How Can I Deep Copy Polymorphic Objects in C Effectively?

Copying Polymorphic Objects in C

When working with polymorphic objects, creating a deep copy of the object is often necessary. However, the traditional methods of using copy constructors and overloading operator= may not be suitable when the specific derived class type is unknown at compile time.

The Clone Method Approach

One common approach involves implementing a virtual Clone method in the base class. Each derived class then implements its own version of Clone that creates a new instance of the specific derived class and copies the data members.

Example:

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

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

Covariant Return Types

However, there is a more idiomatic C way to handle this: covariant return types. This allows derived classes to return a pointer to their own type from the Clone method.

Revised Example:

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

class Derivedn : public Base {
public:
  // Covariant return type, returns Derivedn*
  Derivedn* Clone() {
    return new Derivedn(*this);
  }
private:
  Derivedn(const Derivedn&); // Copy constructor (possibly implicit or private)
};

By using covariant return types, the copy constructor is used implicitly to create the new instance of the derived class. This simplifies the implementation and avoids the need for explicit member copying.

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