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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-exception7//8//===----------------------------------------------------------------------===//9 10#ifndef _LIBCPP___RANGES_NON_PROPAGATING_CACHE_H11#define _LIBCPP___RANGES_NON_PROPAGATING_CACHE_H12 13#include <__config>14#include <__iterator/concepts.h> // indirectly_readable15#include <__iterator/iterator_traits.h> // iter_reference_t16#include <__memory/addressof.h>17#include <__utility/forward.h>18#include <optional>19 20#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)21# pragma GCC system_header22#endif23 24_LIBCPP_BEGIN_NAMESPACE_STD25 26#if _LIBCPP_STD_VER >= 2027 28namespace ranges {29// __non_propagating_cache is a helper type that allows storing an optional value in it,30// but which does not copy the source's value when it is copy constructed/assigned to,31// and which resets the source's value when it is moved-from.32//33// This type is used as an implementation detail of some views that need to cache the34// result of `begin()` in order to provide an amortized O(1) begin() method. Typically,35// we don't want to propagate the value of the cache upon copy because the cached iterator36// may refer to internal details of the source view.37template <class _Tp>38 requires is_object_v<_Tp>39class __non_propagating_cache {40 struct __from_tag {};41 struct __forward_tag {};42 43 // This helper class is needed to perform copy and move elision when44 // constructing the contained type from an iterator.45 struct __wrapper {46 template <class... _Args>47 _LIBCPP_HIDE_FROM_ABI constexpr explicit __wrapper(__forward_tag, _Args&&... __args)48 : __t_(std::forward<_Args>(__args)...) {}49 template <class _Fn>50 _LIBCPP_HIDE_FROM_ABI constexpr explicit __wrapper(__from_tag, _Fn const& __f) : __t_(__f()) {}51 _Tp __t_;52 };53 54 optional<__wrapper> __value_ = nullopt;55 56public:57 _LIBCPP_HIDE_FROM_ABI __non_propagating_cache() = default;58 59 _LIBCPP_HIDE_FROM_ABI constexpr __non_propagating_cache(__non_propagating_cache const&) noexcept60 : __value_(nullopt) {}61 62 _LIBCPP_HIDE_FROM_ABI constexpr __non_propagating_cache(__non_propagating_cache&& __other) noexcept63 : __value_(nullopt) {64 __other.__value_.reset();65 }66 67 _LIBCPP_HIDE_FROM_ABI constexpr __non_propagating_cache& operator=(__non_propagating_cache const& __other) noexcept {68 if (this != std::addressof(__other)) {69 __value_.reset();70 }71 return *this;72 }73 74 _LIBCPP_HIDE_FROM_ABI constexpr __non_propagating_cache& operator=(__non_propagating_cache&& __other) noexcept {75 __value_.reset();76 __other.__value_.reset();77 return *this;78 }79 80 _LIBCPP_HIDE_FROM_ABI constexpr _Tp& operator*() { return __value_->__t_; }81 _LIBCPP_HIDE_FROM_ABI constexpr _Tp const& operator*() const { return __value_->__t_; }82 83 _LIBCPP_HIDE_FROM_ABI constexpr bool __has_value() const { return __value_.has_value(); }84 85 template <class _Fn>86 _LIBCPP_HIDE_FROM_ABI constexpr _Tp& __emplace_from(_Fn const& __f) {87 return __value_.emplace(__from_tag{}, __f).__t_;88 }89 90 template <class... _Args>91 _LIBCPP_HIDE_FROM_ABI constexpr _Tp& __emplace(_Args&&... __args) {92 return __value_.emplace(__forward_tag{}, std::forward<_Args>(__args)...).__t_;93 }94};95 96struct __empty_cache {};97} // namespace ranges98 99#endif // _LIBCPP_STD_VER >= 20100 101_LIBCPP_END_NAMESPACE_STD102 103#endif // _LIBCPP___RANGES_NON_PROPAGATING_CACHE_H104