“std::shared_from_this”不能被它的派生类继承吗？ [复制]答案

【问题标题】：Can't "std::shared_from_this" be inherited by its derived classes? [duplicate]“std::shared_from_this”不能被它的派生类继承吗？ [复制]
【发布时间】：2020-06-11 05:53:08
【问题描述】：

std::shared_from_this 不能被其派生类继承吗？

为什么这段代码 sn-p 不能编译（检查http://cpp.sh/7llcr）？我已经仔细阅读了文档（https://en.cppreference.com/w/cpp/memory/enable_shared_from_this），但仍然找不到任何线索。我想了很久。如果能在这个问题上得到一些帮助，我将不胜感激。

这里是无法编译的代码sn-p：

// enable_shared_from_this example
#include <iostream>
#include <memory>

struct C : std::enable_shared_from_this<C> {};

struct D : public C {};

int main () {
  std::shared_ptr<D> foo, bar;

  foo = std::make_shared<D>();

  bar = foo->shared_from_this();


  return 0;
}

编译器抱怨：

In file included from /usr/include/c++/4.9/bits/shared_ptr.h:52:0,
                 from /usr/include/c++/4.9/memory:82,
                 from 3:
/usr/include/c++/4.9/bits/shared_ptr_base.h: In instantiation of 'std::__shared_ptr<_Tp, _Lp>& std::__shared_ptr<_Tp, _Lp>::operator=(std::__shared_ptr<_Tp1, _Lp>&&) [with _Tp1 = C; _Tp = D; __gnu_cxx::_Lock_policy _Lp = (__gnu_cxx::_Lock_policy)2u]':
/usr/include/c++/4.9/bits/shared_ptr.h:299:4:   required from 'std::shared_ptr<_Tp>& std::shared_ptr<_Tp>::operator=(std::shared_ptr<_Tp1>&&) [with _Tp1 = C; _Tp = D]'
14:7:   required from here
/usr/include/c++/4.9/bits/shared_ptr_base.h:1001:4: error: no matching function for call to 'std::__shared_ptr<D, (__gnu_cxx::_Lock_policy)2u>::__shared_ptr(std::remove_reference<std::__shared_ptr<C, (__gnu_cxx::_Lock_policy)2u>&>::type)'
    __shared_ptr(std::move(__r)).swap(*this);
    ^
/usr/include/c++/4.9/bits/shared_ptr_base.h:1001:4: note: candidates are:
/usr/include/c++/4.9/bits/shared_ptr_base.h:1145:7: note: std::__shared_ptr<_Tp, _Lp>::__shared_ptr(const std::__weak_ptr<_Tp, _Lp>&, std::nothrow_t) [with _Tp = D; __gnu_cxx::_Lock_policy _Lp = (__gnu_cxx::_Lock_policy)2u]
       __shared_ptr(const __weak_ptr<_Tp, _Lp>& __r, std::nothrow_t)
       ^
/usr/include/c++/4.9/bits/shared_ptr_base.h:1145:7: note:   candidate expects 2 arguments, 1 provided
/usr/include/c++/4.9/bits/shared_ptr_base.h:1087:2: note: template<class _Alloc, class ... _Args> std::__shared_ptr<_Tp, _Lp>::__shared_ptr(std::_Sp_make_shared_tag, const _Alloc&, _Args&& ...)
  __shared_ptr(_Sp_make_shared_tag __tag, const _Alloc& __a,
  ^
/usr/include/c++/4.9/bits/shared_ptr_base.h:1087:2: note:   template argument deduction/substitution failed:
/usr/include/c++/4.9/bits/shared_ptr_base.h:1001:4: note:   cannot convert 'std::move<std::__shared_ptr<C, (__gnu_cxx::_Lock_policy)2u>&>((* & __r))' (type 'std::remove_reference<std::__shared_ptr<C, (__gnu_cxx::_Lock_policy)2u>&>::type {aka std::__shared_ptr<C, (__gnu_cxx::_Lock_policy)2u>}') to type 'std::_Sp_make_shared_tag'
    __shared_ptr(std::move(__r)).swap(*this);
    ^
/usr/include/c++/4.9/bits/shared_ptr_base.h:967:17: note: constexpr std::__shared_ptr<_Tp, _Lp>::__shared_ptr(std::nullptr_t) [with _Tp = D; __gnu_cxx::_Lock_policy _Lp = (__gnu_cxx::_Lock_policy)2u; std::nullptr_t = std::nullptr_t]
       constexpr __shared_ptr(nullptr_t) noexcept
                 ^
/usr/include/c++/4.9/bits/shared_ptr_base.h:967:17: note:   no known conversion for argument 1 from 'std::remove_reference<std::__shared_ptr<C, (__gnu_cxx::_Lock_policy)2u>&>::type {aka std::__shared_ptr<C, (__gnu_cxx::_Lock_policy)2u>}' to 'std::nullptr_t'
/usr/include/c++/4.9/bits/shared_ptr_base.h:963:2: note: template<class _Tp1> std::__shared_ptr<_Tp, _Lp>::__shared_ptr(std::auto_ptr<_Up>&&)
  __shared_ptr(std::auto_ptr<_Tp1>&& __r);
  ^
/usr/include/c++/4.9/bits/shared_ptr_base.h:963:2: note:   template argument deduction/substitution failed:
/usr/include/c++/4.9/bits/shared_ptr_base.h:1001:4: note:   'std::remove_reference<std::__shared_ptr<C, (__gnu_cxx::_Lock_policy)2u>&>::type {aka std::__shared_ptr<C, (__gnu_cxx::_Lock_policy)2u>}' is not derived from 'std::auto_ptr<_Up>'
    __shared_ptr(std::move(__r)).swap(*this);
    ^
/usr/include/c++/4.9/bits/shared_ptr_base.h:951:2: note: template<class _Tp1, class _Del> std::__shared_ptr<_Tp, _Lp>::__shared_ptr(std::unique_ptr<_Up, _Ep>&&)
  __shared_ptr(std::unique_ptr<_Tp1, _Del>&& __r)
  ^
/usr/include/c++/4.9/bits/shared_ptr_base.h:951:2: note:   template argument deduction/substitution failed:
/usr/include/c++/4.9/bits/shared_ptr_base.h:1001:4: note:   'std::remove_reference<std::__shared_ptr<C, (__gnu_cxx::_Lock_policy)2u>&>::type {aka std::__shared_ptr<C, (__gnu_cxx::_Lock_policy)2u>}' is not derived from 'std::unique_ptr<_Tp, _Dp>'
    __shared_ptr(std::move(__r)).swap(*this);
    ^
/usr/include/c++/4.9/bits/shared_ptr_base.h:939:11: note: template<class _Tp1> std::__shared_ptr<_Tp, _Lp>::__shared_ptr(const std::__weak_ptr<_Tp1, _Lp>&)
  explicit __shared_ptr(const __weak_ptr<_Tp1, _Lp>& __r)
           ^
/usr/include/c++/4.9/bits/shared_ptr_base.h:939:11: note:   template argument deduction/substitution failed:
/usr/include/c++/4.9/bits/shared_ptr_base.h:1001:4: note:   'std::remove_reference<std::__shared_ptr<C, (__gnu_cxx::_Lock_policy)2u>&>::type {aka std::__shared_ptr<C, (__gnu_cxx::_Lock_policy)2u>}' is not derived from 'const std::__weak_ptr<_Tp>'
    __shared_ptr(std::move(__r)).swap(*this);
    ^
/usr/include/c++/4.9/bits/shared_ptr_base.h:931:2: note: template<class _Tp1, class> std::__shared_ptr<_Tp, _Lp>::__shared_ptr(std::__shared_ptr<_Tp1, _Lp>&&)
  __shared_ptr(__shared_ptr<_Tp1, _Lp>&& __r) noexcept
  ^
/usr/include/c++/4.9/bits/shared_ptr_base.h:931:2: note:   template argument deduction/substitution failed:
/usr/include/c++/4.9/bits/shared_ptr_base.h:929:31: error: no type named 'type' in 'struct std::enable_if<false, void>'
       template<typename _Tp1, typename = typename
                               ^
/usr/include/c++/4.9/bits/shared_ptr_base.h:922:7: note: std::__shared_ptr<_Tp, _Lp>::__shared_ptr(std::__shared_ptr<_Tp, _Lp>&&) [with _Tp = D; __gnu_cxx::_Lock_policy _Lp = (__gnu_cxx::_Lock_policy)2u]
       __shared_ptr(__shared_ptr&& __r) noexcept
       ^
/usr/include/c++/4.9/bits/shared_ptr_base.h:922:7: note:   no known conversion for argument 1 from 'std::remove_reference<std::__shared_ptr<C, (__gnu_cxx::_Lock_policy)2u>&>::type {aka std::__shared_ptr<C, (__gnu_cxx::_Lock_policy)2u>}' to 'std::__shared_ptr<D, (__gnu_cxx::_Lock_policy)2u>&&'
/usr/include/c++/4.9/bits/shared_ptr_base.h:918:2: note: template<class _Tp1, class> std::__shared_ptr<_Tp, _Lp>::__shared_ptr(const std::__shared_ptr<_Tp1, _Lp>&)
  __shared_ptr(const __shared_ptr<_Tp1, _Lp>& __r) noexcept
  ^
/usr/include/c++/4.9/bits/shared_ptr_base.h:918:2: note:   template argument deduction/substitution failed:
/usr/include/c++/4.9/bits/shared_ptr_base.h:916:31: error: no type named 'type' in 'struct std::enable_if<false, void>'
       template<typename _Tp1, typename = typename
                               ^
/usr/include/c++/4.9/bits/shared_ptr_base.h:912:7: note: std::__shared_ptr<_Tp, _Lp>::__shared_ptr(const std::__shared_ptr<_Tp, _Lp>&) [with _Tp = D; __gnu_cxx::_Lock_policy _Lp = (__gnu_cxx::_Lock_policy)2u]
       __shared_ptr(const __shared_ptr&) noexcept = default;
       ^
/usr/include/c++/4.9/bits/shared_ptr_base.h:912:7: note:   no known conversion for argument 1 from 'std::remove_reference<std::__shared_ptr<C, (__gnu_cxx::_Lock_policy)2u>&>::type {aka std::__shared_ptr<C, (__gnu_cxx::_Lock_policy)2u>}' to 'const std::__shared_ptr<D, (__gnu_cxx::_Lock_policy)2u>&'
/usr/include/c++/4.9/bits/shared_ptr_base.h:908:2: note: template<class _Tp1> std::__shared_ptr<_Tp, _Lp>::__shared_ptr(const std::__shared_ptr<_Tp1, _Lp>&, _Tp*)
  __shared_ptr(const __shared_ptr<_Tp1, _Lp>& __r, _Tp* __p) noexcept
  ^
/usr/include/c++/4.9/bits/shared_ptr_base.h:908:2: note:   template argument deduction/substitution failed:
/usr/include/c++/4.9/bits/shared_ptr_base.h:1001:4: note:   candidate expects 2 arguments, 1 provided
    __shared_ptr(std::move(__r)).swap(*this);
    ^
/usr/include/c++/4.9/bits/shared_ptr_base.h:903:9: note: template<class _Deleter, class _Alloc> std::__shared_ptr<_Tp, _Lp>::__shared_ptr(std::nullptr_t, _Deleter, _Alloc)
         __shared_ptr(nullptr_t __p, _Deleter __d, _Alloc __a)
         ^
/usr/include/c++/4.9/bits/shared_ptr_base.h:903:9: note:   template argument deduction/substitution failed:
/usr/include/c++/4.9/bits/shared_ptr_base.h:1001:4: note:   cannot convert 'std::move<std::__shared_ptr<C, (__gnu_cxx::_Lock_policy)2u>&>((* & __r))' (type 'std::remove_reference<std::__shared_ptr<C, (__gnu_cxx::_Lock_policy)2u>&>::type {aka std::__shared_ptr<C, (__gnu_cxx::_Lock_policy)2u>}') to type 'std::nullptr_t'
    __shared_ptr(std::move(__r)).swap(*this);
    ^
/usr/include/c++/4.9/bits/shared_ptr_base.h:898:2: note: template<class _Deleter> std::__shared_ptr<_Tp, _Lp>::__shared_ptr(std::nullptr_t, _Deleter)
  __shared_ptr(nullptr_t __p, _Deleter __d)
  ^
/usr/include/c++/4.9/bits/shared_ptr_base.h:898:2: note:   template argument deduction/substitution failed:
/usr/include/c++/4.9/bits/shared_ptr_base.h:1001:4: note:   cannot convert 'std::move<std::__shared_ptr<C, (__gnu_cxx::_Lock_policy)2u>&>((* & __r))' (type 'std::remove_reference<std::__shared_ptr<C, (__gnu_cxx::_Lock_policy)2u>&>::type {aka std::__shared_ptr<C, (__gnu_cxx::_Lock_policy)2u>}') to type 'std::nullptr_t'
    __shared_ptr(std::move(__r)).swap(*this);
    ^
/usr/include/c++/4.9/bits/shared_ptr_base.h:889:2: note: template<class _Tp1, class _Deleter, class _Alloc> std::__shared_ptr<_Tp, _Lp>::__shared_ptr(_Tp1*, _Deleter, _Alloc)
  __shared_ptr(_Tp1* __p, _Deleter __d, _Alloc __a)
  ^
/usr/include/c++/4.9/bits/shared_ptr_base.h:889:2: note:   template argument deduction/substitution failed:
/usr/include/c++/4.9/bits/shared_ptr_base.h:1001:4: note:   mismatched types '_Tp1*' and 'std::__shared_ptr<C, (__gnu_cxx::_Lock_policy)2u>'
    __shared_ptr(std::move(__r)).swap(*this);
    ^
/usr/include/c++/4.9/bits/shared_ptr_base.h:880:2: note: template<class _Tp1, class _Deleter> std::__shared_ptr<_Tp, _Lp>::__shared_ptr(_Tp1*, _Deleter)
  __shared_ptr(_Tp1* __p, _Deleter __d)
  ^
/usr/include/c++/4.9/bits/shared_ptr_base.h:880:2: note:   template argument deduction/substitution failed:
/usr/include/c++/4.9/bits/shared_ptr_base.h:1001:4: note:   mismatched types '_Tp1*' and 'std::__shared_ptr<C, (__gnu_cxx::_Lock_policy)2u>'
    __shared_ptr(std::move(__r)).swap(*this);
    ^
/usr/include/c++/4.9/bits/shared_ptr_base.h:870:11: note: template<class _Tp1> std::__shared_ptr<_Tp, _Lp>::__shared_ptr(_Tp1*)
  explicit __shared_ptr(_Tp1* __p)
           ^
/usr/include/c++/4.9/bits/shared_ptr_base.h:870:11: note:   template argument deduction/substitution failed:
/usr/include/c++/4.9/bits/shared_ptr_base.h:1001:4: note:   mismatched types '_Tp1*' and 'std::__shared_ptr<C, (__gnu_cxx::_Lock_policy)2u>'
    __shared_ptr(std::move(__r)).swap(*this);
    ^
/usr/include/c++/4.9/bits/shared_ptr_base.h:865:17: note: constexpr std::__shared_ptr<_Tp, _Lp>::__shared_ptr() [with _Tp = D; __gnu_cxx::_Lock_policy _Lp = (__gnu_cxx::_Lock_policy)2u]
       constexpr __shared_ptr() noexcept
                 ^
/usr/include/c++/4.9/bits/shared_ptr_base.h:865:17: note:   candidate expects 0 arguments, 1 provided

【问题讨论】：

标签： c++ shared-ptr smart-pointers

【解决方案1】：

它是继承的：

// enable_shared_from_this example
#include <iostream>
#include <memory>

struct C : std::enable_shared_from_this<C> {};

struct D : public C {};

int main () {
  std::shared_ptr<D> foo;

  foo = std::make_shared<D>();

  std::shared_ptr<C> bar = foo->shared_from_this();

  return 0;
}

您在class C 中定义shared_from_this：因此对象返回std::shated_ptr<C>。没有惊喜。您仍然可以将其转换为 std::shared_ptr<D> 和 std::dynamic_pointer_cast。

【讨论】：

谢谢。为什么std::shared_from_this 不能像普通类那样被继承？例如：class Base base{public: void* ptr;}; class Drived:public Base {}; int main{Drived d; d->ptr = NULL}。我想知道是什么导致它有点奇怪。
std::enable_shared_from_this<C> 被继承。如果struct D 从C 继承std::enable_shared_from_this<C>，那么D::shared_from_this 来自std::enable_shared_from_this<C>，所以它将构造C。如果你想让D::shared_from_this 返回D，你必须在某个地方继承std::enable_shared_from_this<D>。
@KamilCuk 我明白了。谢谢你的澄清。还有一个问题，因为foo和bar都是左值，理论上我可以得到它们的地址，但是&bar和&foo不起作用（它们得到了类D的新实例化的地址）。如何获取foo本身的地址？
一个类可以重载operator&，但std::shared_ptr 不应该。您可以使用std::addressof 来真正获取变量的地址。

【解决方案2】：

正如@DimityKuzminov 已经指出的那样，shared_from_this() 在从std::shared_ptr<D> 调用时也将始终返回std::shared_pointer<C>。为了使用std::dynmaic_pointer_cast，你的基础还需要有一个虚拟函数，比如虚拟析构函数（否则编译器会告诉你source type is not polymorphic）。这是一个例子：

// enable_shared_from_this example
#include <iostream>
#include <memory>

struct C : std::enable_shared_from_this<C> {
    virtual ~C() = default;
};

template<typename T, typename U>
std::shared_ptr<T> 
shared_from(const std::shared_ptr<U> &a)
{
    return std::dynamic_pointer_cast<T>(a->shared_from_this());
}

struct D : public C {};

int main () {
  std::shared_ptr<D> foo, bar;

  foo = std::make_shared<D>();

  bar = shared_from<D>(foo);

  return 0;
}

【讨论】：

谢谢你，很好的评论。但是任何虚函数都足够了，可能没有虚析构函数。
是的，你是对的。我会进行编辑。
@DmitryKuzminov & StefanKssmr 我想知道为什么可能没有虚拟析构函数？我确实见过很多次。我哪里错了？
@John，您可以定义虚拟析构函数、常规非虚拟析构函数或具有默认析构函数：如果构造正确，每个选项都可以使用智能指针。如果您不使用智能指针或使用类型擦除构造 shared_ptr，则需要虚拟析构函数。但是要使用动态类型转换，您需要基类中的 any 虚函数。
@John 确实，您经常看到它们，但没有必要为了从类派生而使用虚拟析构函数。这个answer 可能会让你感兴趣。