

That doesn't work because the compiler can't find a suitable << operator with a B as the right operand - just as the error message says. It's worth asking yourself what you expected it to do with that.

If you want to print the value of x in the result, perhaps you want:

cout << (a + b).get_x();

Answer 2

Just overload the << operator:

std::ostream& operator<<(std::ostream& out, const B& b)
{
    return out << b.x;
}

Answer 3

Add an operator << definition to your B class. Something like

class B : public A {
    public: B(int i) : A(i) {}

    B operator +(const B & b) const { return x + b.x; }

    friend std::ostream & operator <<(std::ostream & out, const B & obj) {
        out << obj.x;
        return out;
    }
};

Answer 4

Can be done like this:

#include<iostream>
using namespace std;

class A
{
        protected: int x;
        public: A(int i) :x(i) {}
                int get_x() { return x; }
};

class B : public A
{
public: B(int i) :A(i) {}
        B operator+(const B& b) const { return x + b.x; }
};

ostream & operator << (ostream &object,B &data)
{
    object << data.get_x();
    return object;
}

int main()
{
    const B a(22), b(-12);
    cout << (a + b);
    system("Pause");
}