Derived curiously recurring templates and covariance - c++

Question

Suppose I have a base class which cloning of derived classes:
class Base
{
public:
virtual Base * clone()
{
return new Base();
}
// ...
};
I have a set of derived classes which are implemented using a curiously recurring template pattern:
template <class T>
class CRTP : public Base
{
public:
virtual T * clone()
{
return new T();
}
// ...
};
And I attempt to derive from that further like this:
class Derived : public CRTP<Derived>
{
public:
// ...
};
I get compilation errors to the effect of:
error C2555: 'CRTP<T>::clone': overriding virtual function return type differs and is not covariant from 'Base::clone'
I realize this is probably a result of the compiler not fully knowing the inheritance tree for Derived when instantiating CRTP. Furthermore, replacing the return type (T*) with (Base*) also compiles. However, I would like to know if there is a work around which retains the above semantics.

Answer 1

A not-so-pretty workaround.
class Base
{
protected:
virtual Base * clone_p()
{
return new Base();
}
};
template <class T>
class CRTP : public Base
{
protected:
virtual CRTP* clone_p()
{
return new T;
}
public:
T* clone()
{
CRTP* res = clone_p();
return static_cast<T*>(res);
}
};
class Derived : public CRTP<Derived>
{
public:
};
Use dynamic_cast<> instead of static if you feel it's safer.

Answer 2

If you can live with having to use a different syntax for specifying complete types, you might do the following (warning: untested code):
Let's first start with the machinery:
// this gives the complete type which needs to be used to create objects
// and provides the implementation of clone()
template<typename T> class Cloneable:
public T
{
public:
template<typename... U> Cloneable(U&&... u): T(std::forward<U>(u) ...) {}
T* clone() { return new Cloneable(*this); }
private:
// this makes the class complete
// Note: T:: to make it type dependent, so it can be found despite not yet defined
typename T::CloneableBase::CloneableKey unlock() {}
};
// this provides the clone function prototype and also makes sure that only
// Cloneable<T> can be instantiated
class CloneableBase
{
template<typename T> friend class Cloneable;
// this type is only accessible to Clonerable instances
struct CloneableKey {};
// this has to be implemented to complete the class; only Cloneable instances can do that
virtual CloneableKey unlock() = 0;
public:
virtual CloneableBase* clone() = 0;
virtual ~CloneableBase() {}
};
OK, now the actual class hierarchy. That one is pretty standard; no CRTP intermediates or other complications. However no class implements the clone function, but all inherit the declaration (directly or indirectly) from CloneableBase.
// Base inherits clone() from CloneableBase
class Base:
public CloneableBase
{
// ...
};
// Derived can inherit normally from Base, nothing special here
class Derived:
public Base
{
// ...
};
Here's how you then create objects:
// However, to create new instances, we actually need to use Cloneable<Derived>
Cloneable<Derived> someObject;
Derived* ptr = new Cloneable<Derived>(whatever);
// Now we clone the objects
Derived* clone1 = someObject.clone();
Derived* clone2 = ptr->clone();
// we can get rid og the objects the usual way:
delete ptr;
delete clone1;
delete clone2;
Note that a Cloneable<Derived> is-a Derived (it is a subclass), therefore you need to use Cloneable only for construction, and can otherwise pretend to work with Derived objects (well, tyepinfo will also identify it as Cloneable<Derived>).