How to Create Copies of Derived Class Instances from Polymorphic Base Class Pointers: A Solution Using Virtual Clone Methods and CRTP

Copying Derived Class Instances from Polymorphic Base Class Pointers

For many, this programming conundrum has proven elusive. Let's explore the complexities involved and discover an efficient solution.

Consider the following scenario: you have classes Base, Derived1, Derived2, etc., where Derived classes inherit from Base. Given a pointer to a Base object, the goal is to create a dynamically allocated copy of the underlying Derived object, avoiding issues such as "returning address of a temporary."

Traditionally, this has been addressed through a litany of typeids or dynamic_casts within conditional statements. However, we seek a more elegant solution.

Virtual Clone Method

The key lies in introducing a virtual clone() method to the Base class, implemented for each Derived type. This method returns a copy of the object. If Base is not abstract, you can invoke its copy constructor, but this approach carries the risk of slicing if it's not implemented properly in the Derived classes.

CRTP Idiom for Code Reuse

To avoid code duplication, the CRTP (Curiously Recurring Template Pattern) idiom can be employed. Consider the following generic template:

template <class Derived>
class DerivationHelper : public Base
{
public:
  virtual Base* clone() const
  {
    return new Derived(static_cast<const Derived&>(*this));
  }
};

By utilizing this template, multiple Derived classes can inherit from DerivationHelper and implement the clone() method using their respective copy constructors.

Implementation and Advantages

This solution offers several benefits:

• Simplicity: The code is concise and easy to implement.
• Extensibility: Adding new Derived classes does not require modifying the Base class.
• Performance: Virtual function calls are relatively efficient.

By embracing the virtual clone() method and the CRTP idiom, programmers can effectively achieve the desired functionality without the need for convoluted or error-prone approaches.

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