__cxx03/__stop_token/intrusive_shared_ptr.h

0001 // -*- C++ -*-
0002 //===----------------------------------------------------------------------===//
0003 //
0004 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
0005 // See https://llvm.org/LICENSE.txt for license information.
0006 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
0007 //
0008 //===----------------------------------------------------------------------===//
0009
0010 #ifndef _LIBCPP___CXX03___STOP_TOKEN_INTRUSIVE_SHARED_PTR_H
0011 #define _LIBCPP___CXX03___STOP_TOKEN_INTRUSIVE_SHARED_PTR_H
0012
0013 #include <__cxx03/__atomic/atomic.h>
0014 #include <__cxx03/__atomic/memory_order.h>
0015 #include <__cxx03/__config>
0016 #include <__cxx03/__type_traits/is_reference.h>
0017 #include <__cxx03/__utility/move.h>
0018 #include <__cxx03/__utility/swap.h>
0019 #include <__cxx03/cstddef>
0020
0021 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
0022 #  pragma GCC system_header
0023 #endif
0024
0025 _LIBCPP_PUSH_MACROS
0026 #include <__cxx03/__undef_macros>
0027
0028 _LIBCPP_BEGIN_NAMESPACE_STD
0029
0030 #if _LIBCPP_STD_VER >= 20
0031
0032 // For intrusive_shared_ptr to work with a type T, specialize __intrusive_shared_ptr_traits<T> and implement
0033 // the following function:
0034 //
0035 // static std::atomic<U>& __get_atomic_ref_count(T&);
0036 //
0037 // where U must be an integral type representing the number of references to the object.
0038 template <class _Tp>
0039 struct __intrusive_shared_ptr_traits;
0040
0041 // A reference counting shared_ptr for types whose reference counter
0042 // is stored inside the class _Tp itself.
0043 // When the reference count goes to zero, the destructor of _Tp will be called
0044 template <class _Tp>
0045 struct __intrusive_shared_ptr {
0046   _LIBCPP_HIDE_FROM_ABI __intrusive_shared_ptr() = default;
0047
0048   _LIBCPP_HIDE_FROM_ABI explicit __intrusive_shared_ptr(_Tp* __raw_ptr) : __raw_ptr_(__raw_ptr) {
0049     if (__raw_ptr_)
0050       __increment_ref_count(*__raw_ptr_);
0051   }
0052
0053   _LIBCPP_HIDE_FROM_ABI __intrusive_shared_ptr(const __intrusive_shared_ptr& __other) noexcept
0054       : __raw_ptr_(__other.__raw_ptr_) {
0055     if (__raw_ptr_)
0056       __increment_ref_count(*__raw_ptr_);
0057   }
0058
0059   _LIBCPP_HIDE_FROM_ABI __intrusive_shared_ptr(__intrusive_shared_ptr&& __other) noexcept
0060       : __raw_ptr_(__other.__raw_ptr_) {
0061     __other.__raw_ptr_ = nullptr;
0062   }
0063
0064   _LIBCPP_HIDE_FROM_ABI __intrusive_shared_ptr& operator=(const __intrusive_shared_ptr& __other) noexcept {
0065     if (__other.__raw_ptr_ != __raw_ptr_) {
0066       if (__other.__raw_ptr_) {
0067         __increment_ref_count(*__other.__raw_ptr_);
0068       }
0069       if (__raw_ptr_) {
0070         __decrement_ref_count(*__raw_ptr_);
0071       }
0072       __raw_ptr_ = __other.__raw_ptr_;
0073     }
0074     return *this;
0075   }
0076
0077   _LIBCPP_HIDE_FROM_ABI __intrusive_shared_ptr& operator=(__intrusive_shared_ptr&& __other) noexcept {
0078     __intrusive_shared_ptr(std::move(__other)).swap(*this);
0079     return *this;
0080   }
0081
0082   _LIBCPP_HIDE_FROM_ABI ~__intrusive_shared_ptr() {
0083     if (__raw_ptr_) {
0084       __decrement_ref_count(*__raw_ptr_);
0085     }
0086   }
0087
0088   _LIBCPP_HIDE_FROM_ABI _Tp* operator->() const noexcept { return __raw_ptr_; }
0089   _LIBCPP_HIDE_FROM_ABI _Tp& operator*() const noexcept { return *__raw_ptr_; }
0090   _LIBCPP_HIDE_FROM_ABI explicit operator bool() const noexcept { return __raw_ptr_ != nullptr; }
0091
0092   _LIBCPP_HIDE_FROM_ABI void swap(__intrusive_shared_ptr& __other) { std::swap(__raw_ptr_, __other.__raw_ptr_); }
0093
0094   _LIBCPP_HIDE_FROM_ABI friend void swap(__intrusive_shared_ptr& __lhs, __intrusive_shared_ptr& __rhs) {
0095     __lhs.swap(__rhs);
0096   }
0097
0098   _LIBCPP_HIDE_FROM_ABI friend bool constexpr
0099   operator==(const __intrusive_shared_ptr&, const __intrusive_shared_ptr&) = default;
0100
0101   _LIBCPP_HIDE_FROM_ABI friend bool constexpr operator==(const __intrusive_shared_ptr& __ptr, std::nullptr_t) {
0102     return __ptr.__raw_ptr_ == nullptr;
0103   }
0104
0105 private:
0106   _Tp* __raw_ptr_ = nullptr;
0107
0108   // the memory order for increment/decrement the counter is the same for shared_ptr
0109   // increment is relaxed and decrement is acq_rel
0110   _LIBCPP_HIDE_FROM_ABI static void __increment_ref_count(_Tp& __obj) {
0111     __get_atomic_ref_count(__obj).fetch_add(1, std::memory_order_relaxed);
0112   }
0113
0114   _LIBCPP_HIDE_FROM_ABI static void __decrement_ref_count(_Tp& __obj) {
0115     if (__get_atomic_ref_count(__obj).fetch_sub(1, std::memory_order_acq_rel) == 1) {
0116       delete &__obj;
0117     }
0118   }
0119
0120   _LIBCPP_HIDE_FROM_ABI static decltype(auto) __get_atomic_ref_count(_Tp& __obj) {
0121     using __ret_type = decltype(__intrusive_shared_ptr_traits<_Tp>::__get_atomic_ref_count(__obj));
0122     static_assert(
0123         std::is_reference_v<__ret_type>, "__get_atomic_ref_count should return a reference to the atomic counter");
0124     return __intrusive_shared_ptr_traits<_Tp>::__get_atomic_ref_count(__obj);
0125   }
0126 };
0127
0128 #endif // _LIBCPP_STD_VER >= 20
0129
0130 _LIBCPP_END_NAMESPACE_STD
0131
0132 _LIBCPP_POP_MACROS
0133
0134 #endif // _LIBCPP___CXX03___STOP_TOKEN_INTRUSIVE_SHARED_PTR_H