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

问题

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.

回答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> { ... };

回答2:

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

来源：https://stackoverflow.com/questions/5731217/how-to-copy-create-derived-class-instance-from-a-pointer-to-a-polymorphic-base-c