How to adapt deep copy patern so it returns by value

Question

I am apologize if it is a stupid problem.

I want to do deep copy of a derived class.

I have do a search and found there already exist the topic.

C++: Deep copying a Base class pointer

Copying derived entities using only base class pointers, (without exhaustive testing!) - C++

Some elegant advice has been given, such as using virtual copy pattern.

struct base {
   virtual ~base() {}                // Remember to provide a virtual destructor
   virtual base* clone() const = 0;
};
struct derived : base {
   virtual derived* clone() const {
      return new derived(*this);
   }
};

In my case, I do not want to use the pointer. I do not know can we do something similar via reference?

Suppose a base class should define a deepCopy interface, to make sure all derived class can do deep copy.

class Base
{
public:
    virtual Base &deepCopy() = 0; // may be not good practice, you can change the deep copy interface
    virtual void doSomeThing() = 0;
};

and the derived class may be something like this，but I have no idea how to write that.

class Derived : public Base
{
public:
    // should overide interface, and I have no idea how to do that.
    Base &deepCopy() override
    {
    }

    void doSomeThing() override
    {
    // Omitted
    }
    // do shallow copy
    Derived& operator=(const Derived&rhs)
    {
     // omitted
     return (*this);
    }
private:
    int size;
    double *data;
};

If you have good idea, change base interface is ok, as long as not using pointer.

I want using deep copy by follow way:

Derived a;
Derived b = a.deepCopy();

Thanks for your time.

edit

Thanks many advice have been given on the comment. Thanks for your all time.

I am rethink my problem.

I want

(1) polymorphism

(2) enforce derived class implement deepCopy interface.

(3) a = interface for shallow copy and a deepCopy interface for deep copy.

Actually, pointer solution can settle (1),(2),(3) while template solution from walnut can settle (2)(3). They are all good enough.

I am still confused that how to using reference to relize (1)(2)(3) above. May be it is a solution solve the (1)(2)(3) in another pespective.

I am now wonder if it is my fault that I am too adhere to reference ...