c++ - "std::shared_from_this"不能被它的派生类继承吗？

Question

这个问题在这里已经有了答案:





How to enable_shared_from_this of both parent and derived

(4 个回答)


1年前关闭。




不能std::shared_from_this被其派生类继承？

为什么这段代码不能编译(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。我将非常感激能在这个问题上得到一些帮助。

这是无法编译的代码片段:

// 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;
}

编译器提示:

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

Answer 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;
}

您定义 shared_from_this在 class C : 所以对象返回 std::shated_ptr<C> .没有惊喜。您仍然可以将其转换为 std::shared_ptr<D>与 std::dynamic_pointer_cast .