Question

Suppose I have a base class and two classes derived from it:

class Base
{
protected:
    double value;
public:
    virtual ~Base();

    Base(double value) : value(value) {}
    Base(const Base& B) { value=B.value; }

    Base operator+ (const Base& B) const { 
        return Base(value+B.value); 
    }

};

class final Derived1 : public Base {
public:
    Derived1(double value) : Base(value) {}
};

class final Derived2 : public Base {
public:
    Derived2(double value) : Base(value) {}
};

I want to accomplish the following:

int main(int argc, char *argv[])
{
    Derived1 a = Derived1(4.0);
    Derived2 b = Derived2(3.0);

    a+a; // this should return a Derived1 object
    b+b; // this should return a Derived2 object

    a+b; // this should FAIL AT COMPILE TIME

    return 0;
}

In other words, I want to guarantee that the inherited operator+ only operates on objects of the same type as the calling instance.

How do I do this cleanly? I found myself re-defining the operator for each class:

class final Derived1 : public Base {
    ...
    Derived1 operator+ (const Derived1& D1) const {
        return Derived1(value+D1.value);
    }
    ...
};

class final Derived2 : public Base {
    ...            
    Derived2 operator+ (const Derived2& D1) const {
        return Derived2(value+D1.value);
    }
    ...
};

But that’s just a pain. Moreover, it doesn’t seem like proper code re-use to me.

What is the proper technique to use here?

Answer 1

If you can make sure Derived1 and Derived2 are leaf classes (i.e. no other class can derive from them) you can do this with the curiously recurring template pattern:

template <typename T>
class BaseWithAddition : public Base {
    T operator+(T const& rhs) const {
        return T(value + rhs.value);
    }
};

class final Derived1 : public BaseWithAddition<Derived1> {
    // blah blah
};

class final Derived2 : public BaseWithAddition<Derived2> {
    // blah blah
};

(final is a C++11 feature that prevents further derivation.)

If you allow derivation from Derived1 and Derived2 then you get trouble:

class Derived3 : public Derived1 {};
Derived3 d3;
Derived1 d1;
Derived1& d3_disguised = d3;
d1 + d3_disguised; // oooops, this is allowed

There’s no way to prevent this at compile-time. And even if you want to allow it, it’s not easy to get decent semantics for this operation without multiple dispatch.