智能指针

shared_ptr是线程安全的吗？

Boost官网：shared_ptr_thread_safety

shared_ptr与内建类型（比如int, char；std::string不是内建类型）具有相同的线程安全等级：

• 同一个shared_ptr可以被并发读；
• 不同的shared_ptr可以被并发写（析构也是写）。
• 其他并发访问是未定义的。

Examples:

shared_ptr<int> p(new int(42));

Code Example 4. Reading a shared_ptr from two threads

// thread A
shared_ptr<int> p2(p); // reads p

// thread B
shared_ptr<int> p3(p); // OK, multiple reads are safe

读取同一个shared_ptr是线程安全的。
同时增加引用计数，安全。

Code Example 5. Writing different shared_ptr instances from two threads

// thread A
p.reset(new int(1912)); // writes p

// thread B
p2.reset(); // OK, writes p2

写不同的shared_ptr实例是线程安全的。
不同的实例各自更新自己的引用计数控制块指针，安全。
对共同掌管的资源的引用计数原子减1，安全。

Code Example 6. Reading and writing a shared_ptr from two threads

// thread A
p = p3; // reads p3, writes p

// thread B
p3.reset(); // writes p3; undefined, simultaneous read/write

同时读写同一个shared_ptr实例，是线程不安全的。
同时对引用计数进行增减，如果先看到减到0，那么可能资源已经被释放。

Code Example 7. Reading and destroying a shared_ptr from two threads

// thread A
p3 = p2; // reads p2, writes p3

// thread B
// p2 goes out of scope: undefined, the destructor is considered a "write access"

同时读写（析构也是写）同一个shared_ptr实例，是线程不安全的。

Code Example 8. Writing a shared_ptr from two threads

// thread A
p3.reset(new int(1));

// thread B
p3.reset(new int(2)); // undefined, multiple writes

同时写同一个shared_ptr，可能对引用计数操作多次（应该只能操作一次），线程不安全。

测试：模拟潜在竞态条件：

最后一个引用即将销毁时，另一个线程试图复制

示例1：复制构造和reset()竞争

race1.cppview raw

#include <chrono>
#include <iostream>
#include <memory>
#include <thread>

struct Test {
    Test() { std::cout << "Test constructed\n"; }
    ~Test() { std::cout << "Test destroyed\n"; }
};

void thread_copy(std::shared_ptr<Test>& ptr) {
    // 模拟延迟，试图在主线程销毁后复制
    std::this_thread::sleep_for(std::chrono::milliseconds(100));
    std::cout << "Thread attempting to copy shared_ptr\n";
    std::shared_ptr<Test> local_copy = ptr;  // 资源可能已经释放
    if (local_copy) {      // shared_ptr 还存在，所以可以用来检查资源是否有效
        std::cout << "Thread copied shared_ptr\n";
    } else {
        std::cout << "Thread failed to copy shared_ptr\n";
    }
}

int main() {
    std::shared_ptr<Test> ptr = std::make_shared<Test>();

    std::thread t(thread_copy, std::ref(ptr));

    // 主线程销毁 shared_ptr
    std::this_thread::sleep_for(std::chrono::milliseconds(50));
    ptr.reset();  // 引用计数降为 0，销毁对象

    t.join();
    return 0;
}

总结： reset() 与复制构造需要加锁保护。

示例2：复制构造和析构竞争

race2.cppview raw

#include <chrono>
#include <iostream>
#include <memory>
#include <thread>

struct Test {
    Test() { std::cout << "Test constructed\n"; }
    ~Test() { std::cout << "Test destroyed\n"; }
};

void thread_copy(std::shared_ptr<Test>& ptr) {
    // 模拟延迟，试图在主线程销毁后复制
    std::this_thread::sleep_for(std::chrono::milliseconds(100));
    std::cout << "Thread attempting to copy shared_ptr\n";
    std::shared_ptr<Test> local_copy = ptr;  // 如果 ptr 已经销毁，这里是未定义行为
    if (local_copy) {                        // 可能“成功”复制一个已经销毁的 shared_ptr
        std::cout << "Thread copied shared_ptr\n";
        std::cout << "Use count in thread: "
                  << local_copy.use_count() << "\n";  // 引用计数控制块是一个野指针
    } else {
        std::cout << "Thread failed to copy shared_ptr\n";
    }
}

int main() {
    std::thread t;

    {
        std::shared_ptr<Test> ptr = std::make_shared<Test>();

        t = std::thread(thread_copy, std::ref(ptr));

        // 主线程销毁 shared_ptr
        std::this_thread::sleep_for(std::chrono::milliseconds(50));

        // 析构，引用计数降为 0，销毁对象
    }

    t.join();
    return 0;
}

总结：

• 不能以左值引用来复制构造shared_ptr，否则会存在竞争。
• 可以使用const引用，因为可以延续shared_ptr的生命期，析构被推迟，间接消除了竞态。
• 程序中shared_ptr读写操作同时存在时，需要加锁。
• 要防止shared_ptr析构：在shared_ptr副本的整个作用域内加锁同样可以保护这种情况。
• C++20 开始支持 atomic<shared_ptr<T>>，可以安全地在多个线程之间共享和更新 shared_ptr。
• 如果 thread A 只是想观察 p2 是否还存在，可以使用 std::weak_ptr 来观察。

shared_ptr为什么要隐式自持一个weak count？

每个 shared_ptr 和 weak_ptr 都指向一个 控制块（control block），这个控制块通常包含：

名称	作用
strong count	当前活跃的 shared_ptr 数量
weak count	当前活跃的 weak_ptr 数量 + 当前活跃的 shared_ptr 数量
指向对象的指针	被管理的资源

控制块的生命周期由 weak count 决定，只要还有 weak_ptr 或 shared_ptr 存在，控制块就不能被销毁。

• weak_ptr和shared_ptr可能存在只有两者之一的情况，所以他们都必须要维护控制块的生命周期。
• 如果只有weak_ptr对weak count进行计数，那么可能出现shared_ptr和weak_ptr共同看到weak count为0的情况：
  • weak_ptr析构将weak count降低为0，准备释放控制块；
  • shared_ptr析构或reset，检测到weak count为0，准备释放控制块；
  • 这样就释放了两次，因为他们都持有指针，且没有设置相互通知的机制。
• 但是，如果weak_ptr和shared_ptr都对weak count进行计数，那么就不可能同时观察到weak count为0的情况。

动手实现一个shared_ptr

SimpleSharedPtr

SimpleSharedPtr.cppview raw

#include <iostream>
#include <atomic>
#include <functional>
#include <stdexcept>

template<typename T>
class SimpleSharedPtr;  // 先声明

template<typename T>
class SimpleWeakPtr;

template<typename T>
class ControlBlock {
public:
    std::atomic<int> shared_count;
    std::atomic<int> weak_count;
    T* ptr;
    std::function<void(T*)> deleter;

    // 每个shared_ptr自持一个weak count
    ControlBlock(T* p, std::function<void(T*)> d)
        : shared_count(1), weak_count(1), ptr(p), deleter(d ? d : [](T* p){ delete p; }) {}

    ~ControlBlock() = default;
};

template<typename T>
class SimpleSharedPtr {
private:
    ControlBlock<T>* ctrl;

    void release() {
        if (ctrl) {
            // 减shared_count
            if (ctrl->shared_count.fetch_sub(1, std::memory_order_acq_rel) == 1) {
                // 最后一个shared_ptr，释放资源
                try {
                    ctrl->deleter(ctrl->ptr);
                } catch (...) {
                    std::cerr << "Exception in deleter\n";
                }
                ctrl->ptr = nullptr;

                // 再减少weak_count，自己占有一个weak计数，减少它
                if (ctrl->weak_count.fetch_sub(1, std::memory_order_acq_rel) == 1) {
                    delete ctrl;
                }
            }
            ctrl = nullptr;
        }
    }

public:
    explicit SimpleSharedPtr(T* p = nullptr, std::function<void(T*)> d = nullptr) {
        if (p) {
            ctrl = new ControlBlock<T>(p, d);
            // shared_count=1, weak_count=0
        } else {
            ctrl = nullptr;
        }
    }

    // 拷贝构造
    SimpleSharedPtr(const SimpleSharedPtr& other) : ctrl(other.ctrl) {
        if (ctrl) {
            ctrl->shared_count.fetch_add(1, std::memory_order_relaxed);
        }
    }

    // 移动构造
    SimpleSharedPtr(SimpleSharedPtr&& other) noexcept : ctrl(other.ctrl) {
        other.ctrl = nullptr;
    }

    // 析构
    ~SimpleSharedPtr() {
        release();
    }

    // 拷贝赋值
    SimpleSharedPtr& operator=(const SimpleSharedPtr& other) {
        if (this != &other) {
            release();
            ctrl = other.ctrl;
            if (ctrl) {
                ctrl->shared_count.fetch_add(1, std::memory_order_relaxed);
            }
        }
        return *this;
    }

    // 移动赋值
    SimpleSharedPtr& operator=(SimpleSharedPtr&& other) noexcept {
        if (this != &other) {
            release();
            ctrl = other.ctrl;
            other.ctrl = nullptr;
        }
        return *this;
    }

    T* get() const {
        return ctrl ? ctrl->ptr : nullptr;
    }

    T& operator*() const {
        if (!get()) throw std::runtime_error("Dereferencing null pointer");
        return *(ctrl->ptr);
    }

    T* operator->() const {
        return get();
    }

    int use_count() const {
        return ctrl ? ctrl->shared_count.load(std::memory_order_relaxed) : 0;
    }

    bool unique() const {
        return use_count() == 1;
    }

    explicit operator bool() const {
        return get() != nullptr;
    }

    void reset(T* p = nullptr, std::function<void(T*)> d = nullptr) {
        release();
        if (p) {
            ctrl = new ControlBlock<T>(p, d);
        } else {
            ctrl = nullptr;
        }
    }

    // 给 SimpleWeakPtr 访问 ControlBlock 指针权限
    friend class SimpleWeakPtr<T>;

private:
    // SimpleSharedPtr增加一个私有构造，能从控制块构造shared_ptr（配合weak_ptr的lock）
    SimpleSharedPtr(ControlBlock<T>* c) : ctrl(c) {}
};

template<typename T>
class SimpleWeakPtr {
private:
    ControlBlock<T>* ctrl;

    void release() {
        if (ctrl) {
            if (ctrl->weak_count.fetch_sub(1, std::memory_order_acq_rel) == 1) {
                // 只有弱引用且没有shared_ptr存在时，销毁控制块
                if (ctrl->shared_count.load(std::memory_order_acquire) == 0) {
                    delete ctrl;
                }
            }
            ctrl = nullptr;
        }
    }

public:
    SimpleWeakPtr() : ctrl(nullptr) {}

    // 从 shared_ptr 构造 weak_ptr
    SimpleWeakPtr(const SimpleSharedPtr<T>& shared) : ctrl(shared.ctrl) {
        if (ctrl) {
            ctrl->weak_count.fetch_add(1, std::memory_order_relaxed);
        }
    }

    // 拷贝构造
    SimpleWeakPtr(const SimpleWeakPtr& other) : ctrl(other.ctrl) {
        if (ctrl) {
            ctrl->weak_count.fetch_add(1, std::memory_order_relaxed);
        }
    }

    // 移动构造
    SimpleWeakPtr(SimpleWeakPtr&& other) noexcept : ctrl(other.ctrl) {
        other.ctrl = nullptr;
    }

    ~SimpleWeakPtr() {
        release();
    }

    // 拷贝赋值
    SimpleWeakPtr& operator=(const SimpleWeakPtr& other) {
        if (this != &other) {
            release();
            ctrl = other.ctrl;
            if (ctrl) {
                ctrl->weak_count.fetch_add(1, std::memory_order_relaxed);
            }
        }
        return *this;
    }

    // 移动赋值
    SimpleWeakPtr& operator=(SimpleWeakPtr&& other) noexcept {
        if (this != &other) {
            release();
            ctrl = other.ctrl;
            other.ctrl = nullptr;
        }
        return *this;
    }

    int use_count() const {
        return ctrl ? ctrl->shared_count.load(std::memory_order_relaxed) : 0;
    }

    bool expired() const {
        return use_count() == 0;
    }

    // 尝试获取shared_ptr，资源已被销毁则返回空shared_ptr
    SimpleSharedPtr<T> lock() const {
        if (expired()) {
            return SimpleSharedPtr<T>();
        } else {
            // 尝试增加shared_count
            int old_count = ctrl->shared_count.load(std::memory_order_relaxed);
            while (old_count != 0) {
                if (ctrl->shared_count.compare_exchange_weak(old_count, old_count + 1,
                    std::memory_order_acq_rel, std::memory_order_relaxed)) {
                    // 成功增加shared_count
                    return SimpleSharedPtr<T>(ctrl);
                }
                // old_count 已被更新，继续循环
            }
            return SimpleSharedPtr<T>();
        }
    }

private:
    // 让 SimpleSharedPtr 可以通过控制块构造 shared_ptr
    friend class SimpleSharedPtr<T>;

    explicit SimpleWeakPtr(ControlBlock<T>* c) : ctrl(c) {}
};


int main() {
    SimpleSharedPtr<int> sp1(new int(42));
    SimpleWeakPtr<int> wp1 = sp1;

    std::cout << "use_count: " << sp1.use_count() << "\n"; // 1
    std::cout << "expired: " << wp1.expired() << "\n";    // 0

    if (auto sp2 = wp1.lock()) {
        std::cout << "locked value: " << *sp2 << "\n";    // 42
        std::cout << "use_count after lock: " << sp2.use_count() << "\n"; // 2
    }

    sp1.reset();

    std::cout << "expired after reset: " << wp1.expired() << "\n"; // 1

    if (auto sp3 = wp1.lock()) {
        std::cout << "Should not print\n";
    } else {
        std::cout << "lock failed, resource expired\n";
    }

    return 0;
}

附录：shared_ptr源码

• /usr/include/c++/11/bits/shared_ptr.h
• /usr/include/c++/11/bits/shared_ptr_base.h
• /usr/include/c++/11/bits/shared_ptr_atomic.h

shared_ptr.h

shared_ptr.hview raw

// shared_ptr and weak_ptr implementation -*- C++ -*-

// Copyright (C) 2007-2021 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
 *  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 <iosfwd>           	  // std::basic_ostream
#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

  /// Write the stored pointer to an ostream.
  /// @relates shared_ptr
  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;
    }

  template<typename _Del, typename _Tp, _Lock_policy _Lp>
    inline _Del*
    get_deleter(const __shared_ptr<_Tp, _Lp>& __p) noexcept
    {
#if __cpp_rtti
      return static_cast<_Del*>(__p._M_get_deleter(typeid(_Del)));
#else
      return 0;
#endif
    }

  /// 20.7.2.2.10 shared_ptr get_deleter

  /// If `__p` has a deleter of type `_Del`, return a pointer to it.
  /// @relates shared_ptr
  template<typename _Del, typename _Tp>
    inline _Del*
    get_deleter(const shared_ptr<_Tp>& __p) noexcept
    {
#if __cpp_rtti
      return static_cast<_Del*>(__p._M_get_deleter(typeid(_Del)));
#else
      return 0;
#endif
    }

  /**
   *  @brief  A smart pointer with reference-counted copy semantics.
   *
   * A `shared_ptr` object is either empty or _owns_ a pointer passed
   * to the constructor. Copies of a `shared_ptr` share ownership of
   * the same pointer. When the last `shared_ptr` that owns the pointer
   * is destroyed or reset, the owned pointer is freed (either by `delete`
   * or by invoking a custom deleter that was passed to the constructor).
   *
   * A `shared_ptr` also stores another pointer, which is usually
   * (but not always) the same pointer as it owns. The stored pointer
   * can be retrieved by calling the `get()` member function.
   *
   * The equality and relational operators for `shared_ptr` only compare
   * the stored pointer returned by `get()`, not the owned pointer.
   * To test whether two `shared_ptr` objects share ownership of the same
   * pointer see `std::shared_ptr::owner_before` and `std::owner_less`.
  */
  template<typename _Tp>
    class shared_ptr : public __shared_ptr<_Tp>
    {
      template<typename... _Args>
	using _Constructible = typename enable_if<
	  is_constructible<__shared_ptr<_Tp>, _Args...>::value
	>::type;

      template<typename _Arg>
	using _Assignable = typename enable_if<
	  is_assignable<__shared_ptr<_Tp>&, _Arg>::value, shared_ptr&
	>::type;

    public:

      /// The type pointed to by the stored pointer, remove_extent_t<_Tp>
      using element_type = typename __shared_ptr<_Tp>::element_type;

#if __cplusplus >= 201703L
# define __cpp_lib_shared_ptr_weak_type 201606
      /// The corresponding weak_ptr type for this shared_ptr
      using weak_type = weak_ptr<_Tp>;
#endif
      /**
       *  @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; ///< Copy constructor

      /**
       *  @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 _Yp, typename = _Constructible<_Yp*>>
	explicit
	shared_ptr(_Yp* __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 _Yp, typename _Deleter,
	       typename = _Constructible<_Yp*, _Deleter>>
	shared_ptr(_Yp* __p, _Deleter __d)
        : __shared_ptr<_Tp>(__p, std::move(__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, std::move(__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 _Yp, typename _Deleter, typename _Alloc,
	       typename = _Constructible<_Yp*, _Deleter, _Alloc>>
	shared_ptr(_Yp* __p, _Deleter __d, _Alloc __a)
	: __shared_ptr<_Tp>(__p, std::move(__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, std::move(__d), std::move(__a)) { }

      // Aliasing constructor

      /**
       *  @brief  Constructs a `shared_ptr` instance that stores `__p`
       *          and shares ownership with `__r`.
       *  @param  __r  A `shared_ptr`.
       *  @param  __p  A pointer that will remain valid while `*__r` is valid.
       *  @post   `get() == __p && use_count() == __r.use_count()`
       *
       *  This can be used to construct a `shared_ptr` to a sub-object
       *  of an object managed by an existing `shared_ptr`. The complete
       *  object will remain valid while any `shared_ptr` owns it, even
       *  if they don't store a pointer to the complete object.
       *
       * @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 _Yp>
	shared_ptr(const shared_ptr<_Yp>& __r, element_type* __p) noexcept
	: __shared_ptr<_Tp>(__r, __p) { }

#if __cplusplus > 201703L
      // _GLIBCXX_RESOLVE_LIB_DEFECTS
      // 2996. Missing rvalue overloads for shared_ptr operations
      /**
       *  @brief  Constructs a `shared_ptr` instance that stores `__p`
       *          and shares ownership with `__r`.
       *  @param  __r  A `shared_ptr`.
       *  @param  __p  A pointer that will remain valid while `*__r` is valid.
       *  @post   `get() == __p && !__r.use_count() && !__r.get()`
       *
       *  This can be used to construct a `shared_ptr` to a sub-object
       *  of an object managed by an existing `shared_ptr`. The complete
       *  object will remain valid while any `shared_ptr` owns it, even
       *  if they don't store a pointer to the complete object.
       *
       * @code
       * shared_ptr<pair<int,int>> pii(new pair<int,int>());
       * shared_ptr<int> pi1(pii, &pii->first);
       * assert(pii.use_count() == 2);
       * shared_ptr<int> pi2(std::move(pii), &pii->second);
       * assert(pii.use_count() == 0);
       * @endcode
       */
      template<typename _Yp>
	shared_ptr(shared_ptr<_Yp>&& __r, element_type* __p) noexcept
	: __shared_ptr<_Tp>(std::move(__r), __p) { }
#endif
      /**
       *  @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 _Yp,
	       typename = _Constructible<const shared_ptr<_Yp>&>>
	shared_ptr(const shared_ptr<_Yp>& __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 _Yp, typename = _Constructible<shared_ptr<_Yp>>>
	shared_ptr(shared_ptr<_Yp>&& __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 _Yp, typename = _Constructible<const weak_ptr<_Yp>&>>
	explicit shared_ptr(const weak_ptr<_Yp>& __r)
	: __shared_ptr<_Tp>(__r) { }

#if _GLIBCXX_USE_DEPRECATED
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
      template<typename _Yp, typename = _Constructible<auto_ptr<_Yp>>>
	shared_ptr(auto_ptr<_Yp>&& __r);
#pragma GCC diagnostic pop
#endif

      // _GLIBCXX_RESOLVE_LIB_DEFECTS
      // 2399. shared_ptr's constructor from unique_ptr should be constrained
      template<typename _Yp, typename _Del,
	       typename = _Constructible<unique_ptr<_Yp, _Del>>>
	shared_ptr(unique_ptr<_Yp, _Del>&& __r)
	: __shared_ptr<_Tp>(std::move(__r)) { }

#if __cplusplus <= 201402L && _GLIBCXX_USE_DEPRECATED
      // This non-standard constructor exists to support conversions that
      // were possible in C++11 and C++14 but are ill-formed in C++17.
      // If an exception is thrown this constructor has no effect.
      template<typename _Yp, typename _Del,
		_Constructible<unique_ptr<_Yp, _Del>, __sp_array_delete>* = 0>
	shared_ptr(unique_ptr<_Yp, _Del>&& __r)
	: __shared_ptr<_Tp>(std::move(__r), __sp_array_delete()) { }
#endif

      /**
       *  @brief  Construct an empty %shared_ptr.
       *  @post   use_count() == 0 && get() == nullptr
       */
      constexpr shared_ptr(nullptr_t) noexcept : shared_ptr() { }

      shared_ptr& operator=(const shared_ptr&) noexcept = default;

      template<typename _Yp>
	_Assignable<const shared_ptr<_Yp>&>
	operator=(const shared_ptr<_Yp>& __r) noexcept
	{
	  this->__shared_ptr<_Tp>::operator=(__r);
	  return *this;
	}

#if _GLIBCXX_USE_DEPRECATED
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
      template<typename _Yp>
	_Assignable<auto_ptr<_Yp>>
	operator=(auto_ptr<_Yp>&& __r)
	{
	  this->__shared_ptr<_Tp>::operator=(std::move(__r));
	  return *this;
	}
#pragma GCC diagnostic pop
#endif

      shared_ptr&
      operator=(shared_ptr&& __r) noexcept
      {
	this->__shared_ptr<_Tp>::operator=(std::move(__r));
	return *this;
      }

      template<class _Yp>
	_Assignable<shared_ptr<_Yp>>
	operator=(shared_ptr<_Yp>&& __r) noexcept
	{
	  this->__shared_ptr<_Tp>::operator=(std::move(__r));
	  return *this;
	}

      template<typename _Yp, typename _Del>
	_Assignable<unique_ptr<_Yp, _Del>>
	operator=(unique_ptr<_Yp, _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_alloc_shared_tag<_Alloc> __tag, _Args&&... __args)
	: __shared_ptr<_Tp>(__tag, std::forward<_Args>(__args)...)
	{ }

      template<typename _Yp, typename _Alloc, typename... _Args>
	friend shared_ptr<_Yp>
	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) noexcept
      : __shared_ptr<_Tp>(__r, std::nothrow) { }

      friend class weak_ptr<_Tp>;
    };

#if __cpp_deduction_guides >= 201606
  template<typename _Tp>
    shared_ptr(weak_ptr<_Tp>) ->  shared_ptr<_Tp>;
  template<typename _Tp, typename _Del>
    shared_ptr(unique_ptr<_Tp, _Del>) ->  shared_ptr<_Tp>;
#endif

  // 20.7.2.2.7 shared_ptr comparisons

  /// @relates shared_ptr @{

  /// Equality operator for shared_ptr objects, compares the stored pointers
  template<typename _Tp, typename _Up>
    _GLIBCXX_NODISCARD inline bool
    operator==(const shared_ptr<_Tp>& __a, const shared_ptr<_Up>& __b) noexcept
    { return __a.get() == __b.get(); }

  /// shared_ptr comparison with nullptr
  template<typename _Tp>
    _GLIBCXX_NODISCARD inline bool
    operator==(const shared_ptr<_Tp>& __a, nullptr_t) noexcept
    { return !__a; }

#ifdef __cpp_lib_three_way_comparison
  template<typename _Tp, typename _Up>
    inline strong_ordering
    operator<=>(const shared_ptr<_Tp>& __a,
		const shared_ptr<_Up>& __b) noexcept
    { return compare_three_way()(__a.get(), __b.get()); }

  template<typename _Tp>
    inline strong_ordering
    operator<=>(const shared_ptr<_Tp>& __a, nullptr_t) noexcept
    {
      using pointer = typename shared_ptr<_Tp>::element_type*;
      return compare_three_way()(__a.get(), static_cast<pointer>(nullptr));
    }
#else
  /// shared_ptr comparison with nullptr
  template<typename _Tp>
    _GLIBCXX_NODISCARD inline bool
    operator==(nullptr_t, const shared_ptr<_Tp>& __a) noexcept
    { return !__a; }

  /// Inequality operator for shared_ptr objects, compares the stored pointers
  template<typename _Tp, typename _Up>
    _GLIBCXX_NODISCARD inline bool
    operator!=(const shared_ptr<_Tp>& __a, const shared_ptr<_Up>& __b) noexcept
    { return __a.get() != __b.get(); }

  /// shared_ptr comparison with nullptr
  template<typename _Tp>
    _GLIBCXX_NODISCARD inline bool
    operator!=(const shared_ptr<_Tp>& __a, nullptr_t) noexcept
    { return (bool)__a; }

  /// shared_ptr comparison with nullptr
  template<typename _Tp>
    _GLIBCXX_NODISCARD inline bool
    operator!=(nullptr_t, const shared_ptr<_Tp>& __a) noexcept
    { return (bool)__a; }

  /// Relational operator for shared_ptr objects, compares the stored pointers
  template<typename _Tp, typename _Up>
    _GLIBCXX_NODISCARD inline bool
    operator<(const shared_ptr<_Tp>& __a, const shared_ptr<_Up>& __b) noexcept
    {
      using _Tp_elt = typename shared_ptr<_Tp>::element_type;
      using _Up_elt = typename shared_ptr<_Up>::element_type;
      using _Vp = typename common_type<_Tp_elt*, _Up_elt*>::type;
      return less<_Vp>()(__a.get(), __b.get());
    }

  /// shared_ptr comparison with nullptr
  template<typename _Tp>
    _GLIBCXX_NODISCARD inline bool
    operator<(const shared_ptr<_Tp>& __a, nullptr_t) noexcept
    {
      using _Tp_elt = typename shared_ptr<_Tp>::element_type;
      return less<_Tp_elt*>()(__a.get(), nullptr);
    }

  /// shared_ptr comparison with nullptr
  template<typename _Tp>
    _GLIBCXX_NODISCARD inline bool
    operator<(nullptr_t, const shared_ptr<_Tp>& __a) noexcept
    {
      using _Tp_elt = typename shared_ptr<_Tp>::element_type;
      return less<_Tp_elt*>()(nullptr, __a.get());
    }

  /// Relational operator for shared_ptr objects, compares the stored pointers
  template<typename _Tp, typename _Up>
    _GLIBCXX_NODISCARD inline bool
    operator<=(const shared_ptr<_Tp>& __a, const shared_ptr<_Up>& __b) noexcept
    { return !(__b < __a); }

  /// shared_ptr comparison with nullptr
  template<typename _Tp>
    _GLIBCXX_NODISCARD inline bool
    operator<=(const shared_ptr<_Tp>& __a, nullptr_t) noexcept
    { return !(nullptr < __a); }

  /// shared_ptr comparison with nullptr
  template<typename _Tp>
    _GLIBCXX_NODISCARD inline bool
    operator<=(nullptr_t, const shared_ptr<_Tp>& __a) noexcept
    { return !(__a < nullptr); }

  /// Relational operator for shared_ptr objects, compares the stored pointers
  template<typename _Tp, typename _Up>
    _GLIBCXX_NODISCARD inline bool
    operator>(const shared_ptr<_Tp>& __a, const shared_ptr<_Up>& __b) noexcept
    { return (__b < __a); }

  /// shared_ptr comparison with nullptr
  template<typename _Tp>
    _GLIBCXX_NODISCARD inline bool
    operator>(const shared_ptr<_Tp>& __a, nullptr_t) noexcept
    { return nullptr < __a; }

  /// shared_ptr comparison with nullptr
  template<typename _Tp>
    _GLIBCXX_NODISCARD inline bool
    operator>(nullptr_t, const shared_ptr<_Tp>& __a) noexcept
    { return __a < nullptr; }

  /// Relational operator for shared_ptr objects, compares the stored pointers
  template<typename _Tp, typename _Up>
    _GLIBCXX_NODISCARD inline bool
    operator>=(const shared_ptr<_Tp>& __a, const shared_ptr<_Up>& __b) noexcept
    { return !(__a < __b); }

  /// shared_ptr comparison with nullptr
  template<typename _Tp>
    _GLIBCXX_NODISCARD inline bool
    operator>=(const shared_ptr<_Tp>& __a, nullptr_t) noexcept
    { return !(__a < nullptr); }

  /// shared_ptr comparison with nullptr
  template<typename _Tp>
    _GLIBCXX_NODISCARD inline bool
    operator>=(nullptr_t, const shared_ptr<_Tp>& __a) noexcept
    { return !(nullptr < __a); }
#endif

  // 20.7.2.2.8 shared_ptr specialized algorithms.

  /// Swap overload for shared_ptr
  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.

  /// Convert type of `shared_ptr`, via `static_cast`
  template<typename _Tp, typename _Up>
    inline shared_ptr<_Tp>
    static_pointer_cast(const shared_ptr<_Up>& __r) noexcept
    {
      using _Sp = shared_ptr<_Tp>;
      return _Sp(__r, static_cast<typename _Sp::element_type*>(__r.get()));
    }

  /// Convert type of `shared_ptr`, via `const_cast`
  template<typename _Tp, typename _Up>
    inline shared_ptr<_Tp>
    const_pointer_cast(const shared_ptr<_Up>& __r) noexcept
    {
      using _Sp = shared_ptr<_Tp>;
      return _Sp(__r, const_cast<typename _Sp::element_type*>(__r.get()));
    }

  /// Convert type of `shared_ptr`, via `dynamic_cast`
  template<typename _Tp, typename _Up>
    inline shared_ptr<_Tp>
    dynamic_pointer_cast(const shared_ptr<_Up>& __r) noexcept
    {
      using _Sp = shared_ptr<_Tp>;
      if (auto* __p = dynamic_cast<typename _Sp::element_type*>(__r.get()))
	return _Sp(__r, __p);
      return _Sp();
    }

#if __cplusplus >= 201703L
  /// Convert type of `shared_ptr`, via `reinterpret_cast`
  template<typename _Tp, typename _Up>
    inline shared_ptr<_Tp>
    reinterpret_pointer_cast(const shared_ptr<_Up>& __r) noexcept
    {
      using _Sp = shared_ptr<_Tp>;
      return _Sp(__r, reinterpret_cast<typename _Sp::element_type*>(__r.get()));
    }

#if __cplusplus > 201703L
  // _GLIBCXX_RESOLVE_LIB_DEFECTS
  // 2996. Missing rvalue overloads for shared_ptr operations

  /// Convert type of `shared_ptr` rvalue, via `static_cast`
  template<typename _Tp, typename _Up>
    inline shared_ptr<_Tp>
    static_pointer_cast(shared_ptr<_Up>&& __r) noexcept
    {
      using _Sp = shared_ptr<_Tp>;
      return _Sp(std::move(__r),
		 static_cast<typename _Sp::element_type*>(__r.get()));
    }

  /// Convert type of `shared_ptr` rvalue, via `const_cast`
  template<typename _Tp, typename _Up>
    inline shared_ptr<_Tp>
    const_pointer_cast(shared_ptr<_Up>&& __r) noexcept
    {
      using _Sp = shared_ptr<_Tp>;
      return _Sp(std::move(__r),
		 const_cast<typename _Sp::element_type*>(__r.get()));
    }

  /// Convert type of `shared_ptr` rvalue, via `dynamic_cast`
  template<typename _Tp, typename _Up>
    inline shared_ptr<_Tp>
    dynamic_pointer_cast(shared_ptr<_Up>&& __r) noexcept
    {
      using _Sp = shared_ptr<_Tp>;
      if (auto* __p = dynamic_cast<typename _Sp::element_type*>(__r.get()))
	return _Sp(std::move(__r), __p);
      return _Sp();
    }

  /// Convert type of `shared_ptr` rvalue, via `reinterpret_cast`
  template<typename _Tp, typename _Up>
    inline shared_ptr<_Tp>
    reinterpret_pointer_cast(shared_ptr<_Up>&& __r) noexcept
    {
      using _Sp = shared_ptr<_Tp>;
      return _Sp(std::move(__r),
		 reinterpret_cast<typename _Sp::element_type*>(__r.get()));
    }
#endif // C++20
#endif // C++17

  /// @}

  /**
   * @brief  A non-owning observer for a pointer owned by a shared_ptr
   *
   * A weak_ptr provides a safe alternative to a raw pointer when you want
   * a non-owning reference to an object that is managed by a shared_ptr.
   *
   * Unlike a raw pointer, a weak_ptr can be converted to a new shared_ptr
   * that shares ownership with every other shared_ptr that already owns
   * the pointer. In other words you can upgrade from a non-owning "weak"
   * reference to an owning shared_ptr, without having access to any of
   * the existing shared_ptr objects.
   *
   * Also unlike a raw pointer, a weak_ptr does not become "dangling" after
   * the object it points to has been destroyed. Instead, a weak_ptr
   * becomes _expired_ and can no longer be converted to a shared_ptr that
   * owns the freed pointer, so you cannot accidentally access the pointed-to
   * object after it has been destroyed.
   */
  template<typename _Tp>
    class weak_ptr : public __weak_ptr<_Tp>
    {
      template<typename _Arg>
	using _Constructible = typename enable_if<
	  is_constructible<__weak_ptr<_Tp>, _Arg>::value
	>::type;

      template<typename _Arg>
	using _Assignable = typename enable_if<
	  is_assignable<__weak_ptr<_Tp>&, _Arg>::value, weak_ptr&
	>::type;

    public:
      constexpr weak_ptr() noexcept = default;

      template<typename _Yp,
	       typename = _Constructible<const shared_ptr<_Yp>&>>
	weak_ptr(const shared_ptr<_Yp>& __r) noexcept
	: __weak_ptr<_Tp>(__r) { }

      weak_ptr(const weak_ptr&) noexcept = default;

      template<typename _Yp, typename = _Constructible<const weak_ptr<_Yp>&>>
	weak_ptr(const weak_ptr<_Yp>& __r) noexcept
	: __weak_ptr<_Tp>(__r) { }

      weak_ptr(weak_ptr&&) noexcept = default;

      template<typename _Yp, typename = _Constructible<weak_ptr<_Yp>>>
	weak_ptr(weak_ptr<_Yp>&& __r) noexcept
	: __weak_ptr<_Tp>(std::move(__r)) { }

      weak_ptr&
      operator=(const weak_ptr& __r) noexcept = default;

      template<typename _Yp>
	_Assignable<const weak_ptr<_Yp>&>
	operator=(const weak_ptr<_Yp>& __r) noexcept
	{
	  this->__weak_ptr<_Tp>::operator=(__r);
	  return *this;
	}

      template<typename _Yp>
	_Assignable<const shared_ptr<_Yp>&>
	operator=(const shared_ptr<_Yp>& __r) noexcept
	{
	  this->__weak_ptr<_Tp>::operator=(__r);
	  return *this;
	}

      weak_ptr&
      operator=(weak_ptr&& __r) noexcept = default;

      template<typename _Yp>
	_Assignable<weak_ptr<_Yp>>
	operator=(weak_ptr<_Yp>&& __r) noexcept
	{
	  this->__weak_ptr<_Tp>::operator=(std::move(__r));
	  return *this;
	}

      shared_ptr<_Tp>
      lock() const noexcept
      { return shared_ptr<_Tp>(*this, std::nothrow); }
    };

#if __cpp_deduction_guides >= 201606
  template<typename _Tp>
    weak_ptr(shared_ptr<_Tp>) ->  weak_ptr<_Tp>;
#endif

  // 20.7.2.3.6 weak_ptr specialized algorithms.
  /// Swap overload for weak_ptr
  /// @relates weak_ptr
  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 = void>
    struct owner_less;

  /// Void specialization of owner_less compares either shared_ptr or weak_ptr
  template<>
    struct owner_less<void> : _Sp_owner_less<void, void>
    { };

  /// 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); }

#if __cplusplus > 201402L || !defined(__STRICT_ANSI__) // c++1z or gnu++11
#define __cpp_lib_enable_shared_from_this 201603
      weak_ptr<_Tp>
      weak_from_this() noexcept
      { return this->_M_weak_this; }

      weak_ptr<const _Tp>
      weak_from_this() const noexcept
      { return this->_M_weak_this; }
#endif

    private:
      template<typename _Tp1>
	void
	_M_weak_assign(_Tp1* __p, const __shared_count<>& __n) const noexcept
	{ _M_weak_this._M_assign(__p, __n); }

      // Found by ADL when this is an associated class.
      friend const enable_shared_from_this*
      __enable_shared_from_this_base(const __shared_count<>&,
				     const enable_shared_from_this* __p)
      { return __p; }

      template<typename, _Lock_policy>
	friend class __shared_ptr;

      mutable weak_ptr<_Tp>  _M_weak_this;
    };

  /// @relates shared_ptr @{

  /**
   *  @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)
    {
      static_assert(!is_array<_Tp>::value, "make_shared<T[]> not supported");

      return shared_ptr<_Tp>(_Sp_alloc_shared_tag<_Alloc>{__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_cv<_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<typename shared_ptr<_Tp>::element_type*>()(__s.get());
      }
    };

  /// @} relates shared_ptr
  /// @} group pointer_abstractions

#if __cplusplus >= 201703L
  namespace __detail::__variant
  {
    template<typename> struct _Never_valueless_alt; // see <variant>

    // Provide the strong exception-safety guarantee when emplacing a
    // shared_ptr into a variant.
    template<typename _Tp>
      struct _Never_valueless_alt<std::shared_ptr<_Tp>>
      : std::true_type
      { };

    // Provide the strong exception-safety guarantee when emplacing a
    // weak_ptr into a variant.
    template<typename _Tp>
      struct _Never_valueless_alt<std::weak_ptr<_Tp>>
      : std::true_type
      { };
  }  // namespace __detail::__variant
#endif // C++17

_GLIBCXX_END_NAMESPACE_VERSION
} // namespace

#endif // _SHARED_PTR_H

shared_ptr_base.h

shared_ptr_base.hview raw

// shared_ptr and weak_ptr implementation details -*- C++ -*-

// Copyright (C) 2007-2021 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_base.h
 *  This is an internal header file, included by other library headers.
 *  Do not attempt to use it directly. @headername{memory}
 */

#ifndef _SHARED_PTR_BASE_H
#define _SHARED_PTR_BASE_H 1

#include <typeinfo>
#include <bits/allocated_ptr.h>
#include <bits/allocator.h>
#include <bits/exception_defines.h>
#include <bits/functional_hash.h>
#include <bits/refwrap.h>
#include <bits/stl_function.h>  // std::less
#include <bits/unique_ptr.h>
#include <ext/aligned_buffer.h>
#include <ext/atomicity.h>
#include <ext/concurrence.h>
#if __cplusplus > 201703L
# include <compare>
#endif

namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION

#if _GLIBCXX_USE_DEPRECATED
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
  template<typename> class auto_ptr;
#pragma GCC diagnostic pop
#endif

 /**
   *  @brief  Exception possibly thrown by @c shared_ptr.
   *  @ingroup exceptions
   */
  class bad_weak_ptr : public std::exception
  {
  public:
    virtual char const* what() const noexcept;

    virtual ~bad_weak_ptr() noexcept;
  };

  // Substitute for bad_weak_ptr object in the case of -fno-exceptions.
  inline void
  __throw_bad_weak_ptr()
  { _GLIBCXX_THROW_OR_ABORT(bad_weak_ptr()); }

  using __gnu_cxx::_Lock_policy;
  using __gnu_cxx::__default_lock_policy;
  using __gnu_cxx::_S_single;
  using __gnu_cxx::_S_mutex;
  using __gnu_cxx::_S_atomic;

  // Empty helper class except when the template argument is _S_mutex.
  template<_Lock_policy _Lp>
    class _Mutex_base
    {
    protected:
      // The atomic policy uses fully-fenced builtins, single doesn't care.
      enum { _S_need_barriers = 0 };
    };

  template<>
    class _Mutex_base<_S_mutex>
    : public __gnu_cxx::__mutex
    {
    protected:
      // This policy is used when atomic builtins are not available.
      // The replacement atomic operations might not have the necessary
      // memory barriers.
      enum { _S_need_barriers = 1 };
    };

  template<_Lock_policy _Lp = __default_lock_policy>
    class _Sp_counted_base
    : public _Mutex_base<_Lp>
    {
    public:
      _Sp_counted_base() noexcept
      : _M_use_count(1), _M_weak_count(1) { }

      virtual
      ~_Sp_counted_base() noexcept
      { }

      // Called when _M_use_count drops to zero, to release the resources
      // managed by *this.
      virtual void
      _M_dispose() noexcept = 0;

      // Called when _M_weak_count drops to zero.
      virtual void
      _M_destroy() noexcept
      { delete this; }

      virtual void*
      _M_get_deleter(const std::type_info&) noexcept = 0;

      void
      _M_add_ref_copy()
      { __gnu_cxx::__atomic_add_dispatch(&_M_use_count, 1); }

      void
      _M_add_ref_lock()
      {
	if (!_M_add_ref_lock_nothrow())
	  __throw_bad_weak_ptr();
      }

      bool
      _M_add_ref_lock_nothrow() noexcept;

      void
      _M_release() noexcept
      {
        // Be race-detector-friendly.  For more info see bits/c++config.
        _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&_M_use_count);
	if (__gnu_cxx::__exchange_and_add_dispatch(&_M_use_count, -1) == 1)
	  {
            _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_use_count);
	    _M_dispose();
	    // There must be a memory barrier between dispose() and destroy()
	    // to ensure that the effects of dispose() are observed in the
	    // thread that runs destroy().
	    // See http://gcc.gnu.org/ml/libstdc++/2005-11/msg00136.html
	    if (_Mutex_base<_Lp>::_S_need_barriers)
	      {
		__atomic_thread_fence (__ATOMIC_ACQ_REL);
	      }

            // Be race-detector-friendly.  For more info see bits/c++config.
            _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&_M_weak_count);
	    if (__gnu_cxx::__exchange_and_add_dispatch(&_M_weak_count,
						       -1) == 1)
              {
                _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_weak_count);
	        _M_destroy();
              }
	  }
      }

      void
      _M_weak_add_ref() noexcept
      { __gnu_cxx::__atomic_add_dispatch(&_M_weak_count, 1); }

      void
      _M_weak_release() noexcept
      {
        // Be race-detector-friendly. For more info see bits/c++config.
        _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&_M_weak_count);
	if (__gnu_cxx::__exchange_and_add_dispatch(&_M_weak_count, -1) == 1)
	  {
            _GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_M_weak_count);
	    if (_Mutex_base<_Lp>::_S_need_barriers)
	      {
	        // See _M_release(),
	        // destroy() must observe results of dispose()
		__atomic_thread_fence (__ATOMIC_ACQ_REL);
	      }
	    _M_destroy();
	  }
      }

      long
      _M_get_use_count() const noexcept
      {
        // No memory barrier is used here so there is no synchronization
        // with other threads.
        return __atomic_load_n(&_M_use_count, __ATOMIC_RELAXED);
      }

    private:
      _Sp_counted_base(_Sp_counted_base const&) = delete;
      _Sp_counted_base& operator=(_Sp_counted_base const&) = delete;

      _Atomic_word  _M_use_count;     // #shared
      _Atomic_word  _M_weak_count;    // #weak + (#shared != 0)
    };

  template<>
    inline bool
    _Sp_counted_base<_S_single>::
    _M_add_ref_lock_nothrow() noexcept
    {
      if (_M_use_count == 0)
	return false;
      ++_M_use_count;
      return true;
    }

  template<>
    inline bool
    _Sp_counted_base<_S_mutex>::
    _M_add_ref_lock_nothrow() noexcept
    {
      __gnu_cxx::__scoped_lock sentry(*this);
      if (__gnu_cxx::__exchange_and_add_dispatch(&_M_use_count, 1) == 0)
	{
	  _M_use_count = 0;
	  return false;
	}
      return true;
    }

  template<>
    inline bool
    _Sp_counted_base<_S_atomic>::
    _M_add_ref_lock_nothrow() noexcept
    {
      // Perform lock-free add-if-not-zero operation.
      _Atomic_word __count = _M_get_use_count();
      do
	{
	  if (__count == 0)
	    return false;
	  // Replace the current counter value with the old value + 1, as
	  // long as it's not changed meanwhile.
	}
      while (!__atomic_compare_exchange_n(&_M_use_count, &__count, __count + 1,
					  true, __ATOMIC_ACQ_REL,
					  __ATOMIC_RELAXED));
      return true;
    }

  template<>
    inline void
    _Sp_counted_base<_S_single>::_M_add_ref_copy()
    { ++_M_use_count; }

  template<>
    inline void
    _Sp_counted_base<_S_single>::_M_release() noexcept
    {
      if (--_M_use_count == 0)
        {
          _M_dispose();
          if (--_M_weak_count == 0)
            _M_destroy();
        }
    }

  template<>
    inline void
    _Sp_counted_base<_S_single>::_M_weak_add_ref() noexcept
    { ++_M_weak_count; }

  template<>
    inline void
    _Sp_counted_base<_S_single>::_M_weak_release() noexcept
    {
      if (--_M_weak_count == 0)
        _M_destroy();
    }

  template<>
    inline long
    _Sp_counted_base<_S_single>::_M_get_use_count() const noexcept
    { return _M_use_count; }


  // Forward declarations.
  template<typename _Tp, _Lock_policy _Lp = __default_lock_policy>
    class __shared_ptr;

  template<typename _Tp, _Lock_policy _Lp = __default_lock_policy>
    class __weak_ptr;

  template<typename _Tp, _Lock_policy _Lp = __default_lock_policy>
    class __enable_shared_from_this;

  template<typename _Tp>
    class shared_ptr;

  template<typename _Tp>
    class weak_ptr;

  template<typename _Tp>
    struct owner_less;

  template<typename _Tp>
    class enable_shared_from_this;

  template<_Lock_policy _Lp = __default_lock_policy>
    class __weak_count;

  template<_Lock_policy _Lp = __default_lock_policy>
    class __shared_count;


  // Counted ptr with no deleter or allocator support
  template<typename _Ptr, _Lock_policy _Lp>
    class _Sp_counted_ptr final : public _Sp_counted_base<_Lp>
    {
    public:
      explicit
      _Sp_counted_ptr(_Ptr __p) noexcept
      : _M_ptr(__p) { }

      virtual void
      _M_dispose() noexcept
      { delete _M_ptr; }

      virtual void
      _M_destroy() noexcept
      { delete this; }

      virtual void*
      _M_get_deleter(const std::type_info&) noexcept
      { return nullptr; }

      _Sp_counted_ptr(const _Sp_counted_ptr&) = delete;
      _Sp_counted_ptr& operator=(const _Sp_counted_ptr&) = delete;

    private:
      _Ptr             _M_ptr;
    };

  template<>
    inline void
    _Sp_counted_ptr<nullptr_t, _S_single>::_M_dispose() noexcept { }

  template<>
    inline void
    _Sp_counted_ptr<nullptr_t, _S_mutex>::_M_dispose() noexcept { }

  template<>
    inline void
    _Sp_counted_ptr<nullptr_t, _S_atomic>::_M_dispose() noexcept { }

  template<int _Nm, typename _Tp,
	   bool __use_ebo = !__is_final(_Tp) && __is_empty(_Tp)>
    struct _Sp_ebo_helper;

  /// Specialization using EBO.
  template<int _Nm, typename _Tp>
    struct _Sp_ebo_helper<_Nm, _Tp, true> : private _Tp
    {
      explicit _Sp_ebo_helper(const _Tp& __tp) : _Tp(__tp) { }
      explicit _Sp_ebo_helper(_Tp&& __tp) : _Tp(std::move(__tp)) { }

      static _Tp&
      _S_get(_Sp_ebo_helper& __eboh) { return static_cast<_Tp&>(__eboh); }
    };

  /// Specialization not using EBO.
  template<int _Nm, typename _Tp>
    struct _Sp_ebo_helper<_Nm, _Tp, false>
    {
      explicit _Sp_ebo_helper(const _Tp& __tp) : _M_tp(__tp) { }
      explicit _Sp_ebo_helper(_Tp&& __tp) : _M_tp(std::move(__tp)) { }

      static _Tp&
      _S_get(_Sp_ebo_helper& __eboh)
      { return __eboh._M_tp; }

    private:
      _Tp _M_tp;
    };

  // Support for custom deleter and/or allocator
  template<typename _Ptr, typename _Deleter, typename _Alloc, _Lock_policy _Lp>
    class _Sp_counted_deleter final : public _Sp_counted_base<_Lp>
    {
      class _Impl : _Sp_ebo_helper<0, _Deleter>, _Sp_ebo_helper<1, _Alloc>
      {
	typedef _Sp_ebo_helper<0, _Deleter>	_Del_base;
	typedef _Sp_ebo_helper<1, _Alloc>	_Alloc_base;

      public:
	_Impl(_Ptr __p, _Deleter __d, const _Alloc& __a) noexcept
	: _Del_base(std::move(__d)), _Alloc_base(__a), _M_ptr(__p)
	{ }

	_Deleter& _M_del() noexcept { return _Del_base::_S_get(*this); }
	_Alloc& _M_alloc() noexcept { return _Alloc_base::_S_get(*this); }

	_Ptr _M_ptr;
      };

    public:
      using __allocator_type = __alloc_rebind<_Alloc, _Sp_counted_deleter>;

      // __d(__p) must not throw.
      _Sp_counted_deleter(_Ptr __p, _Deleter __d) noexcept
      : _M_impl(__p, std::move(__d), _Alloc()) { }

      // __d(__p) must not throw.
      _Sp_counted_deleter(_Ptr __p, _Deleter __d, const _Alloc& __a) noexcept
      : _M_impl(__p, std::move(__d), __a) { }

      ~_Sp_counted_deleter() noexcept { }

      virtual void
      _M_dispose() noexcept
      { _M_impl._M_del()(_M_impl._M_ptr); }

      virtual void
      _M_destroy() noexcept
      {
	__allocator_type __a(_M_impl._M_alloc());
	__allocated_ptr<__allocator_type> __guard_ptr{ __a, this };
	this->~_Sp_counted_deleter();
      }

      virtual void*
      _M_get_deleter(const type_info& __ti [[__gnu__::__unused__]]) noexcept
      {
#if __cpp_rtti
	// _GLIBCXX_RESOLVE_LIB_DEFECTS
	// 2400. shared_ptr's get_deleter() should use addressof()
        return __ti == typeid(_Deleter)
	  ? std::__addressof(_M_impl._M_del())
	  : nullptr;
#else
        return nullptr;
#endif
      }

    private:
      _Impl _M_impl;
    };

  // helpers for make_shared / allocate_shared

  struct _Sp_make_shared_tag
  {
  private:
    template<typename _Tp, typename _Alloc, _Lock_policy _Lp>
      friend class _Sp_counted_ptr_inplace;

    static const type_info&
    _S_ti() noexcept _GLIBCXX_VISIBILITY(default)
    {
      alignas(type_info) static constexpr char __tag[sizeof(type_info)] = { };
      return reinterpret_cast<const type_info&>(__tag);
    }

    static bool _S_eq(const type_info&) noexcept;
  };

  template<typename _Alloc>
    struct _Sp_alloc_shared_tag
    {
      const _Alloc& _M_a;
    };

  template<typename _Tp, typename _Alloc, _Lock_policy _Lp>
    class _Sp_counted_ptr_inplace final : public _Sp_counted_base<_Lp>
    {
      class _Impl : _Sp_ebo_helper<0, _Alloc>
      {
	typedef _Sp_ebo_helper<0, _Alloc>	_A_base;

      public:
	explicit _Impl(_Alloc __a) noexcept : _A_base(__a) { }

	_Alloc& _M_alloc() noexcept { return _A_base::_S_get(*this); }

	__gnu_cxx::__aligned_buffer<_Tp> _M_storage;
      };

    public:
      using __allocator_type = __alloc_rebind<_Alloc, _Sp_counted_ptr_inplace>;

      // Alloc parameter is not a reference so doesn't alias anything in __args
      template<typename... _Args>
	_Sp_counted_ptr_inplace(_Alloc __a, _Args&&... __args)
	: _M_impl(__a)
	{
	  // _GLIBCXX_RESOLVE_LIB_DEFECTS
	  // 2070.  allocate_shared should use allocator_traits<A>::construct
	  allocator_traits<_Alloc>::construct(__a, _M_ptr(),
	      std::forward<_Args>(__args)...); // might throw
	}

      ~_Sp_counted_ptr_inplace() noexcept { }

      virtual void
      _M_dispose() noexcept
      {
	allocator_traits<_Alloc>::destroy(_M_impl._M_alloc(), _M_ptr());
      }

      // Override because the allocator needs to know the dynamic type
      virtual void
      _M_destroy() noexcept
      {
	__allocator_type __a(_M_impl._M_alloc());
	__allocated_ptr<__allocator_type> __guard_ptr{ __a, this };
	this->~_Sp_counted_ptr_inplace();
      }

    private:
      friend class __shared_count<_Lp>; // To be able to call _M_ptr().

      // No longer used, but code compiled against old libstdc++ headers
      // might still call it from __shared_ptr ctor to get the pointer out.
      virtual void*
      _M_get_deleter(const std::type_info& __ti) noexcept override
      {
	auto __ptr = const_cast<typename remove_cv<_Tp>::type*>(_M_ptr());
	// Check for the fake type_info first, so we don't try to access it
	// as a real type_info object. Otherwise, check if it's the real
	// type_info for this class. With RTTI enabled we can check directly,
	// or call a library function to do it.
	if (&__ti == &_Sp_make_shared_tag::_S_ti()
	    ||
#if __cpp_rtti
	    __ti == typeid(_Sp_make_shared_tag)
#else
	    _Sp_make_shared_tag::_S_eq(__ti)
#endif
	   )
	  return __ptr;
	return nullptr;
      }

      _Tp* _M_ptr() noexcept { return _M_impl._M_storage._M_ptr(); }

      _Impl _M_impl;
    };

  // The default deleter for shared_ptr<T[]> and shared_ptr<T[N]>.
  struct __sp_array_delete
  {
    template<typename _Yp>
      void operator()(_Yp* __p) const { delete[] __p; }
  };

  template<_Lock_policy _Lp>
    class __shared_count
    {
      template<typename _Tp>
	struct __not_alloc_shared_tag { using type = void; };

      template<typename _Tp>
	struct __not_alloc_shared_tag<_Sp_alloc_shared_tag<_Tp>> { };

    public:
      constexpr __shared_count() noexcept : _M_pi(0)
      { }

      template<typename _Ptr>
        explicit
	__shared_count(_Ptr __p) : _M_pi(0)
	{
	  __try
	    {
	      _M_pi = new _Sp_counted_ptr<_Ptr, _Lp>(__p);
	    }
	  __catch(...)
	    {
	      delete __p;
	      __throw_exception_again;
	    }
	}

      template<typename _Ptr>
	__shared_count(_Ptr __p, /* is_array = */ false_type)
	: __shared_count(__p)
	{ }

      template<typename _Ptr>
	__shared_count(_Ptr __p, /* is_array = */ true_type)
	: __shared_count(__p, __sp_array_delete{}, allocator<void>())
	{ }

      template<typename _Ptr, typename _Deleter,
	       typename = typename __not_alloc_shared_tag<_Deleter>::type>
	__shared_count(_Ptr __p, _Deleter __d)
	: __shared_count(__p, std::move(__d), allocator<void>())
	{ }

      template<typename _Ptr, typename _Deleter, typename _Alloc,
	       typename = typename __not_alloc_shared_tag<_Deleter>::type>
	__shared_count(_Ptr __p, _Deleter __d, _Alloc __a) : _M_pi(0)
	{
	  typedef _Sp_counted_deleter<_Ptr, _Deleter, _Alloc, _Lp> _Sp_cd_type;
	  __try
	    {
	      typename _Sp_cd_type::__allocator_type __a2(__a);
	      auto __guard = std::__allocate_guarded(__a2);
	      _Sp_cd_type* __mem = __guard.get();
	      ::new (__mem) _Sp_cd_type(__p, std::move(__d), std::move(__a));
	      _M_pi = __mem;
	      __guard = nullptr;
	    }
	  __catch(...)
	    {
	      __d(__p); // Call _Deleter on __p.
	      __throw_exception_again;
	    }
	}

      template<typename _Tp, typename _Alloc, typename... _Args>
	__shared_count(_Tp*& __p, _Sp_alloc_shared_tag<_Alloc> __a,
		       _Args&&... __args)
	{
	  typedef _Sp_counted_ptr_inplace<_Tp, _Alloc, _Lp> _Sp_cp_type;
	  typename _Sp_cp_type::__allocator_type __a2(__a._M_a);
	  auto __guard = std::__allocate_guarded(__a2);
	  _Sp_cp_type* __mem = __guard.get();
	  auto __pi = ::new (__mem)
	    _Sp_cp_type(__a._M_a, std::forward<_Args>(__args)...);
	  __guard = nullptr;
	  _M_pi = __pi;
	  __p = __pi->_M_ptr();
	}

#if _GLIBCXX_USE_DEPRECATED
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
      // Special case for auto_ptr<_Tp> to provide the strong guarantee.
      template<typename _Tp>
        explicit
	__shared_count(std::auto_ptr<_Tp>&& __r);
#pragma GCC diagnostic pop
#endif

      // Special case for unique_ptr<_Tp,_Del> to provide the strong guarantee.
      template<typename _Tp, typename _Del>
        explicit
	__shared_count(std::unique_ptr<_Tp, _Del>&& __r) : _M_pi(0)
	{
	  // _GLIBCXX_RESOLVE_LIB_DEFECTS
	  // 2415. Inconsistency between unique_ptr and shared_ptr
	  if (__r.get() == nullptr)
	    return;

	  using _Ptr = typename unique_ptr<_Tp, _Del>::pointer;
	  using _Del2 = typename conditional<is_reference<_Del>::value,
	      reference_wrapper<typename remove_reference<_Del>::type>,
	      _Del>::type;
	  using _Sp_cd_type
	    = _Sp_counted_deleter<_Ptr, _Del2, allocator<void>, _Lp>;
	  using _Alloc = allocator<_Sp_cd_type>;
	  using _Alloc_traits = allocator_traits<_Alloc>;
	  _Alloc __a;
	  _Sp_cd_type* __mem = _Alloc_traits::allocate(__a, 1);
	  // _GLIBCXX_RESOLVE_LIB_DEFECTS
	  // 3548. shared_ptr construction from unique_ptr should move
	  // (not copy) the deleter
	  _Alloc_traits::construct(__a, __mem, __r.release(),
				   std::forward<_Del>(__r.get_deleter()));
	  _M_pi = __mem;
	}

      // Throw bad_weak_ptr when __r._M_get_use_count() == 0.
      explicit __shared_count(const __weak_count<_Lp>& __r);

      // Does not throw if __r._M_get_use_count() == 0, caller must check.
      explicit
      __shared_count(const __weak_count<_Lp>& __r, std::nothrow_t) noexcept;

      ~__shared_count() noexcept
      {
	if (_M_pi != nullptr)
	  _M_pi->_M_release();
      }

      __shared_count(const __shared_count& __r) noexcept
      : _M_pi(__r._M_pi)
      {
	if (_M_pi != nullptr)
	  _M_pi->_M_add_ref_copy();
      }

      __shared_count&
      operator=(const __shared_count& __r) noexcept
      {
	_Sp_counted_base<_Lp>* __tmp = __r._M_pi;
	if (__tmp != _M_pi)
	  {
	    if (__tmp != nullptr)
	      __tmp->_M_add_ref_copy();
	    if (_M_pi != nullptr)
	      _M_pi->_M_release();
	    _M_pi = __tmp;
	  }
	return *this;
      }

      void
      _M_swap(__shared_count& __r) noexcept
      {
	_Sp_counted_base<_Lp>* __tmp = __r._M_pi;
	__r._M_pi = _M_pi;
	_M_pi = __tmp;
      }

      long
      _M_get_use_count() const noexcept
      { return _M_pi ? _M_pi->_M_get_use_count() : 0; }

      bool
      _M_unique() const noexcept
      { return this->_M_get_use_count() == 1; }

      void*
      _M_get_deleter(const std::type_info& __ti) const noexcept
      { return _M_pi ? _M_pi->_M_get_deleter(__ti) : nullptr; }

      bool
      _M_less(const __shared_count& __rhs) const noexcept
      { return std::less<_Sp_counted_base<_Lp>*>()(this->_M_pi, __rhs._M_pi); }

      bool
      _M_less(const __weak_count<_Lp>& __rhs) const noexcept
      { return std::less<_Sp_counted_base<_Lp>*>()(this->_M_pi, __rhs._M_pi); }

      // Friend function injected into enclosing namespace and found by ADL
      friend inline bool
      operator==(const __shared_count& __a, const __shared_count& __b) noexcept
      { return __a._M_pi == __b._M_pi; }

    private:
      friend class __weak_count<_Lp>;

      _Sp_counted_base<_Lp>*  _M_pi;
    };


  template<_Lock_policy _Lp>
    class __weak_count
    {
    public:
      constexpr __weak_count() noexcept : _M_pi(nullptr)
      { }

      __weak_count(const __shared_count<_Lp>& __r) noexcept
      : _M_pi(__r._M_pi)
      {
	if (_M_pi != nullptr)
	  _M_pi->_M_weak_add_ref();
      }

      __weak_count(const __weak_count& __r) noexcept
      : _M_pi(__r._M_pi)
      {
	if (_M_pi != nullptr)
	  _M_pi->_M_weak_add_ref();
      }

      __weak_count(__weak_count&& __r) noexcept
      : _M_pi(__r._M_pi)
      { __r._M_pi = nullptr; }

      ~__weak_count() noexcept
      {
	if (_M_pi != nullptr)
	  _M_pi->_M_weak_release();
      }

      __weak_count&
      operator=(const __shared_count<_Lp>& __r) noexcept
      {
	_Sp_counted_base<_Lp>* __tmp = __r._M_pi;
	if (__tmp != nullptr)
	  __tmp->_M_weak_add_ref();
	if (_M_pi != nullptr)
	  _M_pi->_M_weak_release();
	_M_pi = __tmp;
	return *this;
      }

      __weak_count&
      operator=(const __weak_count& __r) noexcept
      {
	_Sp_counted_base<_Lp>* __tmp = __r._M_pi;
	if (__tmp != nullptr)
	  __tmp->_M_weak_add_ref();
	if (_M_pi != nullptr)
	  _M_pi->_M_weak_release();
	_M_pi = __tmp;
	return *this;
      }

      __weak_count&
      operator=(__weak_count&& __r) noexcept
      {
	if (_M_pi != nullptr)
	  _M_pi->_M_weak_release();
	_M_pi = __r._M_pi;
        __r._M_pi = nullptr;
	return *this;
      }

      void
      _M_swap(__weak_count& __r) noexcept
      {
	_Sp_counted_base<_Lp>* __tmp = __r._M_pi;
	__r._M_pi = _M_pi;
	_M_pi = __tmp;
      }

      long
      _M_get_use_count() const noexcept
      { return _M_pi != nullptr ? _M_pi->_M_get_use_count() : 0; }

      bool
      _M_less(const __weak_count& __rhs) const noexcept
      { return std::less<_Sp_counted_base<_Lp>*>()(this->_M_pi, __rhs._M_pi); }

      bool
      _M_less(const __shared_count<_Lp>& __rhs) const noexcept
      { return std::less<_Sp_counted_base<_Lp>*>()(this->_M_pi, __rhs._M_pi); }

      // Friend function injected into enclosing namespace and found by ADL
      friend inline bool
      operator==(const __weak_count& __a, const __weak_count& __b) noexcept
      { return __a._M_pi == __b._M_pi; }

    private:
      friend class __shared_count<_Lp>;

      _Sp_counted_base<_Lp>*  _M_pi;
    };

  // Now that __weak_count is defined we can define this constructor:
  template<_Lock_policy _Lp>
    inline
    __shared_count<_Lp>::__shared_count(const __weak_count<_Lp>& __r)
    : _M_pi(__r._M_pi)
    {
      if (_M_pi == nullptr || !_M_pi->_M_add_ref_lock_nothrow())
	__throw_bad_weak_ptr();
    }

  // Now that __weak_count is defined we can define this constructor:
  template<_Lock_policy _Lp>
    inline
    __shared_count<_Lp>::
    __shared_count(const __weak_count<_Lp>& __r, std::nothrow_t) noexcept
    : _M_pi(__r._M_pi)
    {
      if (_M_pi && !_M_pi->_M_add_ref_lock_nothrow())
	_M_pi = nullptr;
    }

#define __cpp_lib_shared_ptr_arrays 201611L

  // Helper traits for shared_ptr of array:

  // A pointer type Y* is said to be compatible with a pointer type T* when
  // either Y* is convertible to T* or Y is U[N] and T is U cv [].
  template<typename _Yp_ptr, typename _Tp_ptr>
    struct __sp_compatible_with
    : false_type
    { };

  template<typename _Yp, typename _Tp>
    struct __sp_compatible_with<_Yp*, _Tp*>
    : is_convertible<_Yp*, _Tp*>::type
    { };

  template<typename _Up, size_t _Nm>
    struct __sp_compatible_with<_Up(*)[_Nm], _Up(*)[]>
    : true_type
    { };

  template<typename _Up, size_t _Nm>
    struct __sp_compatible_with<_Up(*)[_Nm], const _Up(*)[]>
    : true_type
    { };

  template<typename _Up, size_t _Nm>
    struct __sp_compatible_with<_Up(*)[_Nm], volatile _Up(*)[]>
    : true_type
    { };

  template<typename _Up, size_t _Nm>
    struct __sp_compatible_with<_Up(*)[_Nm], const volatile _Up(*)[]>
    : true_type
    { };

  // Test conversion from Y(*)[N] to U(*)[N] without forming invalid type Y[N].
  template<typename _Up, size_t _Nm, typename _Yp, typename = void>
    struct __sp_is_constructible_arrN
    : false_type
    { };

  template<typename _Up, size_t _Nm, typename _Yp>
    struct __sp_is_constructible_arrN<_Up, _Nm, _Yp, __void_t<_Yp[_Nm]>>
    : is_convertible<_Yp(*)[_Nm], _Up(*)[_Nm]>::type
    { };

  // Test conversion from Y(*)[] to U(*)[] without forming invalid type Y[].
  template<typename _Up, typename _Yp, typename = void>
    struct __sp_is_constructible_arr
    : false_type
    { };

  template<typename _Up, typename _Yp>
    struct __sp_is_constructible_arr<_Up, _Yp, __void_t<_Yp[]>>
    : is_convertible<_Yp(*)[], _Up(*)[]>::type
    { };

  // Trait to check if shared_ptr<T> can be constructed from Y*.
  template<typename _Tp, typename _Yp>
    struct __sp_is_constructible;

  // When T is U[N], Y(*)[N] shall be convertible to T*;
  template<typename _Up, size_t _Nm, typename _Yp>
    struct __sp_is_constructible<_Up[_Nm], _Yp>
    : __sp_is_constructible_arrN<_Up, _Nm, _Yp>::type
    { };

  // when T is U[], Y(*)[] shall be convertible to T*;
  template<typename _Up, typename _Yp>
    struct __sp_is_constructible<_Up[], _Yp>
    : __sp_is_constructible_arr<_Up, _Yp>::type
    { };

  // otherwise, Y* shall be convertible to T*.
  template<typename _Tp, typename _Yp>
    struct __sp_is_constructible
    : is_convertible<_Yp*, _Tp*>::type
    { };


  // Define operator* and operator-> for shared_ptr<T>.
  template<typename _Tp, _Lock_policy _Lp,
	   bool = is_array<_Tp>::value, bool = is_void<_Tp>::value>
    class __shared_ptr_access
    {
    public:
      using element_type = _Tp;

      element_type&
      operator*() const noexcept
      {
	__glibcxx_assert(_M_get() != nullptr);
	return *_M_get();
      }

      element_type*
      operator->() const noexcept
      {
	_GLIBCXX_DEBUG_PEDASSERT(_M_get() != nullptr);
	return _M_get();
      }

    private:
      element_type*
      _M_get() const noexcept
      { return static_cast<const __shared_ptr<_Tp, _Lp>*>(this)->get(); }
    };

  // Define operator-> for shared_ptr<cv void>.
  template<typename _Tp, _Lock_policy _Lp>
    class __shared_ptr_access<_Tp, _Lp, false, true>
    {
    public:
      using element_type = _Tp;

      element_type*
      operator->() const noexcept
      {
	auto __ptr = static_cast<const __shared_ptr<_Tp, _Lp>*>(this)->get();
	_GLIBCXX_DEBUG_PEDASSERT(__ptr != nullptr);
	return __ptr;
      }
    };

  // Define operator[] for shared_ptr<T[]> and shared_ptr<T[N]>.
  template<typename _Tp, _Lock_policy _Lp>
    class __shared_ptr_access<_Tp, _Lp, true, false>
    {
    public:
      using element_type = typename remove_extent<_Tp>::type;

#if __cplusplus <= 201402L
      [[__deprecated__("shared_ptr<T[]>::operator* is absent from C++17")]]
      element_type&
      operator*() const noexcept
      {
	__glibcxx_assert(_M_get() != nullptr);
	return *_M_get();
      }

      [[__deprecated__("shared_ptr<T[]>::operator-> is absent from C++17")]]
      element_type*
      operator->() const noexcept
      {
	_GLIBCXX_DEBUG_PEDASSERT(_M_get() != nullptr);
	return _M_get();
      }
#endif

      element_type&
      operator[](ptrdiff_t __i) const
      {
	__glibcxx_assert(_M_get() != nullptr);
	__glibcxx_assert(!extent<_Tp>::value || __i < extent<_Tp>::value);
	return _M_get()[__i];
      }

    private:
      element_type*
      _M_get() const noexcept
      { return static_cast<const __shared_ptr<_Tp, _Lp>*>(this)->get(); }
    };

  template<typename _Tp, _Lock_policy _Lp>
    class __shared_ptr
    : public __shared_ptr_access<_Tp, _Lp>
    {
    public:
      using element_type = typename remove_extent<_Tp>::type;

    private:
      // Constraint for taking ownership of a pointer of type _Yp*:
      template<typename _Yp>
	using _SafeConv
	  = typename enable_if<__sp_is_constructible<_Tp, _Yp>::value>::type;

      // Constraint for construction from shared_ptr and weak_ptr:
      template<typename _Yp, typename _Res = void>
	using _Compatible = typename
	  enable_if<__sp_compatible_with<_Yp*, _Tp*>::value, _Res>::type;

      // Constraint for assignment from shared_ptr and weak_ptr:
      template<typename _Yp>
	using _Assignable = _Compatible<_Yp, __shared_ptr&>;

      // Constraint for construction from unique_ptr:
      template<typename _Yp, typename _Del, typename _Res = void,
	       typename _Ptr = typename unique_ptr<_Yp, _Del>::pointer>
	using _UniqCompatible = __enable_if_t<__and_<
	  __sp_compatible_with<_Yp*, _Tp*>,
	  is_convertible<_Ptr, element_type*>,
	  is_move_constructible<_Del>
	  >::value, _Res>;

      // Constraint for assignment from unique_ptr:
      template<typename _Yp, typename _Del>
	using _UniqAssignable = _UniqCompatible<_Yp, _Del, __shared_ptr&>;

    public:

#if __cplusplus > 201402L
      using weak_type = __weak_ptr<_Tp, _Lp>;
#endif

      constexpr __shared_ptr() noexcept
      : _M_ptr(0), _M_refcount()
      { }

      template<typename _Yp, typename = _SafeConv<_Yp>>
	explicit
	__shared_ptr(_Yp* __p)
	: _M_ptr(__p), _M_refcount(__p, typename is_array<_Tp>::type())
	{
	  static_assert( !is_void<_Yp>::value, "incomplete type" );
	  static_assert( sizeof(_Yp) > 0, "incomplete type" );
	  _M_enable_shared_from_this_with(__p);
	}

      template<typename _Yp, typename _Deleter, typename = _SafeConv<_Yp>>
	__shared_ptr(_Yp* __p, _Deleter __d)
	: _M_ptr(__p), _M_refcount(__p, std::move(__d))
	{
	  static_assert(__is_invocable<_Deleter&, _Yp*&>::value,
	      "deleter expression d(p) is well-formed");
	  _M_enable_shared_from_this_with(__p);
	}

      template<typename _Yp, typename _Deleter, typename _Alloc,
	       typename = _SafeConv<_Yp>>
	__shared_ptr(_Yp* __p, _Deleter __d, _Alloc __a)
	: _M_ptr(__p), _M_refcount(__p, std::move(__d), std::move(__a))
	{
	  static_assert(__is_invocable<_Deleter&, _Yp*&>::value,
	      "deleter expression d(p) is well-formed");
	  _M_enable_shared_from_this_with(__p);
	}

      template<typename _Deleter>
	__shared_ptr(nullptr_t __p, _Deleter __d)
	: _M_ptr(0), _M_refcount(__p, std::move(__d))
	{ }

      template<typename _Deleter, typename _Alloc>
        __shared_ptr(nullptr_t __p, _Deleter __d, _Alloc __a)
	: _M_ptr(0), _M_refcount(__p, std::move(__d), std::move(__a))
	{ }

      // Aliasing constructor
      template<typename _Yp>
	__shared_ptr(const __shared_ptr<_Yp, _Lp>& __r,
		     element_type* __p) noexcept
	: _M_ptr(__p), _M_refcount(__r._M_refcount) // never throws
	{ }

      // Aliasing constructor
      template<typename _Yp>
	__shared_ptr(__shared_ptr<_Yp, _Lp>&& __r,
		     element_type* __p) noexcept
	: _M_ptr(__p), _M_refcount()
	{
	  _M_refcount._M_swap(__r._M_refcount);
	  __r._M_ptr = nullptr;
	}

      __shared_ptr(const __shared_ptr&) noexcept = default;
      __shared_ptr& operator=(const __shared_ptr&) noexcept = default;
      ~__shared_ptr() = default;

      template<typename _Yp, typename = _Compatible<_Yp>>
	__shared_ptr(const __shared_ptr<_Yp, _Lp>& __r) noexcept
	: _M_ptr(__r._M_ptr), _M_refcount(__r._M_refcount)
	{ }

      __shared_ptr(__shared_ptr&& __r) noexcept
      : _M_ptr(__r._M_ptr), _M_refcount()
      {
	_M_refcount._M_swap(__r._M_refcount);
	__r._M_ptr = nullptr;
      }

      template<typename _Yp, typename = _Compatible<_Yp>>
	__shared_ptr(__shared_ptr<_Yp, _Lp>&& __r) noexcept
	: _M_ptr(__r._M_ptr), _M_refcount()
	{
	  _M_refcount._M_swap(__r._M_refcount);
	  __r._M_ptr = nullptr;
	}

      template<typename _Yp, typename = _Compatible<_Yp>>
	explicit __shared_ptr(const __weak_ptr<_Yp, _Lp>& __r)
	: _M_refcount(__r._M_refcount) // may throw
	{
	  // It is now safe to copy __r._M_ptr, as
	  // _M_refcount(__r._M_refcount) did not throw.
	  _M_ptr = __r._M_ptr;
	}

      // If an exception is thrown this constructor has no effect.
      template<typename _Yp, typename _Del,
	       typename = _UniqCompatible<_Yp, _Del>>
	__shared_ptr(unique_ptr<_Yp, _Del>&& __r)
	: _M_ptr(__r.get()), _M_refcount()
	{
	  auto __raw = __to_address(__r.get());
	  _M_refcount = __shared_count<_Lp>(std::move(__r));
	  _M_enable_shared_from_this_with(__raw);
	}

#if __cplusplus <= 201402L && _GLIBCXX_USE_DEPRECATED
    protected:
      // If an exception is thrown this constructor has no effect.
      template<typename _Tp1, typename _Del,
	       typename enable_if<__and_<
		 __not_<is_array<_Tp>>, is_array<_Tp1>,
	         is_convertible<typename unique_ptr<_Tp1, _Del>::pointer, _Tp*>
	       >::value, bool>::type = true>
	__shared_ptr(unique_ptr<_Tp1, _Del>&& __r, __sp_array_delete)
	: _M_ptr(__r.get()), _M_refcount()
	{
	  auto __raw = __to_address(__r.get());
	  _M_refcount = __shared_count<_Lp>(std::move(__r));
	  _M_enable_shared_from_this_with(__raw);
	}
    public:
#endif

#if _GLIBCXX_USE_DEPRECATED
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
      // Postcondition: use_count() == 1 and __r.get() == 0
      template<typename _Yp, typename = _Compatible<_Yp>>
	__shared_ptr(auto_ptr<_Yp>&& __r);
#pragma GCC diagnostic pop
#endif

      constexpr __shared_ptr(nullptr_t) noexcept : __shared_ptr() { }

      template<typename _Yp>
	_Assignable<_Yp>
	operator=(const __shared_ptr<_Yp, _Lp>& __r) noexcept
	{
	  _M_ptr = __r._M_ptr;
	  _M_refcount = __r._M_refcount; // __shared_count::op= doesn't throw
	  return *this;
	}

#if _GLIBCXX_USE_DEPRECATED
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
      template<typename _Yp>
	_Assignable<_Yp>
	operator=(auto_ptr<_Yp>&& __r)
	{
	  __shared_ptr(std::move(__r)).swap(*this);
	  return *this;
	}
#pragma GCC diagnostic pop
#endif

      __shared_ptr&
      operator=(__shared_ptr&& __r) noexcept
      {
	__shared_ptr(std::move(__r)).swap(*this);
	return *this;
      }

      template<class _Yp>
	_Assignable<_Yp>
	operator=(__shared_ptr<_Yp, _Lp>&& __r) noexcept
	{
	  __shared_ptr(std::move(__r)).swap(*this);
	  return *this;
	}

      template<typename _Yp, typename _Del>
	_UniqAssignable<_Yp, _Del>
	operator=(unique_ptr<_Yp, _Del>&& __r)
	{
	  __shared_ptr(std::move(__r)).swap(*this);
	  return *this;
	}

      void
      reset() noexcept
      { __shared_ptr().swap(*this); }

      template<typename _Yp>
	_SafeConv<_Yp>
	reset(_Yp* __p) // _Yp must be complete.
	{
	  // Catch self-reset errors.
	  __glibcxx_assert(__p == nullptr || __p != _M_ptr);
	  __shared_ptr(__p).swap(*this);
	}

      template<typename _Yp, typename _Deleter>
	_SafeConv<_Yp>
	reset(_Yp* __p, _Deleter __d)
	{ __shared_ptr(__p, std::move(__d)).swap(*this); }

      template<typename _Yp, typename _Deleter, typename _Alloc>
	_SafeConv<_Yp>
	reset(_Yp* __p, _Deleter __d, _Alloc __a)
        { __shared_ptr(__p, std::move(__d), std::move(__a)).swap(*this); }

      /// Return the stored pointer.
      element_type*
      get() const noexcept
      { return _M_ptr; }

      /// Return true if the stored pointer is not null.
      explicit operator bool() const noexcept
      { return _M_ptr != nullptr; }

      /// Return true if use_count() == 1.
      bool
      unique() const noexcept
      { return _M_refcount._M_unique(); }

      /// If *this owns a pointer, return the number of owners, otherwise zero.
      long
      use_count() const noexcept
      { return _M_refcount._M_get_use_count(); }

      /// Exchange both the owned pointer and the stored pointer.
      void
      swap(__shared_ptr<_Tp, _Lp>& __other) noexcept
      {
	std::swap(_M_ptr, __other._M_ptr);
	_M_refcount._M_swap(__other._M_refcount);
      }

      /** @brief Define an ordering based on ownership.
       *
       * This function defines a strict weak ordering between two shared_ptr
       * or weak_ptr objects, such that one object is less than the other
       * unless they share ownership of the same pointer, or are both empty.
       * @{
      */
      template<typename _Tp1>
	bool
	owner_before(__shared_ptr<_Tp1, _Lp> const& __rhs) const noexcept
	{ return _M_refcount._M_less(__rhs._M_refcount); }

      template<typename _Tp1>
	bool
	owner_before(__weak_ptr<_Tp1, _Lp> const& __rhs) const noexcept
	{ return _M_refcount._M_less(__rhs._M_refcount); }
      /// @}

    protected:
      // This constructor is non-standard, it is used by allocate_shared.
      template<typename _Alloc, typename... _Args>
	__shared_ptr(_Sp_alloc_shared_tag<_Alloc> __tag, _Args&&... __args)
	: _M_ptr(), _M_refcount(_M_ptr, __tag, std::forward<_Args>(__args)...)
	{ _M_enable_shared_from_this_with(_M_ptr); }

      template<typename _Tp1, _Lock_policy _Lp1, typename _Alloc,
	       typename... _Args>
	friend __shared_ptr<_Tp1, _Lp1>
	__allocate_shared(const _Alloc& __a, _Args&&... __args);

      // This constructor is used by __weak_ptr::lock() and
      // shared_ptr::shared_ptr(const weak_ptr&, std::nothrow_t).
      __shared_ptr(const __weak_ptr<_Tp, _Lp>& __r, std::nothrow_t) noexcept
      : _M_refcount(__r._M_refcount, std::nothrow)
      {
	_M_ptr = _M_refcount._M_get_use_count() ? __r._M_ptr : nullptr;
      }

      friend class __weak_ptr<_Tp, _Lp>;

    private:

      template<typename _Yp>
	using __esft_base_t = decltype(__enable_shared_from_this_base(
	      std::declval<const __shared_count<_Lp>&>(),
	      std::declval<_Yp*>()));

      // Detect an accessible and unambiguous enable_shared_from_this base.
      template<typename _Yp, typename = void>
	struct __has_esft_base
	: false_type { };

      template<typename _Yp>
	struct __has_esft_base<_Yp, __void_t<__esft_base_t<_Yp>>>
	: __not_<is_array<_Tp>> { }; // No enable shared_from_this for arrays

      template<typename _Yp, typename _Yp2 = typename remove_cv<_Yp>::type>
	typename enable_if<__has_esft_base<_Yp2>::value>::type
	_M_enable_shared_from_this_with(_Yp* __p) noexcept
	{
	  if (auto __base = __enable_shared_from_this_base(_M_refcount, __p))
	    __base->_M_weak_assign(const_cast<_Yp2*>(__p), _M_refcount);
	}

      template<typename _Yp, typename _Yp2 = typename remove_cv<_Yp>::type>
	typename enable_if<!__has_esft_base<_Yp2>::value>::type
	_M_enable_shared_from_this_with(_Yp*) noexcept
	{ }

      void*
      _M_get_deleter(const std::type_info& __ti) const noexcept
      { return _M_refcount._M_get_deleter(__ti); }

      template<typename _Tp1, _Lock_policy _Lp1> friend class __shared_ptr;
      template<typename _Tp1, _Lock_policy _Lp1> friend class __weak_ptr;

      template<typename _Del, typename _Tp1, _Lock_policy _Lp1>
	friend _Del* get_deleter(const __shared_ptr<_Tp1, _Lp1>&) noexcept;

      template<typename _Del, typename _Tp1>
	friend _Del* get_deleter(const shared_ptr<_Tp1>&) noexcept;

      element_type*	   _M_ptr;         // Contained pointer.
      __shared_count<_Lp>  _M_refcount;    // Reference counter.
    };


  // 20.7.2.2.7 shared_ptr comparisons
  template<typename _Tp1, typename _Tp2, _Lock_policy _Lp>
    inline bool
    operator==(const __shared_ptr<_Tp1, _Lp>& __a,
	       const __shared_ptr<_Tp2, _Lp>& __b) noexcept
    { return __a.get() == __b.get(); }

  template<typename _Tp, _Lock_policy _Lp>
    inline bool
    operator==(const __shared_ptr<_Tp, _Lp>& __a, nullptr_t) noexcept
    { return !__a; }

#ifdef __cpp_lib_three_way_comparison
  template<typename _Tp, typename _Up, _Lock_policy _Lp>
    inline strong_ordering
    operator<=>(const __shared_ptr<_Tp, _Lp>& __a,
		const __shared_ptr<_Up, _Lp>& __b) noexcept
    { return compare_three_way()(__a.get(), __b.get()); }

  template<typename _Tp, _Lock_policy _Lp>
    inline strong_ordering
    operator<=>(const __shared_ptr<_Tp, _Lp>& __a, nullptr_t) noexcept
    {
      using pointer = typename __shared_ptr<_Tp, _Lp>::element_type*;
      return compare_three_way()(__a.get(), static_cast<pointer>(nullptr));
    }
#else
  template<typename _Tp, _Lock_policy _Lp>
    inline bool
    operator==(nullptr_t, const __shared_ptr<_Tp, _Lp>& __a) noexcept
    { return !__a; }

  template<typename _Tp1, typename _Tp2, _Lock_policy _Lp>
    inline bool
    operator!=(const __shared_ptr<_Tp1, _Lp>& __a,
	       const __shared_ptr<_Tp2, _Lp>& __b) noexcept
    { return __a.get() != __b.get(); }

  template<typename _Tp, _Lock_policy _Lp>
    inline bool
    operator!=(const __shared_ptr<_Tp, _Lp>& __a, nullptr_t) noexcept
    { return (bool)__a; }

  template<typename _Tp, _Lock_policy _Lp>
    inline bool
    operator!=(nullptr_t, const __shared_ptr<_Tp, _Lp>& __a) noexcept
    { return (bool)__a; }

  template<typename _Tp, typename _Up, _Lock_policy _Lp>
    inline bool
    operator<(const __shared_ptr<_Tp, _Lp>& __a,
	      const __shared_ptr<_Up, _Lp>& __b) noexcept
    {
      using _Tp_elt = typename __shared_ptr<_Tp, _Lp>::element_type;
      using _Up_elt = typename __shared_ptr<_Up, _Lp>::element_type;
      using _Vp = typename common_type<_Tp_elt*, _Up_elt*>::type;
      return less<_Vp>()(__a.get(), __b.get());
    }

  template<typename _Tp, _Lock_policy _Lp>
    inline bool
    operator<(const __shared_ptr<_Tp, _Lp>& __a, nullptr_t) noexcept
    {
      using _Tp_elt = typename __shared_ptr<_Tp, _Lp>::element_type;
      return less<_Tp_elt*>()(__a.get(), nullptr);
    }

  template<typename _Tp, _Lock_policy _Lp>
    inline bool
    operator<(nullptr_t, const __shared_ptr<_Tp, _Lp>& __a) noexcept
    {
      using _Tp_elt = typename __shared_ptr<_Tp, _Lp>::element_type;
      return less<_Tp_elt*>()(nullptr, __a.get());
    }

  template<typename _Tp1, typename _Tp2, _Lock_policy _Lp>
    inline bool
    operator<=(const __shared_ptr<_Tp1, _Lp>& __a,
	       const __shared_ptr<_Tp2, _Lp>& __b) noexcept
    { return !(__b < __a); }

  template<typename _Tp, _Lock_policy _Lp>
    inline bool
    operator<=(const __shared_ptr<_Tp, _Lp>& __a, nullptr_t) noexcept
    { return !(nullptr < __a); }

  template<typename _Tp, _Lock_policy _Lp>
    inline bool
    operator<=(nullptr_t, const __shared_ptr<_Tp, _Lp>& __a) noexcept
    { return !(__a < nullptr); }

  template<typename _Tp1, typename _Tp2, _Lock_policy _Lp>
    inline bool
    operator>(const __shared_ptr<_Tp1, _Lp>& __a,
	      const __shared_ptr<_Tp2, _Lp>& __b) noexcept
    { return (__b < __a); }

  template<typename _Tp, _Lock_policy _Lp>
    inline bool
    operator>(const __shared_ptr<_Tp, _Lp>& __a, nullptr_t) noexcept
    { return nullptr < __a; }

  template<typename _Tp, _Lock_policy _Lp>
    inline bool
    operator>(nullptr_t, const __shared_ptr<_Tp, _Lp>& __a) noexcept
    { return __a < nullptr; }

  template<typename _Tp1, typename _Tp2, _Lock_policy _Lp>
    inline bool
    operator>=(const __shared_ptr<_Tp1, _Lp>& __a,
	       const __shared_ptr<_Tp2, _Lp>& __b) noexcept
    { return !(__a < __b); }

  template<typename _Tp, _Lock_policy _Lp>
    inline bool
    operator>=(const __shared_ptr<_Tp, _Lp>& __a, nullptr_t) noexcept
    { return !(__a < nullptr); }

  template<typename _Tp, _Lock_policy _Lp>
    inline bool
    operator>=(nullptr_t, const __shared_ptr<_Tp, _Lp>& __a) noexcept
    { return !(nullptr < __a); }
#endif // three-way comparison

  // 20.7.2.2.8 shared_ptr specialized algorithms.
  template<typename _Tp, _Lock_policy _Lp>
    inline void
    swap(__shared_ptr<_Tp, _Lp>& __a, __shared_ptr<_Tp, _Lp>& __b) noexcept
    { __a.swap(__b); }

  // 20.7.2.2.9 shared_ptr casts

  // The seemingly equivalent code:
  // shared_ptr<_Tp, _Lp>(static_cast<_Tp*>(__r.get()))
  // will eventually result in undefined behaviour, attempting to
  // delete the same object twice.
  /// static_pointer_cast
  template<typename _Tp, typename _Tp1, _Lock_policy _Lp>
    inline __shared_ptr<_Tp, _Lp>
    static_pointer_cast(const __shared_ptr<_Tp1, _Lp>& __r) noexcept
    {
      using _Sp = __shared_ptr<_Tp, _Lp>;
      return _Sp(__r, static_cast<typename _Sp::element_type*>(__r.get()));
    }

  // The seemingly equivalent code:
  // shared_ptr<_Tp, _Lp>(const_cast<_Tp*>(__r.get()))
  // will eventually result in undefined behaviour, attempting to
  // delete the same object twice.
  /// const_pointer_cast
  template<typename _Tp, typename _Tp1, _Lock_policy _Lp>
    inline __shared_ptr<_Tp, _Lp>
    const_pointer_cast(const __shared_ptr<_Tp1, _Lp>& __r) noexcept
    {
      using _Sp = __shared_ptr<_Tp, _Lp>;
      return _Sp(__r, const_cast<typename _Sp::element_type*>(__r.get()));
    }

  // The seemingly equivalent code:
  // shared_ptr<_Tp, _Lp>(dynamic_cast<_Tp*>(__r.get()))
  // will eventually result in undefined behaviour, attempting to
  // delete the same object twice.
  /// dynamic_pointer_cast
  template<typename _Tp, typename _Tp1, _Lock_policy _Lp>
    inline __shared_ptr<_Tp, _Lp>
    dynamic_pointer_cast(const __shared_ptr<_Tp1, _Lp>& __r) noexcept
    {
      using _Sp = __shared_ptr<_Tp, _Lp>;
      if (auto* __p = dynamic_cast<typename _Sp::element_type*>(__r.get()))
	return _Sp(__r, __p);
      return _Sp();
    }

#if __cplusplus > 201402L
  template<typename _Tp, typename _Tp1, _Lock_policy _Lp>
    inline __shared_ptr<_Tp, _Lp>
    reinterpret_pointer_cast(const __shared_ptr<_Tp1, _Lp>& __r) noexcept
    {
      using _Sp = __shared_ptr<_Tp, _Lp>;
      return _Sp(__r, reinterpret_cast<typename _Sp::element_type*>(__r.get()));
    }
#endif

  template<typename _Tp, _Lock_policy _Lp>
    class __weak_ptr
    {
      template<typename _Yp, typename _Res = void>
	using _Compatible = typename
	  enable_if<__sp_compatible_with<_Yp*, _Tp*>::value, _Res>::type;

      // Constraint for assignment from shared_ptr and weak_ptr:
      template<typename _Yp>
	using _Assignable = _Compatible<_Yp, __weak_ptr&>;

    public:
      using element_type = typename remove_extent<_Tp>::type;

      constexpr __weak_ptr() noexcept
      : _M_ptr(nullptr), _M_refcount()
      { }

      __weak_ptr(const __weak_ptr&) noexcept = default;

      ~__weak_ptr() = default;

      // The "obvious" converting constructor implementation:
      //
      //  template<typename _Tp1>
      //    __weak_ptr(const __weak_ptr<_Tp1, _Lp>& __r)
      //    : _M_ptr(__r._M_ptr), _M_refcount(__r._M_refcount) // never throws
      //    { }
      //
      // has a serious problem.
      //
      //  __r._M_ptr may already have been invalidated. The _M_ptr(__r._M_ptr)
      //  conversion may require access to *__r._M_ptr (virtual inheritance).
      //
      // It is not possible to avoid spurious access violations since
      // in multithreaded programs __r._M_ptr may be invalidated at any point.
      template<typename _Yp, typename = _Compatible<_Yp>>
	__weak_ptr(const __weak_ptr<_Yp, _Lp>& __r) noexcept
	: _M_refcount(__r._M_refcount)
        { _M_ptr = __r.lock().get(); }

      template<typename _Yp, typename = _Compatible<_Yp>>
	__weak_ptr(const __shared_ptr<_Yp, _Lp>& __r) noexcept
	: _M_ptr(__r._M_ptr), _M_refcount(__r._M_refcount)
	{ }

      __weak_ptr(__weak_ptr&& __r) noexcept
      : _M_ptr(__r._M_ptr), _M_refcount(std::move(__r._M_refcount))
      { __r._M_ptr = nullptr; }

      template<typename _Yp, typename = _Compatible<_Yp>>
	__weak_ptr(__weak_ptr<_Yp, _Lp>&& __r) noexcept
	: _M_ptr(__r.lock().get()), _M_refcount(std::move(__r._M_refcount))
        { __r._M_ptr = nullptr; }

      __weak_ptr&
      operator=(const __weak_ptr& __r) noexcept = default;

      template<typename _Yp>
	_Assignable<_Yp>
	operator=(const __weak_ptr<_Yp, _Lp>& __r) noexcept
	{
	  _M_ptr = __r.lock().get();
	  _M_refcount = __r._M_refcount;
	  return *this;
	}

      template<typename _Yp>
	_Assignable<_Yp>
	operator=(const __shared_ptr<_Yp, _Lp>& __r) noexcept
	{
	  _M_ptr = __r._M_ptr;
	  _M_refcount = __r._M_refcount;
	  return *this;
	}

      __weak_ptr&
      operator=(__weak_ptr&& __r) noexcept
      {
	__weak_ptr(std::move(__r)).swap(*this);
	return *this;
      }

      template<typename _Yp>
	_Assignable<_Yp>
	operator=(__weak_ptr<_Yp, _Lp>&& __r) noexcept
	{
	  _M_ptr = __r.lock().get();
	  _M_refcount = std::move(__r._M_refcount);
	  __r._M_ptr = nullptr;
	  return *this;
	}

      __shared_ptr<_Tp, _Lp>
      lock() const noexcept
      { return __shared_ptr<element_type, _Lp>(*this, std::nothrow); }

      long
      use_count() const noexcept
      { return _M_refcount._M_get_use_count(); }

      bool
      expired() const noexcept
      { return _M_refcount._M_get_use_count() == 0; }

      template<typename _Tp1>
	bool
	owner_before(const __shared_ptr<_Tp1, _Lp>& __rhs) const noexcept
	{ return _M_refcount._M_less(__rhs._M_refcount); }

      template<typename _Tp1>
	bool
	owner_before(const __weak_ptr<_Tp1, _Lp>& __rhs) const noexcept
	{ return _M_refcount._M_less(__rhs._M_refcount); }

      void
      reset() noexcept
      { __weak_ptr().swap(*this); }

      void
      swap(__weak_ptr& __s) noexcept
      {
	std::swap(_M_ptr, __s._M_ptr);
	_M_refcount._M_swap(__s._M_refcount);
      }

    private:
      // Used by __enable_shared_from_this.
      void
      _M_assign(_Tp* __ptr, const __shared_count<_Lp>& __refcount) noexcept
      {
	if (use_count() == 0)
	  {
	    _M_ptr = __ptr;
	    _M_refcount = __refcount;
	  }
      }

      template<typename _Tp1, _Lock_policy _Lp1> friend class __shared_ptr;
      template<typename _Tp1, _Lock_policy _Lp1> friend class __weak_ptr;
      friend class __enable_shared_from_this<_Tp, _Lp>;
      friend class enable_shared_from_this<_Tp>;

      element_type*	 _M_ptr;         // Contained pointer.
      __weak_count<_Lp>  _M_refcount;    // Reference counter.
    };

  // 20.7.2.3.6 weak_ptr specialized algorithms.
  template<typename _Tp, _Lock_policy _Lp>
    inline void
    swap(__weak_ptr<_Tp, _Lp>& __a, __weak_ptr<_Tp, _Lp>& __b) noexcept
    { __a.swap(__b); }

  template<typename _Tp, typename _Tp1>
    struct _Sp_owner_less : public binary_function<_Tp, _Tp, bool>
    {
      bool
      operator()(const _Tp& __lhs, const _Tp& __rhs) const noexcept
      { return __lhs.owner_before(__rhs); }

      bool
      operator()(const _Tp& __lhs, const _Tp1& __rhs) const noexcept
      { return __lhs.owner_before(__rhs); }

      bool
      operator()(const _Tp1& __lhs, const _Tp& __rhs) const noexcept
      { return __lhs.owner_before(__rhs); }
    };

  template<>
    struct _Sp_owner_less<void, void>
    {
      template<typename _Tp, typename _Up>
	auto
	operator()(const _Tp& __lhs, const _Up& __rhs) const noexcept
	-> decltype(__lhs.owner_before(__rhs))
	{ return __lhs.owner_before(__rhs); }

      using is_transparent = void;
    };

  template<typename _Tp, _Lock_policy _Lp>
    struct owner_less<__shared_ptr<_Tp, _Lp>>
    : public _Sp_owner_less<__shared_ptr<_Tp, _Lp>, __weak_ptr<_Tp, _Lp>>
    { };

  template<typename _Tp, _Lock_policy _Lp>
    struct owner_less<__weak_ptr<_Tp, _Lp>>
    : public _Sp_owner_less<__weak_ptr<_Tp, _Lp>, __shared_ptr<_Tp, _Lp>>
    { };


  template<typename _Tp, _Lock_policy _Lp>
    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, _Lp>
      shared_from_this()
      { return __shared_ptr<_Tp, _Lp>(this->_M_weak_this); }

      __shared_ptr<const _Tp, _Lp>
      shared_from_this() const
      { return __shared_ptr<const _Tp, _Lp>(this->_M_weak_this); }

#if __cplusplus > 201402L || !defined(__STRICT_ANSI__) // c++1z or gnu++11
      __weak_ptr<_Tp, _Lp>
      weak_from_this() noexcept
      { return this->_M_weak_this; }

      __weak_ptr<const _Tp, _Lp>
      weak_from_this() const noexcept
      { return this->_M_weak_this; }
#endif

    private:
      template<typename _Tp1>
	void
	_M_weak_assign(_Tp1* __p, const __shared_count<_Lp>& __n) const noexcept
	{ _M_weak_this._M_assign(__p, __n); }

      friend const __enable_shared_from_this*
      __enable_shared_from_this_base(const __shared_count<_Lp>&,
				     const __enable_shared_from_this* __p)
      { return __p; }

      template<typename, _Lock_policy>
	friend class __shared_ptr;

      mutable __weak_ptr<_Tp, _Lp>  _M_weak_this;
    };

  template<typename _Tp, _Lock_policy _Lp = __default_lock_policy,
	   typename _Alloc, typename... _Args>
    inline __shared_ptr<_Tp, _Lp>
    __allocate_shared(const _Alloc& __a, _Args&&... __args)
    {
      static_assert(!is_array<_Tp>::value, "make_shared<T[]> not supported");

      return __shared_ptr<_Tp, _Lp>(_Sp_alloc_shared_tag<_Alloc>{__a},
				    std::forward<_Args>(__args)...);
    }

  template<typename _Tp, _Lock_policy _Lp = __default_lock_policy,
	   typename... _Args>
    inline __shared_ptr<_Tp, _Lp>
    __make_shared(_Args&&... __args)
    {
      typedef typename std::remove_const<_Tp>::type _Tp_nc;
      return std::__allocate_shared<_Tp, _Lp>(std::allocator<_Tp_nc>(),
					      std::forward<_Args>(__args)...);
    }

  /// std::hash specialization for __shared_ptr.
  template<typename _Tp, _Lock_policy _Lp>
    struct hash<__shared_ptr<_Tp, _Lp>>
    : public __hash_base<size_t, __shared_ptr<_Tp, _Lp>>
    {
      size_t
      operator()(const __shared_ptr<_Tp, _Lp>& __s) const noexcept
      {
	return hash<typename __shared_ptr<_Tp, _Lp>::element_type*>()(
	    __s.get());
      }
    };

_GLIBCXX_END_NAMESPACE_VERSION
} // namespace

#endif // _SHARED_PTR_BASE_H

shared_ptr_atomic.h

shared_ptr_atomic.hview raw

// shared_ptr atomic access -*- C++ -*-

// Copyright (C) 2014-2021 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/>.

/** @file bits/shared_ptr_atomic.h
 *  This is an internal header file, included by other library headers.
 *  Do not attempt to use it directly. @headername{memory}
 */

#ifndef _SHARED_PTR_ATOMIC_H
#define _SHARED_PTR_ATOMIC_H 1

#include <bits/atomic_base.h>

namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION

  /**
   * @addtogroup pointer_abstractions
   * @{
   */
  /// @relates shared_ptr @{

  /// @cond undocumented

  struct _Sp_locker
  {
    _Sp_locker(const _Sp_locker&) = delete;
    _Sp_locker& operator=(const _Sp_locker&) = delete;

#ifdef __GTHREADS
    explicit
    _Sp_locker(const void*) noexcept;
    _Sp_locker(const void*, const void*) noexcept;
    ~_Sp_locker();

  private:
    unsigned char _M_key1;
    unsigned char _M_key2;
#else
    explicit _Sp_locker(const void*, const void* = nullptr) { }
#endif
  };

  /// @endcond

  /**
   *  @brief  Report whether shared_ptr atomic operations are lock-free.
   *  @param  __p A non-null pointer to a shared_ptr object.
   *  @return True if atomic access to @c *__p is lock-free, false otherwise.
   *  @{
  */
  template<typename _Tp, _Lock_policy _Lp>
    inline bool
    atomic_is_lock_free(const __shared_ptr<_Tp, _Lp>* __p)
    {
#ifdef __GTHREADS
      return __gthread_active_p() == 0;
#else
      return true;
#endif
    }

  template<typename _Tp>
    inline bool
    atomic_is_lock_free(const shared_ptr<_Tp>* __p)
    { return std::atomic_is_lock_free<_Tp, __default_lock_policy>(__p); }

  /// @}

  /**
   *  @brief  Atomic load for shared_ptr objects.
   *  @param  __p A non-null pointer to a shared_ptr object.
   *  @return @c *__p
   *
   *  The memory order shall not be @c memory_order_release or
   *  @c memory_order_acq_rel.
   *  @{
  */
  template<typename _Tp>
    inline shared_ptr<_Tp>
    atomic_load_explicit(const shared_ptr<_Tp>* __p, memory_order)
    {
      _Sp_locker __lock{__p};
      return *__p;
    }

  template<typename _Tp>
    inline shared_ptr<_Tp>
    atomic_load(const shared_ptr<_Tp>* __p)
    { return std::atomic_load_explicit(__p, memory_order_seq_cst); }

  template<typename _Tp, _Lock_policy _Lp>
    inline __shared_ptr<_Tp, _Lp>
    atomic_load_explicit(const __shared_ptr<_Tp, _Lp>* __p, memory_order)
    {
      _Sp_locker __lock{__p};
      return *__p;
    }

  template<typename _Tp, _Lock_policy _Lp>
    inline __shared_ptr<_Tp, _Lp>
    atomic_load(const __shared_ptr<_Tp, _Lp>* __p)
    { return std::atomic_load_explicit(__p, memory_order_seq_cst); }
  /// @}

  /**
   *  @brief  Atomic store for shared_ptr objects.
   *  @param  __p A non-null pointer to a shared_ptr object.
   *  @param  __r The value to store.
   *
   *  The memory order shall not be @c memory_order_acquire or
   *  @c memory_order_acq_rel.
   *  @{
  */
  template<typename _Tp>
    inline void
    atomic_store_explicit(shared_ptr<_Tp>* __p, shared_ptr<_Tp> __r,
			  memory_order)
    {
      _Sp_locker __lock{__p};
      __p->swap(__r); // use swap so that **__p not destroyed while lock held
    }

  template<typename _Tp>
    inline void
    atomic_store(shared_ptr<_Tp>* __p, shared_ptr<_Tp> __r)
    { std::atomic_store_explicit(__p, std::move(__r), memory_order_seq_cst); }

  template<typename _Tp, _Lock_policy _Lp>
    inline void
    atomic_store_explicit(__shared_ptr<_Tp, _Lp>* __p,
			  __shared_ptr<_Tp, _Lp> __r,
			  memory_order)
    {
      _Sp_locker __lock{__p};
      __p->swap(__r); // use swap so that **__p not destroyed while lock held
    }

  template<typename _Tp, _Lock_policy _Lp>
    inline void
    atomic_store(__shared_ptr<_Tp, _Lp>* __p, __shared_ptr<_Tp, _Lp> __r)
    { std::atomic_store_explicit(__p, std::move(__r), memory_order_seq_cst); }
  /// @}

  /**
   *  @brief  Atomic exchange for shared_ptr objects.
   *  @param  __p A non-null pointer to a shared_ptr object.
   *  @param  __r New value to store in @c *__p.
   *  @return The original value of @c *__p
   *  @{
  */
  template<typename _Tp>
    inline shared_ptr<_Tp>
    atomic_exchange_explicit(shared_ptr<_Tp>* __p, shared_ptr<_Tp> __r,
			     memory_order)
    {
      _Sp_locker __lock{__p};
      __p->swap(__r);
      return __r;
    }

  template<typename _Tp>
    inline shared_ptr<_Tp>
    atomic_exchange(shared_ptr<_Tp>* __p, shared_ptr<_Tp> __r)
    {
      return std::atomic_exchange_explicit(__p, std::move(__r),
					   memory_order_seq_cst);
    }

  template<typename _Tp, _Lock_policy _Lp>
    inline __shared_ptr<_Tp, _Lp>
    atomic_exchange_explicit(__shared_ptr<_Tp, _Lp>* __p,
			     __shared_ptr<_Tp, _Lp> __r,
			     memory_order)
    {
      _Sp_locker __lock{__p};
      __p->swap(__r);
      return __r;
    }

  template<typename _Tp, _Lock_policy _Lp>
    inline __shared_ptr<_Tp, _Lp>
    atomic_exchange(__shared_ptr<_Tp, _Lp>* __p, __shared_ptr<_Tp, _Lp> __r)
    {
      return std::atomic_exchange_explicit(__p, std::move(__r),
					   memory_order_seq_cst);
    }
  /// @}

  /**
   *  @brief  Atomic compare-and-swap for shared_ptr objects.
   *  @param  __p A non-null pointer to a shared_ptr object.
   *  @param  __v A non-null pointer to a shared_ptr object.
   *  @param  __w A non-null pointer to a shared_ptr object.
   *  @return True if @c *__p was equivalent to @c *__v, false otherwise.
   *
   *  The memory order for failure shall not be @c memory_order_release or
   *  @c memory_order_acq_rel, or stronger than the memory order for success.
   *  @{
  */
  template<typename _Tp>
    bool
    atomic_compare_exchange_strong_explicit(shared_ptr<_Tp>* __p,
					    shared_ptr<_Tp>* __v,
					    shared_ptr<_Tp> __w,
					    memory_order,
					    memory_order)
    {
      shared_ptr<_Tp> __x; // goes out of scope after __lock
      _Sp_locker __lock{__p, __v};
      owner_less<shared_ptr<_Tp>> __less;
      if (*__p == *__v && !__less(*__p, *__v) && !__less(*__v, *__p))
	{
	  __x = std::move(*__p);
	  *__p = std::move(__w);
	  return true;
	}
      __x = std::move(*__v);
      *__v = *__p;
      return false;
    }

  template<typename _Tp>
    inline bool
    atomic_compare_exchange_strong(shared_ptr<_Tp>* __p, shared_ptr<_Tp>* __v,
				 shared_ptr<_Tp> __w)
    {
      return std::atomic_compare_exchange_strong_explicit(__p, __v,
	  std::move(__w), memory_order_seq_cst, memory_order_seq_cst);
    }

  template<typename _Tp>
    inline bool
    atomic_compare_exchange_weak_explicit(shared_ptr<_Tp>* __p,
					  shared_ptr<_Tp>* __v,
					  shared_ptr<_Tp> __w,
					  memory_order __success,
					  memory_order __failure)
    {
      return std::atomic_compare_exchange_strong_explicit(__p, __v,
	  std::move(__w), __success, __failure);
    }

  template<typename _Tp>
    inline bool
    atomic_compare_exchange_weak(shared_ptr<_Tp>* __p, shared_ptr<_Tp>* __v,
				 shared_ptr<_Tp> __w)
    {
      return std::atomic_compare_exchange_weak_explicit(__p, __v,
	  std::move(__w), memory_order_seq_cst, memory_order_seq_cst);
    }

  template<typename _Tp, _Lock_policy _Lp>
    bool
    atomic_compare_exchange_strong_explicit(__shared_ptr<_Tp, _Lp>* __p,
					    __shared_ptr<_Tp, _Lp>* __v,
					    __shared_ptr<_Tp, _Lp> __w,
					    memory_order,
					    memory_order)
    {
      __shared_ptr<_Tp, _Lp> __x; // goes out of scope after __lock
      _Sp_locker __lock{__p, __v};
      owner_less<__shared_ptr<_Tp, _Lp>> __less;
      if (*__p == *__v && !__less(*__p, *__v) && !__less(*__v, *__p))
	{
	  __x = std::move(*__p);
	  *__p = std::move(__w);
	  return true;
	}
      __x = std::move(*__v);
      *__v = *__p;
      return false;
    }

  template<typename _Tp, _Lock_policy _Lp>
    inline bool
    atomic_compare_exchange_strong(__shared_ptr<_Tp, _Lp>* __p,
				   __shared_ptr<_Tp, _Lp>* __v,
				   __shared_ptr<_Tp, _Lp> __w)
    {
      return std::atomic_compare_exchange_strong_explicit(__p, __v,
	  std::move(__w), memory_order_seq_cst, memory_order_seq_cst);
    }

  template<typename _Tp, _Lock_policy _Lp>
    inline bool
    atomic_compare_exchange_weak_explicit(__shared_ptr<_Tp, _Lp>* __p,
					  __shared_ptr<_Tp, _Lp>* __v,
					  __shared_ptr<_Tp, _Lp> __w,
					  memory_order __success,
					  memory_order __failure)
    {
      return std::atomic_compare_exchange_strong_explicit(__p, __v,
	  std::move(__w), __success, __failure);
    }

  template<typename _Tp, _Lock_policy _Lp>
    inline bool
    atomic_compare_exchange_weak(__shared_ptr<_Tp, _Lp>* __p,
				 __shared_ptr<_Tp, _Lp>* __v,
				 __shared_ptr<_Tp, _Lp> __w)
    {
      return std::atomic_compare_exchange_weak_explicit(__p, __v,
	  std::move(__w), memory_order_seq_cst, memory_order_seq_cst);
    }
  /// @}

  /// @} relates shared_ptr
  /// @} group pointer_abstractions

_GLIBCXX_END_NAMESPACE_VERSION
} // namespace

#endif // _SHARED_PTR_ATOMIC_H