shared_ptr.h
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// shared_ptr and weak_ptr implementation -*- C++ -*-
// Copyright (C) 2007-2014 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library. This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU General Public License for more details.
// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.
// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
// <http://www.gnu.org/licenses/>.
// GCC Note: Based on files from version 1.32.0 of the Boost library.
// shared_count.hpp
// Copyright (c) 2001, 2002, 2003 Peter Dimov and Multi Media Ltd.
// shared_ptr.hpp
// Copyright (C) 1998, 1999 Greg Colvin and Beman Dawes.
// Copyright (C) 2001, 2002, 2003 Peter Dimov
// weak_ptr.hpp
// Copyright (C) 2001, 2002, 2003 Peter Dimov
// enable_shared_from_this.hpp
// Copyright (C) 2002 Peter Dimov
// Distributed under the Boost Software License, Version 1.0. (See
// accompanying file LICENSE_1_0.txt or copy at
// http://www.boost.org/LICENSE_1_0.txt)
/** @file bits/shared_ptr.h
* This is an internal header file, included by other library headers.
* Do not attempt to use it directly. @headername{memory}
*/
#ifndef _SHARED_PTR_H
#define _SHARED_PTR_H 1
#include <bits/shared_ptr_base.h>
namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION
/**
* @addtogroup pointer_abstractions
* @{
*/
/// 20.7.2.2.11 shared_ptr I/O
template<typename _Ch, typename _Tr, typename _Tp, _Lock_policy _Lp>
inline std::basic_ostream<_Ch, _Tr>&
operator<<(std::basic_ostream<_Ch, _Tr>& __os,
const __shared_ptr<_Tp, _Lp>& __p)
{
__os << __p.get();
return __os;
}
/// 20.7.2.2.10 shared_ptr get_deleter
template<typename _Del, typename _Tp, _Lock_policy _Lp>
inline _Del*
get_deleter(const __shared_ptr<_Tp, _Lp>& __p) noexcept
{
#ifdef __GXX_RTTI
return static_cast<_Del*>(__p._M_get_deleter(typeid(_Del)));
#else
return 0;
#endif
}
/**
* @brief A smart pointer with reference-counted copy semantics.
*
* The object pointed to is deleted when the last shared_ptr pointing to
* it is destroyed or reset.
*/
template<typename _Tp>
class shared_ptr : public __shared_ptr<_Tp>
{
public:
/**
* @brief Construct an empty %shared_ptr.
* @post use_count()==0 && get()==0
*/
constexpr shared_ptr() noexcept
: __shared_ptr<_Tp>() { }
shared_ptr(const shared_ptr&) noexcept = default;
/**
* @brief Construct a %shared_ptr that owns the pointer @a __p.
* @param __p A pointer that is convertible to element_type*.
* @post use_count() == 1 && get() == __p
* @throw std::bad_alloc, in which case @c delete @a __p is called.
*/
template<typename _Tp1>
explicit shared_ptr(_Tp1* __p)
: __shared_ptr<_Tp>(__p) { }
/**
* @brief Construct a %shared_ptr that owns the pointer @a __p
* and the deleter @a __d.
* @param __p A pointer.
* @param __d A deleter.
* @post use_count() == 1 && get() == __p
* @throw std::bad_alloc, in which case @a __d(__p) is called.
*
* Requirements: _Deleter's copy constructor and destructor must
* not throw
*
* __shared_ptr will release __p by calling __d(__p)
*/
template<typename _Tp1, typename _Deleter>
shared_ptr(_Tp1* __p, _Deleter __d)
: __shared_ptr<_Tp>(__p, __d) { }
/**
* @brief Construct a %shared_ptr that owns a null pointer
* and the deleter @a __d.
* @param __p A null pointer constant.
* @param __d A deleter.
* @post use_count() == 1 && get() == __p
* @throw std::bad_alloc, in which case @a __d(__p) is called.
*
* Requirements: _Deleter's copy constructor and destructor must
* not throw
*
* The last owner will call __d(__p)
*/
template<typename _Deleter>
shared_ptr(nullptr_t __p, _Deleter __d)
: __shared_ptr<_Tp>(__p, __d) { }
/**
* @brief Construct a %shared_ptr that owns the pointer @a __p
* and the deleter @a __d.
* @param __p A pointer.
* @param __d A deleter.
* @param __a An allocator.
* @post use_count() == 1 && get() == __p
* @throw std::bad_alloc, in which case @a __d(__p) is called.
*
* Requirements: _Deleter's copy constructor and destructor must
* not throw _Alloc's copy constructor and destructor must not
* throw.
*
* __shared_ptr will release __p by calling __d(__p)
*/
template<typename _Tp1, typename _Deleter, typename _Alloc>
shared_ptr(_Tp1* __p, _Deleter __d, _Alloc __a)
: __shared_ptr<_Tp>(__p, __d, std::move(__a)) { }
/**
* @brief Construct a %shared_ptr that owns a null pointer
* and the deleter @a __d.
* @param __p A null pointer constant.
* @param __d A deleter.
* @param __a An allocator.
* @post use_count() == 1 && get() == __p
* @throw std::bad_alloc, in which case @a __d(__p) is called.
*
* Requirements: _Deleter's copy constructor and destructor must
* not throw _Alloc's copy constructor and destructor must not
* throw.
*
* The last owner will call __d(__p)
*/
template<typename _Deleter, typename _Alloc>
shared_ptr(nullptr_t __p, _Deleter __d, _Alloc __a)
: __shared_ptr<_Tp>(__p, __d, std::move(__a)) { }
// Aliasing constructor
/**
* @brief Constructs a %shared_ptr instance that stores @a __p
* and shares ownership with @a __r.
* @param __r A %shared_ptr.
* @param __p A pointer that will remain valid while @a *__r is valid.
* @post get() == __p && use_count() == __r.use_count()
*
* This can be used to construct a @c shared_ptr to a sub-object
* of an object managed by an existing @c shared_ptr.
*
* @code
* shared_ptr< pair<int,int> > pii(new pair<int,int>());
* shared_ptr<int> pi(pii, &pii->first);
* assert(pii.use_count() == 2);
* @endcode
*/
template<typename _Tp1>
shared_ptr(const shared_ptr<_Tp1>& __r, _Tp* __p) noexcept
: __shared_ptr<_Tp>(__r, __p) { }
/**
* @brief If @a __r is empty, constructs an empty %shared_ptr;
* otherwise construct a %shared_ptr that shares ownership
* with @a __r.
* @param __r A %shared_ptr.
* @post get() == __r.get() && use_count() == __r.use_count()
*/
template<typename _Tp1, typename = typename
std::enable_if<std::is_convertible<_Tp1*, _Tp*>::value>::type>
shared_ptr(const shared_ptr<_Tp1>& __r) noexcept
: __shared_ptr<_Tp>(__r) { }
/**
* @brief Move-constructs a %shared_ptr instance from @a __r.
* @param __r A %shared_ptr rvalue.
* @post *this contains the old value of @a __r, @a __r is empty.
*/
shared_ptr(shared_ptr&& __r) noexcept
: __shared_ptr<_Tp>(std::move(__r)) { }
/**
* @brief Move-constructs a %shared_ptr instance from @a __r.
* @param __r A %shared_ptr rvalue.
* @post *this contains the old value of @a __r, @a __r is empty.
*/
template<typename _Tp1, typename = typename
std::enable_if<std::is_convertible<_Tp1*, _Tp*>::value>::type>
shared_ptr(shared_ptr<_Tp1>&& __r) noexcept
: __shared_ptr<_Tp>(std::move(__r)) { }
/**
* @brief Constructs a %shared_ptr that shares ownership with @a __r
* and stores a copy of the pointer stored in @a __r.
* @param __r A weak_ptr.
* @post use_count() == __r.use_count()
* @throw bad_weak_ptr when __r.expired(),
* in which case the constructor has no effect.
*/
template<typename _Tp1>
explicit shared_ptr(const weak_ptr<_Tp1>& __r)
: __shared_ptr<_Tp>(__r) { }
#if _GLIBCXX_USE_DEPRECATED
template<typename _Tp1>
shared_ptr(std::auto_ptr<_Tp1>&& __r);
#endif
template<typename _Tp1, typename _Del>
shared_ptr(std::unique_ptr<_Tp1, _Del>&& __r)
: __shared_ptr<_Tp>(std::move(__r)) { }
/**
* @brief Construct an empty %shared_ptr.
* @param __p A null pointer constant.
* @post use_count() == 0 && get() == nullptr
*/
constexpr shared_ptr(nullptr_t __p) noexcept
: __shared_ptr<_Tp>(__p) { }
shared_ptr& operator=(const shared_ptr&) noexcept = default;
template<typename _Tp1>
shared_ptr&
operator=(const shared_ptr<_Tp1>& __r) noexcept
{
this->__shared_ptr<_Tp>::operator=(__r);
return *this;
}
#if _GLIBCXX_USE_DEPRECATED
template<typename _Tp1>
shared_ptr&
operator=(std::auto_ptr<_Tp1>&& __r)
{
this->__shared_ptr<_Tp>::operator=(std::move(__r));
return *this;
}
#endif
shared_ptr&
operator=(shared_ptr&& __r) noexcept
{
this->__shared_ptr<_Tp>::operator=(std::move(__r));
return *this;
}
template<class _Tp1>
shared_ptr&
operator=(shared_ptr<_Tp1>&& __r) noexcept
{
this->__shared_ptr<_Tp>::operator=(std::move(__r));
return *this;
}
template<typename _Tp1, typename _Del>
shared_ptr&
operator=(std::unique_ptr<_Tp1, _Del>&& __r)
{
this->__shared_ptr<_Tp>::operator=(std::move(__r));
return *this;
}
private:
// This constructor is non-standard, it is used by allocate_shared.
template<typename _Alloc, typename... _Args>
shared_ptr(_Sp_make_shared_tag __tag, const _Alloc& __a,
_Args&&... __args)
: __shared_ptr<_Tp>(__tag, __a, std::forward<_Args>(__args)...)
{ }
template<typename _Tp1, typename _Alloc, typename... _Args>
friend shared_ptr<_Tp1>
allocate_shared(const _Alloc& __a, _Args&&... __args);
// This constructor is non-standard, it is used by weak_ptr::lock().
shared_ptr(const weak_ptr<_Tp>& __r, std::nothrow_t)
: __shared_ptr<_Tp>(__r, std::nothrow) { }
friend class weak_ptr<_Tp>;
};
// 20.7.2.2.7 shared_ptr comparisons
template<typename _Tp1, typename _Tp2>
inline bool
operator==(const shared_ptr<_Tp1>& __a,
const shared_ptr<_Tp2>& __b) noexcept
{ return __a.get() == __b.get(); }
template<typename _Tp>
inline bool
operator==(const shared_ptr<_Tp>& __a, nullptr_t) noexcept
{ return !__a; }
template<typename _Tp>
inline bool
operator==(nullptr_t, const shared_ptr<_Tp>& __a) noexcept
{ return !__a; }
template<typename _Tp1, typename _Tp2>
inline bool
operator!=(const shared_ptr<_Tp1>& __a,
const shared_ptr<_Tp2>& __b) noexcept
{ return __a.get() != __b.get(); }
template<typename _Tp>
inline bool
operator!=(const shared_ptr<_Tp>& __a, nullptr_t) noexcept
{ return (bool)__a; }
template<typename _Tp>
inline bool
operator!=(nullptr_t, const shared_ptr<_Tp>& __a) noexcept
{ return (bool)__a; }
template<typename _Tp1, typename _Tp2>
inline bool
operator<(const shared_ptr<_Tp1>& __a,
const shared_ptr<_Tp2>& __b) noexcept
{
typedef typename std::common_type<_Tp1*, _Tp2*>::type _CT;
return std::less<_CT>()(__a.get(), __b.get());
}
template<typename _Tp>
inline bool
operator<(const shared_ptr<_Tp>& __a, nullptr_t) noexcept
{ return std::less<_Tp*>()(__a.get(), nullptr); }
template<typename _Tp>
inline bool
operator<(nullptr_t, const shared_ptr<_Tp>& __a) noexcept
{ return std::less<_Tp*>()(nullptr, __a.get()); }
template<typename _Tp1, typename _Tp2>
inline bool
operator<=(const shared_ptr<_Tp1>& __a,
const shared_ptr<_Tp2>& __b) noexcept
{ return !(__b < __a); }
template<typename _Tp>
inline bool
operator<=(const shared_ptr<_Tp>& __a, nullptr_t) noexcept
{ return !(nullptr < __a); }
template<typename _Tp>
inline bool
operator<=(nullptr_t, const shared_ptr<_Tp>& __a) noexcept
{ return !(__a < nullptr); }
template<typename _Tp1, typename _Tp2>
inline bool
operator>(const shared_ptr<_Tp1>& __a,
const shared_ptr<_Tp2>& __b) noexcept
{ return (__b < __a); }
template<typename _Tp>
inline bool
operator>(const shared_ptr<_Tp>& __a, nullptr_t) noexcept
{ return std::less<_Tp*>()(nullptr, __a.get()); }
template<typename _Tp>
inline bool
operator>(nullptr_t, const shared_ptr<_Tp>& __a) noexcept
{ return std::less<_Tp*>()(__a.get(), nullptr); }
template<typename _Tp1, typename _Tp2>
inline bool
operator>=(const shared_ptr<_Tp1>& __a,
const shared_ptr<_Tp2>& __b) noexcept
{ return !(__a < __b); }
template<typename _Tp>
inline bool
operator>=(const shared_ptr<_Tp>& __a, nullptr_t) noexcept
{ return !(__a < nullptr); }
template<typename _Tp>
inline bool
operator>=(nullptr_t, const shared_ptr<_Tp>& __a) noexcept
{ return !(nullptr < __a); }
template<typename _Tp>
struct less<shared_ptr<_Tp>> : public _Sp_less<shared_ptr<_Tp>>
{ };
// 20.7.2.2.8 shared_ptr specialized algorithms.
template<typename _Tp>
inline void
swap(shared_ptr<_Tp>& __a, shared_ptr<_Tp>& __b) noexcept
{ __a.swap(__b); }
// 20.7.2.2.9 shared_ptr casts.
template<typename _Tp, typename _Tp1>
inline shared_ptr<_Tp>
static_pointer_cast(const shared_ptr<_Tp1>& __r) noexcept
{ return shared_ptr<_Tp>(__r, static_cast<_Tp*>(__r.get())); }
template<typename _Tp, typename _Tp1>
inline shared_ptr<_Tp>
const_pointer_cast(const shared_ptr<_Tp1>& __r) noexcept
{ return shared_ptr<_Tp>(__r, const_cast<_Tp*>(__r.get())); }
template<typename _Tp, typename _Tp1>
inline shared_ptr<_Tp>
dynamic_pointer_cast(const shared_ptr<_Tp1>& __r) noexcept
{
if (_Tp* __p = dynamic_cast<_Tp*>(__r.get()))
return shared_ptr<_Tp>(__r, __p);
return shared_ptr<_Tp>();
}
/**
* @brief A smart pointer with weak semantics.
*
* With forwarding constructors and assignment operators.
*/
template<typename _Tp>
class weak_ptr : public __weak_ptr<_Tp>
{
public:
constexpr weak_ptr() noexcept
: __weak_ptr<_Tp>() { }
template<typename _Tp1, typename = typename
std::enable_if<std::is_convertible<_Tp1*, _Tp*>::value>::type>
weak_ptr(const weak_ptr<_Tp1>& __r) noexcept
: __weak_ptr<_Tp>(__r) { }
template<typename _Tp1, typename = typename
std::enable_if<std::is_convertible<_Tp1*, _Tp*>::value>::type>
weak_ptr(const shared_ptr<_Tp1>& __r) noexcept
: __weak_ptr<_Tp>(__r) { }
template<typename _Tp1>
weak_ptr&
operator=(const weak_ptr<_Tp1>& __r) noexcept
{
this->__weak_ptr<_Tp>::operator=(__r);
return *this;
}
template<typename _Tp1>
weak_ptr&
operator=(const shared_ptr<_Tp1>& __r) noexcept
{
this->__weak_ptr<_Tp>::operator=(__r);
return *this;
}
shared_ptr<_Tp>
lock() const noexcept
{ return shared_ptr<_Tp>(*this, std::nothrow); }
};
// 20.7.2.3.6 weak_ptr specialized algorithms.
template<typename _Tp>
inline void
swap(weak_ptr<_Tp>& __a, weak_ptr<_Tp>& __b) noexcept
{ __a.swap(__b); }
/// Primary template owner_less
template<typename _Tp>
struct owner_less;
/// Partial specialization of owner_less for shared_ptr.
template<typename _Tp>
struct owner_less<shared_ptr<_Tp>>
: public _Sp_owner_less<shared_ptr<_Tp>, weak_ptr<_Tp>>
{ };
/// Partial specialization of owner_less for weak_ptr.
template<typename _Tp>
struct owner_less<weak_ptr<_Tp>>
: public _Sp_owner_less<weak_ptr<_Tp>, shared_ptr<_Tp>>
{ };
/**
* @brief Base class allowing use of member function shared_from_this.
*/
template<typename _Tp>
class enable_shared_from_this
{
protected:
constexpr enable_shared_from_this() noexcept { }
enable_shared_from_this(const enable_shared_from_this&) noexcept { }
enable_shared_from_this&
operator=(const enable_shared_from_this&) noexcept
{ return *this; }
~enable_shared_from_this() { }
public:
shared_ptr<_Tp>
shared_from_this()
{ return shared_ptr<_Tp>(this->_M_weak_this); }
shared_ptr<const _Tp>
shared_from_this() const
{ return shared_ptr<const _Tp>(this->_M_weak_this); }
private:
template<typename _Tp1>
void
_M_weak_assign(_Tp1* __p, const __shared_count<>& __n) const noexcept
{ _M_weak_this._M_assign(__p, __n); }
template<typename _Tp1>
friend void
__enable_shared_from_this_helper(const __shared_count<>& __pn,
const enable_shared_from_this* __pe,
const _Tp1* __px) noexcept
{
if (__pe != 0)
__pe->_M_weak_assign(const_cast<_Tp1*>(__px), __pn);
}
mutable weak_ptr<_Tp> _M_weak_this;
};
/**
* @brief Create an object that is owned by a shared_ptr.
* @param __a An allocator.
* @param __args Arguments for the @a _Tp object's constructor.
* @return A shared_ptr that owns the newly created object.
* @throw An exception thrown from @a _Alloc::allocate or from the
* constructor of @a _Tp.
*
* A copy of @a __a will be used to allocate memory for the shared_ptr
* and the new object.
*/
template<typename _Tp, typename _Alloc, typename... _Args>
inline shared_ptr<_Tp>
allocate_shared(const _Alloc& __a, _Args&&... __args)
{
return shared_ptr<_Tp>(_Sp_make_shared_tag(), __a,
std::forward<_Args>(__args)...);
}
/**
* @brief Create an object that is owned by a shared_ptr.
* @param __args Arguments for the @a _Tp object's constructor.
* @return A shared_ptr that owns the newly created object.
* @throw std::bad_alloc, or an exception thrown from the
* constructor of @a _Tp.
*/
template<typename _Tp, typename... _Args>
inline shared_ptr<_Tp>
make_shared(_Args&&... __args)
{
typedef typename std::remove_const<_Tp>::type _Tp_nc;
return std::allocate_shared<_Tp>(std::allocator<_Tp_nc>(),
std::forward<_Args>(__args)...);
}
/// std::hash specialization for shared_ptr.
template<typename _Tp>
struct hash<shared_ptr<_Tp>>
: public __hash_base<size_t, shared_ptr<_Tp>>
{
size_t
operator()(const shared_ptr<_Tp>& __s) const noexcept
{ return std::hash<_Tp*>()(__s.get()); }
};
// @} group pointer_abstractions
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace
#endif // _SHARED_PTR_H
std::shared_ptr::shared_ptr
shared_ptr(nullptr_t __p, _Deleter __d, _Alloc __a)
Construct a shared_ptr that owns a null pointer and the deleter __d.
Definition: shared_ptr.h:184
std::shared_ptr::shared_ptr
shared_ptr(_Tp1 *__p, _Deleter __d)
Construct a shared_ptr that owns the pointer __p and the deleter __d.
Definition: shared_ptr.h:129
shared_ptr_base.h
std::make_shared
shared_ptr< _Tp > make_shared(_Args &&...__args)
Create an object that is owned by a shared_ptr.
Definition: shared_ptr.h:600
std::allocate_shared
shared_ptr< _Tp > allocate_shared(const _Alloc &__a, _Args &&...__args)
Create an object that is owned by a shared_ptr.
Definition: shared_ptr.h:585
std::enable_shared_from_this
Base class allowing use of member function shared_from_this.
Definition: shared_ptr.h:531
std::shared_ptr::shared_ptr
shared_ptr(_Tp1 *__p, _Deleter __d, _Alloc __a)
Construct a shared_ptr that owns the pointer __p and the deleter __d.
Definition: shared_ptr.h:165
std::shared_ptr::shared_ptr
shared_ptr(const shared_ptr< _Tp1 > &__r) noexcept
If __r is empty, constructs an empty shared_ptr; otherwise construct a shared_ptr that shares ownersh...
Definition: shared_ptr.h:218
std::unique_ptr
20.7.1.2 unique_ptr for single objects.
Definition: unique_ptr.h:129
std::shared_ptr::shared_ptr
shared_ptr(const weak_ptr< _Tp1 > &__r)
Constructs a shared_ptr that shares ownership with __r and stores a copy of the pointer stored in __r...
Definition: shared_ptr.h:248
std::auto_ptr
A simple smart pointer providing strict ownership semantics.
Definition: auto_ptr.h:87
std::forward
constexpr _Tp && forward(typename std::remove_reference< _Tp >::type &__t) noexcept
Forward an lvalue.
Definition: move.h:76
std::shared_ptr::shared_ptr
constexpr shared_ptr(nullptr_t __p) noexcept
Construct an empty shared_ptr.
Definition: shared_ptr.h:265
std::allocator
The standard allocator, as per [20.4].
Definition: allocator.h:92
std::shared_ptr::shared_ptr
shared_ptr(nullptr_t __p, _Deleter __d)
Construct a shared_ptr that owns a null pointer and the deleter __d.
Definition: shared_ptr.h:146
std::weak_ptr
A smart pointer with weak semantics.
Definition: shared_ptr.h:467
std::hash< _Tp * >
Partial specializations for pointer types.
Definition: functional_hash.h:62
std::shared_ptr::allocate_shared
friend shared_ptr< _Tp1 > allocate_shared(const _Alloc &__a, _Args &&...__args)
Create an object that is owned by a shared_ptr.
Definition: shared_ptr.h:585
std::basic_ostream
Template class basic_ostream.
Definition: iosfwd:86
std::hash
Primary class template hash.
Definition: system_error:115
std::less
One of the comparison functors.
Definition: stl_function.h:363
std::shared_ptr::shared_ptr
shared_ptr(const shared_ptr< _Tp1 > &__r, _Tp *__p) noexcept
Constructs a shared_ptr instance that stores __p and shares ownership with __r.
Definition: shared_ptr.h:206
std::shared_ptr
A smart pointer with reference-counted copy semantics.
Definition: shared_ptr.h:93
std::shared_ptr::shared_ptr
shared_ptr(shared_ptr< _Tp1 > &&__r) noexcept
Move-constructs a shared_ptr instance from __r.
Definition: shared_ptr.h:236
std::shared_ptr::shared_ptr
shared_ptr(shared_ptr &&__r) noexcept
Move-constructs a shared_ptr instance from __r.
Definition: shared_ptr.h:226
std::owner_less
Primary template owner_less.
Definition: shared_ptr.h:513
std::get_deleter
_Del * get_deleter(const __shared_ptr< _Tp, _Lp > &__p) noexcept
20.7.2.2.10 shared_ptr get_deleter
Definition: shared_ptr.h:76
std::shared_ptr::shared_ptr
constexpr shared_ptr() noexcept
Construct an empty shared_ptr.
Definition: shared_ptr.h:100