libcxx/include/__stop_token/intrusive_shared_ptr.h

   1 // -*- C++ -*-
   2 //===----------------------------------------------------------------------===//
   3 //
   4 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   5 // See https://llvm.org/LICENSE.txt for license information.
   6 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   7 //
   8 //===----------------------------------------------------------------------===//
   9
  10 #ifndef _LIBCPP___STOP_TOKEN_INTRUSIVE_SHARED_PTR_H
  11 #define _LIBCPP___STOP_TOKEN_INTRUSIVE_SHARED_PTR_H
  12
  13 #include <__atomic/atomic.h>
  14 #include <__atomic/memory_order.h>
  15 #include <__config>
  16 #include <__cstddef/nullptr_t.h>
  17 #include <__type_traits/is_reference.h>
  18 #include <__utility/move.h>
  19 #include <__utility/swap.h>
  20
  21 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
  22 #  pragma GCC system_header
  23 #endif
  24
  25 _LIBCPP_PUSH_MACROS
  26 #include <__undef_macros>
  27
  28 _LIBCPP_BEGIN_NAMESPACE_STD
  29
  30 #if _LIBCPP_STD_VER >= 20
  31
  32 // For intrusive_shared_ptr to work with a type T, specialize __intrusive_shared_ptr_traits<T> and implement
  33 // the following function:
  34 //
  35 // static std::atomic<U>& __get_atomic_ref_count(T&);
  36 //
  37 // where U must be an integral type representing the number of references to the object.
  38 template <class _Tp>
  39 struct __intrusive_shared_ptr_traits;
  40
  41 // A reference counting shared_ptr for types whose reference counter
  42 // is stored inside the class _Tp itself.
  43 // When the reference count goes to zero, the destructor of _Tp will be called
  44 template <class _Tp>
  45 struct __intrusive_shared_ptr {
  46   _LIBCPP_HIDE_FROM_ABI __intrusive_shared_ptr() = default;
  47
  48   _LIBCPP_HIDE_FROM_ABI explicit __intrusive_shared_ptr(_Tp* __raw_ptr) : __raw_ptr_(__raw_ptr) {
  49     if (__raw_ptr_)
  50       __increment_ref_count(*__raw_ptr_);
  51   }
  52
  53   _LIBCPP_HIDE_FROM_ABI __intrusive_shared_ptr(const __intrusive_shared_ptr& __other) noexcept
  54       : __raw_ptr_(__other.__raw_ptr_) {
  55     if (__raw_ptr_)
  56       __increment_ref_count(*__raw_ptr_);
  57   }
  58
  59   _LIBCPP_HIDE_FROM_ABI __intrusive_shared_ptr(__intrusive_shared_ptr&& __other) noexcept
  60       : __raw_ptr_(__other.__raw_ptr_) {
  61     __other.__raw_ptr_ = nullptr;
  62   }
  63
  64   _LIBCPP_HIDE_FROM_ABI __intrusive_shared_ptr& operator=(const __intrusive_shared_ptr& __other) noexcept {
  65     if (__other.__raw_ptr_ != __raw_ptr_) {
  66       if (__other.__raw_ptr_) {
  67         __increment_ref_count(*__other.__raw_ptr_);
  68       }
  69       if (__raw_ptr_) {
  70         __decrement_ref_count(*__raw_ptr_);
  71       }
  72       __raw_ptr_ = __other.__raw_ptr_;
  73     }
  74     return *this;
  75   }
  76
  77   _LIBCPP_HIDE_FROM_ABI __intrusive_shared_ptr& operator=(__intrusive_shared_ptr&& __other) noexcept {
  78     __intrusive_shared_ptr(std::move(__other)).swap(*this);
  79     return *this;
  80   }
  81
  82   _LIBCPP_HIDE_FROM_ABI ~__intrusive_shared_ptr() {
  83     if (__raw_ptr_) {
  84       __decrement_ref_count(*__raw_ptr_);
  85     }
  86   }
  87
  88   _LIBCPP_HIDE_FROM_ABI _Tp* operator->() const noexcept { return __raw_ptr_; }
  89   _LIBCPP_HIDE_FROM_ABI _Tp& operator*() const noexcept { return *__raw_ptr_; }
  90   _LIBCPP_HIDE_FROM_ABI explicit operator bool() const noexcept { return __raw_ptr_ != nullptr; }
  91
  92   _LIBCPP_HIDE_FROM_ABI void swap(__intrusive_shared_ptr& __other) { std::swap(__raw_ptr_, __other.__raw_ptr_); }
  93
  94   _LIBCPP_HIDE_FROM_ABI friend void swap(__intrusive_shared_ptr& __lhs, __intrusive_shared_ptr& __rhs) {
  95     __lhs.swap(__rhs);
  96   }
  97
  98   _LIBCPP_HIDE_FROM_ABI friend bool constexpr
  99   operator==(const __intrusive_shared_ptr&, const __intrusive_shared_ptr&) = default;
 100
 101   _LIBCPP_HIDE_FROM_ABI friend bool constexpr operator==(const __intrusive_shared_ptr& __ptr, std::nullptr_t) {
 102     return __ptr.__raw_ptr_ == nullptr;
 103   }
 104
 105 private:
 106   _Tp* __raw_ptr_ = nullptr;
 107
 108   // the memory order for increment/decrement the counter is the same for shared_ptr
 109   // increment is relaxed and decrement is acq_rel
 110   _LIBCPP_HIDE_FROM_ABI static void __increment_ref_count(_Tp& __obj) {
 111     __get_atomic_ref_count(__obj).fetch_add(1, std::memory_order_relaxed);
 112   }
 113
 114   _LIBCPP_HIDE_FROM_ABI static void __decrement_ref_count(_Tp& __obj) {
 115     if (__get_atomic_ref_count(__obj).fetch_sub(1, std::memory_order_acq_rel) == 1) {
 116       delete &__obj;
 117     }
 118   }
 119
 120   _LIBCPP_HIDE_FROM_ABI static decltype(auto) __get_atomic_ref_count(_Tp& __obj) {
 121     using __ret_type = decltype(__intrusive_shared_ptr_traits<_Tp>::__get_atomic_ref_count(__obj));
 122     static_assert(
 123         std::is_reference_v<__ret_type>, "__get_atomic_ref_count should return a reference to the atomic counter");
 124     return __intrusive_shared_ptr_traits<_Tp>::__get_atomic_ref_count(__obj);
 125   }
 126 };
 127
 128 #endif // _LIBCPP_STD_VER >= 20
 129
 130 _LIBCPP_END_NAMESPACE_STD
 131
 132 _LIBCPP_POP_MACROS
 133
 134 #endif // _LIBCPP___STOP_TOKEN_INTRUSIVE_SHARED_PTR_H