Compilation issues with smart pointer-based CRTP idiom - c++

Question

I'm trying to compile a minimal working example for the CRTP example given in this blog post, which is based on smart pointers.
Based on the code example, I've written two files, a header and source.
Header (crtp.h):
#include <memory>
class Cloneable
{
public:
virtual ~Cloneable() {}
std::shared_ptr<Cloneable> clone() const
{
return std::shared_ptr<Cloneable>(this->clone_raw());
}
private:
virtual Cloneable* clone_raw() const = 0;
};
template <typename Derived, typename Base>
class CloneInherit<Derived, Base>: public Base
{
public:
std::shared_ptr<Derived> clone() const
{
return std::shared_ptr<Derived>(static_cast<Derived*>(this->clone_raw()));
}
private:
virtual CloneInherit* clone_raw() const override
{
return new Derived(*this);
}
};
class Concrete: public CloneInherit<Concrete, Cloneable> {};
Source (example.cc):
#include <memory>
#include "crtp.h"
int main()
{
std::shared_ptr<Concrete> c = std::make_shared<Concrete>();
std::shared_ptr<Concrete> cc = c->clone();
Cloneable* p = c.get();
std::shared_ptr<Cloneable> pp = p->clone();
return 0;
}
Compilation of this code fails with the following error:
In file included from example.cc:3:
./crtp.h:18:7: error: explicit specialization of non-template class 'CloneInherit'
class CloneInherit<Derived, Base>: public Base
^ ~~~~~~~~~~~~~~~
./crtp.h:29:16: error: no matching constructor for initialization of 'Concrete'
return new Derived(*this);
^ ~~~~~
./crtp.h:33:7: note: in instantiation of member function 'CloneInherit<Concrete, Cloneable>::clone_raw' requested here
class Concrete: public CloneInherit<Concrete, Cloneable>
^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/memory:4411:26: note: in instantiation of member function 'std::__1::__shared_ptr_emplace<Concrete, std::__1::allocator<Concrete> >::__shared_ptr_emplace' requested
here
::new(__hold2.get()) _CntrlBlk(__a2, _VSTD::forward<_Args>(__args)...);
^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/memory:4775:29: note: in instantiation of function template specialization 'std::__1::shared_ptr<Concrete>::make_shared<>' requested here
return shared_ptr<_Tp>::make_shared(_VSTD::forward<_Args>(__args)...);
^
example.cc:7:42: note: in instantiation of function template specialization 'std::__1::make_shared<Concrete>' requested here
std::shared_ptr<Concrete> c = std::make_shared<Concrete>();
^
./crtp.h:33:7: note: candidate constructor (the implicit copy constructor) not viable: no known conversion from 'const CloneInherit<Concrete, Cloneable>' to 'const Concrete' for 1st argument
class Concrete: public CloneInherit<Concrete, Cloneable>
^
./crtp.h:33:7: note: candidate constructor (the implicit move constructor) not viable: no known conversion from 'const CloneInherit<Concrete, Cloneable>' to 'Concrete' for 1st argument
class Concrete: public CloneInherit<Concrete, Cloneable>
^
./crtp.h:33:7: note: candidate constructor (the implicit default constructor) not viable: requires 0 arguments, but 1 was provided
2 errors generated.
There are ways I can fix these errors. I can delete the <Derived, Base> specialization from the declaration for CloneInherit to make the first error go away and change the definition of clone_raw() function of CloneInherit to
virtual CloneInherit* clone_raw() const override
{
return new CloneInherit(*this);
}
but I'm not sure if this will get the same results as what the post originally intended.

Answer 1

There are indeed several typo in his post:
Fixed version:
#include <memory>
class cloneable
{
public:
virtual ~cloneable() {}
std::unique_ptr<cloneable> clone() const
{
return std::unique_ptr<cloneable>(this->clone_impl());
}
private:
virtual cloneable * clone_impl() const = 0;
};
template <typename Derived, typename Base>
class clone_inherit : public Base
{
public:
std::unique_ptr<Derived> clone() const
{
return std::unique_ptr<Derived>(static_cast<Derived*>(this->clone_impl()));
}
private:
clone_inherit* clone_impl() const override
{
return new Derived(*static_cast<const Derived*>(this));
}
};
class concrete : public clone_inherit<concrete, cloneable>
{
};
int main()
{
std::unique_ptr<concrete> c = std::make_unique<concrete>();
std::unique_ptr<concrete> cc = c->clone();
cloneable * p = c.get();
std::unique_ptr<cloneable> pp = p->clone();
}
Demo