Question

I have base-class Base from which is derived Derived1, Derived2 and Derived3.

I have constructed an instance for one of the the derived classes which I store as Base* a. I now need to make a deep copy of the object which I will store as Base* b.

As far as I know, the normal way of copying a class is to use copy constructors and to overload operator=. However since I don’t know whether a is of type Derived1, Derived2 or Derived3, I cannot think of a way of using either the copy constructor or operator=. The only way I can think of to cleanly make this work is to implement something like:

class Base
{
public:
  virtual Base* Clone() = 0;

};

and the implement Clone in in the derived class as in:

class Derivedn : public Base
{
public:
  Base* Clone() 
  {
    Derived1* ret = new Derived1;
    copy all the data members
  }
};

Java tends to use Clone quite a bit is there more of a C++ way of doing this?

Answer 1

This is still how we do stuff in C++ for polymorphic classes, but you don’t need to do the explicit copy of members if you create a copy constructor (possibly implicit or private) for your objects.

class Base
{
public:
  virtual Base* Clone() = 0;
};

class Derivedn : public Base
{
public:
  //This is OK, its called covariant return type.
  Derivedn* Clone() 
  {
    return new Derivedn(*this);
  }
private:
  Derivedn(const Derivedn&) : ... {}
};