How to copy/create derived class instance from a pointer to a polymorphic base class?

Question

I have been struggling with this kind of problem for a long time, so I decided to ask here.

class Base {   virtual ~Base(); }; class Derived1 : public Base { ... }; class Derived2 : public Base { ... }; ...  // Copies the instance of derived class pointed by the *base pointer Base* CreateCopy(Base* base); 

The method should return a dynamically created copy, or at least store the object on stack in some data structure to avoid "returning address of a temporary" problem.

The naive approach to implement the above method would be using multiple typeids or dynamic_casts in a series of if-statements to check for each possible derived type and then use the new operator. Is there any other, better approach?

P.S.: I know, that the this problem can be avoided using smart pointers, but I am interested in the minimalistic approach, without a bunch of libraries.

Answer 1

You add a virtual Base* clone() const = 0; in your base class and implement it appropriately in your Derived classes. If your Base is not abstract, you can of course call its copy-constructor, but that's a bit dangerous: If you forget to implement it in a derived class, you'll get (probably unwanted) slicing.

If you don't want to duplicate that code, you can use the CRTP idiom to implement the function via a template:

template <class Derived> class DerivationHelper : public Base { public:   virtual Base* clone() const   {     return new Derived(static_cast<const Derived&>(*this)); // call the copy ctor.   } };  class Derived1 : public DerivationHelper <Derived1> { ... }; class Derived2 : public DerivationHelper <Derived2> { ... }; 

Answer 2

An alternative is to have a pure virtual CreateCopy() method in the common base that is implemented in each derived class.