[libcxx-commits] [libcxx] [libc++] Implement P0718R2: `atomic<shared_ptr<T>>` (PR #78317)

[A. Jiang via libcxx-commits]

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+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___MEMORY_ATOMIC_SHARED_PTR_H
+#define _LIBCPP___MEMORY_ATOMIC_SHARED_PTR_H
+
+#include <__memory/addressof.h>
+#include <__memory/shared_ptr.h>
+#include <cstddef>
+#if !defined(_LIBCPP_HAS_NO_ATOMIC_HEADER)
+#  include <__atomic/memory_order.h>
+#endif
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if !defined(_LIBCPP_HAS_NO_THREADS)
+
+class _LIBCPP_EXPORTED_FROM_ABI __sp_mut {
+  void* __lx_;
+
+public:
+  void lock() _NOEXCEPT;
+  void unlock() _NOEXCEPT;
+
+private:
+  _LIBCPP_CONSTEXPR __sp_mut(void*) _NOEXCEPT;
+  __sp_mut(const __sp_mut&);
+  __sp_mut& operator=(const __sp_mut&);
+
+  friend _LIBCPP_EXPORTED_FROM_ABI __sp_mut& __get_sp_mut(const void*);
+};
+
+_LIBCPP_EXPORTED_FROM_ABI __sp_mut& __get_sp_mut(const void*);
+
+template <class _Tp>
+_LIBCPP_HIDE_FROM_ABI _Tp __sp_atomic_load(const _Tp* __p) {
+  __sp_mut& __m = std::__get_sp_mut(__p);
+  __m.lock();
+  _Tp __q = *__p;
+  __m.unlock();
+  return __q;
+}
+
+template <class _Tp>
+_LIBCPP_HIDE_FROM_ABI void __sp_atomic_store(_Tp* __p, _Tp& __r) {
+  __sp_mut& __m = std::__get_sp_mut(__p);
+  __m.lock();
+  __p->swap(__r);
+  __m.unlock();
+}
+
+template <class _Tp>
+_LIBCPP_HIDE_FROM_ABI bool __sp_atomic_compare_exchange_strong(_Tp* __p, _Tp* __v, _Tp& __w) {
+  _Tp __temp;
+  __sp_mut& __m = std::__get_sp_mut(__p);
+  __m.lock();
+  if (__p->__owner_equivalent(*__v)) {
+    std::swap(__temp, *__p);
+    *__p = __w;
+    __m.unlock();
+    return true;
+  }
+  std::swap(__temp, *__v);
+  *__v = *__p;
+  __m.unlock();
+  return false;
+}
+
+template <class _Tp>
+_LIBCPP_HIDE_FROM_ABI _Tp __sp_atomic_exchange(_Tp* __p, _Tp& __r) {
+  __sp_mut& __m = std::__get_sp_mut(__p);
+  __m.lock();
+  __p->swap(__r);
+  __m.unlock();
+  return __r;
+}
+
+#  if _LIBCPP_STD_VER >= 20
+template <class _Tp>
+struct atomic;
+
+template <class _Tp>
+struct __sp_atomic_base {
+  using value_type = _Tp;
+
+  static constexpr bool is_always_lock_free = false;
+  _LIBCPP_HIDE_FROM_ABI bool is_lock_free() const noexcept { return false; }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr __sp_atomic_base() noexcept = default;
+  _LIBCPP_HIDE_FROM_ABI __sp_atomic_base(_Tp&& __d) noexcept : __p(std::move(__d)) {}
+  _LIBCPP_HIDE_FROM_ABI __sp_atomic_base(const __sp_atomic_base&) = delete;
+  _LIBCPP_HIDE_FROM_ABI void operator=(const __sp_atomic_base&)   = delete;
+
+  _LIBCPP_HIDE_FROM_ABI _Tp load(memory_order = memory_order_seq_cst) noexcept {
+    return std::__sp_atomic_load(std::addressof(__p));
+  }
+  _LIBCPP_HIDE_FROM_ABI operator _Tp() const noexcept { return load(); }
+  _LIBCPP_HIDE_FROM_ABI void store(_Tp __d, memory_order = memory_order_seq_cst) noexcept {
+    std::__sp_atomic_store(std::addressof(__p), __d);
+  }
+  _LIBCPP_HIDE_FROM_ABI void operator=(_Tp __d) noexcept { std::__sp_atomic_store(std::addressof(__p), __d); }
+  _LIBCPP_HIDE_FROM_ABI void operator=(nullptr_t) noexcept { store(nullptr); }
+
+  _LIBCPP_HIDE_FROM_ABI _Tp exchange(_Tp __d, memory_order = memory_order_seq_cst) noexcept {
+    return std::__sp_atomic_exchange(std::addressof(__p), __d);
+  }
+  _LIBCPP_HIDE_FROM_ABI bool compare_exchange_weak(_Tp& __e, _Tp __d, memory_order, memory_order) noexcept {
+    return std::__sp_atomic_compare_exchange_strong(std::addressof(__p), std::addressof(__e), __d);
+  }
+  _LIBCPP_HIDE_FROM_ABI bool compare_exchange_strong(_Tp& __e, _Tp __d, memory_order, memory_order) noexcept {
+    return std::__sp_atomic_compare_exchange_strong(std::addressof(__p), std::addressof(__e), __d);
+  }
+  _LIBCPP_HIDE_FROM_ABI bool compare_exchange_weak(_Tp& __e, _Tp __d, memory_order = memory_order_seq_cst) noexcept {
+    return std::__sp_atomic_compare_exchange_strong(std::addressof(__p), std::addressof(__e), __d);
+  }
+  _LIBCPP_HIDE_FROM_ABI bool compare_exchange_strong(_Tp& __e, _Tp __d, memory_order = memory_order_seq_cst) noexcept {
+    return std::__sp_atomic_compare_exchange_strong(std::addressof(__p), std::addressof(__e), __d);
+  }
+
+  // P1644R0 not implemented
+  // void wait(_Tp old, memory_order order = memory_order::seq_cst) const noexcept;
+  // void notify_one() noexcept;
+  // void notify_all() noexcept;
+
+private:
+  _Tp __p;
+};
+
+template <class _Tp>
+struct atomic<shared_ptr<_Tp>> : __sp_atomic_base<shared_ptr<_Tp>> {
+  _LIBCPP_HIDE_FROM_ABI constexpr atomic() noexcept = default;
+  _LIBCPP_HIDE_FROM_ABI constexpr atomic(nullptr_t) noexcept : atomic() {}
+  _LIBCPP_HIDE_FROM_ABI atomic(shared_ptr<_Tp> desired) noexcept
+      : __sp_atomic_base<shared_ptr<_Tp>>(std::move(desired)) {}
+  _LIBCPP_HIDE_FROM_ABI atomic(const atomic&) = delete;
+
+  _LIBCPP_HIDE_FROM_ABI void operator=(const atomic&) = delete;
+  using __sp_atomic_base<shared_ptr<_Tp>>::operator=;
+};
+
+template <class _Tp>
+struct atomic<weak_ptr<_Tp>> : __sp_atomic_base<weak_ptr<_Tp>> {
+  _LIBCPP_HIDE_FROM_ABI constexpr atomic() noexcept = default;
+  _LIBCPP_HIDE_FROM_ABI constexpr atomic(nullptr_t) noexcept : atomic() {}
----------------
frederick-vs-ja wrote:

The reason was said in [LWG3611](https://cplusplus.github.io/LWG/issue3661):
> There is no need to also change `atomic<weak_ptr<T>>` because `weak_ptr` doesn't have a constructor taking `nullptr`.

Did you submit an LWG issue for corresponding change of `atomic<weak_ptr<T>>`?

https://github.com/llvm/llvm-project/pull/78317