Is it possible to clone a polymorphic object without manually adding overridden clone method into each derived class in C++?

Question

The typical pattern when you want to copy a polymorphic class is adding a virtual clone method and implement it in each derived class like this:

Base* Derived::clone()
{
    return new Derived(*this);
}

Then in a calling code you can:

Base *x = new Derived();
Base *y = x->clone();

However if you have 50+ derived classes and realize you need polymorphic copy, it's tedious to copy-paste the cloning method into each of them. And it's essentially a boilerplate that works around a language limitation that you have to spell out the actual name to call the constructor.

I haven't keep track with the new features in recent C++ standards... Is there a way to avoid this in modern C++?

Answer 1

You can use this generic CRTP code

template <class Derived, class Base>
struct Clonable : Base {
    virtual Base* do_clone() {
        return new Derived(*static_cast<Derived*>(this));
    }
    Derived* clone() { // not virtual
        return static_cast<Derived*>(do_clone());
    }

    using Base::Base;
};

struct empty {};
struct A : Clonable<A, empty> {};
struct B : Clonable<B, A> {};

It can be generalised to smart pointers and multiple bases if desired.

Answer 2

You could use a CRTP approach, but that has other drawbacks:

struct Base {
    virtual Base* clone() const = 0;
};

template <typename Derived>
class BaseT : public Base {
    // ...
public:
    Base* clone() const override {
        return new Derived(*static_cast<Derived*>(this));
    }
};

Usage:

class DerivedA : public BaseT<DerivedA> {
};

---

Base *x = new DerivedA();
Base *y = x->clone();

---

I haven't keep track with the new features in recent C++ standards... Is there a way to avoid this in modern C++?

This trick is available since the c++98 standard.

Answer 3

If you can control how you pass around the polymorphic type, use type erasure. In particular, the proposed std::polymorphic_value calls the derived copy constructor when it is copied. You can imagine it as something like this:

template <typename B>
class polymorphic_value {
public:
    template <typename D,
        std::enable_if_t<
            std::is_base_of<B, std::decay_t<D>>::value, int> = 0>
    explicit polymorphic_value(D&& value)
        : ptr{std::make_unique<derived_t<std::decay_t<D>>>(std::forward<D>(value))}
    {}

    polymorphic_value(polymorphic_value const& rhs)
        : ptr{rhs.ptr->clone()}
    {}

    B const& get() const { return ptr->get(); }

    B& get() {
        // Safe usage of const_cast, since the actual object is not const:
        return const_cast<B&>(ptr->get());
    }

private:
    struct base_t {
        virtual ~base_t() = default;
        virtual std::unique_ptr<B> clone() const = 0;
        // With more effort, this doesn't have to be a virtual function.
        // For example, rolling our own vtables would make that possible.
        virtual B const& get() const = 0;
    };

    template <typename D>
    struct derived_t final : public base_t {
        explicit derived_t(D const& d)
            : value{d}
        {}

        explicit derived_t(D&& d)
            : value{std::move(d)}
        {}

        std::unique_ptr<B> clone() const override {
            return std::make_unique<D>(value);
        }

        B const& get() const override {
            return value;
        }

        D value;
    };

    std::unique_ptr<base_t> ptr;
};

For a thorough implementation which follows the proposal, see jbcoe's github repository.

Sample usage:

class Base {
public:
    virtual ~Base() = default;
};

class Derived : public Base {
public:
    Derived() = default;
    Derived(Derived const&);
};

int main() {
    polymorphic_value<Base> it{Derived{}};
    auto const copy = it;
}

Live on Godbolt

Answer 4

You can at minimum avoid writing the class name by getting it from the type of the class itself with:

struct A: public Base
{
    Base* Clone() { return new std::remove_reference_t<decltype(*this)>(*this); }
};

Using CRTP will not help avoid duplicating the class name again, as you than have to write the class name inside the template parms for the CRTP base.

Answer 5

You probably have a class where you store the polymorphic object and where you want to clone? Together with your polymorphic object you could store a function-pointer doing the cloning:

template<class Derived>
Base* clone(const Base* b) {
    return new Derived(static_cast<const Derived*>(b));
}

void SampleUsage() {
    Base* b = new Derived;
    Base*(*cloner)(const Base*) = clone<Derived>;
    Base* copy = cloner(b);
}

The type of cloner is independent of Derived. Its like a simplified std::function.

Answer 6

You could use CRTP to add an additional layer between your derived class and base that implements your cloning method.

struct Base {
    virtual ~Base() = default;
    virtual Base* clone() = 0;
};

template <typename T>
struct Base_with_clone : Base {
    Base* clone() {
        return new T(*this);
    }
};

struct Derived : Base_with_clone<Derived> {};