Compilation issues with smart pointer-based CRTP idiom

问题

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.

回答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

来源：https://stackoverflow.com/questions/46915833/compilation-issues-with-smart-pointer-based-crtp-idiom