C++ 基于智能指针的CRTP习惯用法的编译问题

我试图为中给出的CRTP示例编译一个最小的工作示例，该示例基于智能指针

根据代码示例，我编写了两个文件，一个头文件和源文件

标题（

crtp.h

）：

编译此代码失败，出现以下错误：

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.

---

但我不确定这是否会得到与文章最初预期相同的结果。

他的文章中确实有几个拼写错误：

固定版本：

#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();
}

#包括
类可克隆
{
公众：
虚拟~cloneable（）{}
std:：unique_ptr clone（）常量
{
返回std:：unique_ptr（this->clone_impl（））；
}
私人：
虚拟可克隆*clone_impl（）常量=0；
};
模板
类clone\u inherit:public Base
{
公众：
std:：unique_ptr clone（）常量
{
返回std:：unique_ptr（静态_cast（this->clone_impl（））；
}
私人：
clone\u inherit*clone\u impl（）常量覆盖
{
返回新的派生（*static_cast（this））；
}
};
类具体：公共克隆_继承
{
};
int main（）
{
std:：unique_ptr c=std:：make_unique（）；
std:：unique_ptr cc=c->clone（）；
可克隆*p=c.get（）；
std:：unique_ptr pp=p->clone（）；
}

---

您确实应该删除

，因为您在这里不是专门化的，而是定义类。在

clone\u inherit

的

clone\u impl（）

方法中强制转换为

派生*

的原因是什么？此指针作为

clone\u inherit*

返回到

clone（）

，但随后需要再次转换为

派生的*

，这似乎是多余的。我缺少什么？我们确实想要一个签名返回类型

派生*

，但是，派生类的定义还不是一个完整的类型，所以我们不能说派生类继承自基（对于协方差返回类型）

clone\u inherit

继承

Base

，因此它是有效的返回类型。然后，强制转换将修复键入：在该CRTP中，所有

clone\u inherit

都是

派生的
基类。
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.

virtual CloneInherit* clone_raw() const override
{
  return new CloneInherit(*this);
}

#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();
}