CODEWITHSUNDEEP
c++polymorphismassignment-operator
Axel Williot
Axel Williot

Reputation: 505

Polymorphic assignment operator from base and derived class

I would like to be able to copy either a derived or a base object to a derived object, and I would like the correct operator to be chosen polymorphically depending on the type of the copied object.

This code does not work, I would like b1 = (A&)b2; to use B & operator= (B const &other) because b2 is a B, but it uses B & operator= (A const &other):

#include<iostream> 

using namespace std;

class A {
public:
    A & operator= (A const &other) {
        // Here copy A members...
        cout<<"A to A"<<endl;
        return *this;
    }
};

class B: public A {
public: 
    B & operator= (A const &other) {
        A::operator=(other); // Copy A members.
        cout<<"A to B"<<endl;
        return *this;
    }
    B & operator= (B const &other) {
        A::operator=(other); // Copy A members.
        // Here copy B members...
        cout<<"B to B"<<endl; 
        return *this;
    }
};

int main() 
{
    B b1, b2;
    A a2;
    b1 = b2;
    cout<<endl; 
    b1 = (A&)b2;
    cout<<endl; 
    b1 = a2;
    cout<<endl; 
    return 0; 
}

I guess I have to make something virtual but I don't find how.

Upvotes: 1

Views: 513

Answers (1)

R Sahu
R Sahu

Reputation: 206717

I would like to be able to copy either a derived or a base object to a derived object, That is a symptom of poor design.

It's better to strive for a design where the only the leaf-level classes in a class hierarchy are instantiable. This allows you to have clean, non-virtual, assignment operator functions deal only with objects of right type.

class A {
   public:

      // Make A uninstantiable.
      virtual ~A() = 0;

      A & operator= (A const &other) {
         // Here copy A members...
         cout<<"A to A"<<endl;
         return *this;
      }
};

class B: public A {
   public: 

      // Not necessary.
      // B & operator= (A const &other) { ... }

      B & operator= (B const &other) {
         A::operator=(other); // Copy A members.
         // Here copy B members...
         cout<<"B to B"<<endl; 
         return *this;
      }
};

Upvotes: 2

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