我正在尝试为 CRTP 示例编译一个最小的工作示例这篇博文 https://www.fluentcpp.com/2017/09/12/how-to-return-a-smart-pointer-and-use-covariance/,它基于智能指针。
根据代码示例,我编写了两个文件:标头和源代码。
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;
}
此代码编译失败并出现以下错误:
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.
我可以通过一些方法来修复这些错误。我可以删除<Derived, Base>
声明中的专业化CloneInherit
使第一个错误消失并更改定义clone_raw()
的函数CloneInherit
to
virtual CloneInherit* clone_raw() const override
{
return new CloneInherit(*this);
}
但我不确定这是否会得到与帖子最初意图相同的结果。