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___RANGES_NON_PROPAGATING_CACHE_H 11 #define _LIBCPP___RANGES_NON_PROPAGATING_CACHE_H 12 13 #include <__config> 14 #include <__iterator/concepts.h> // indirectly_readable 15 #include <__iterator/iterator_traits.h> // iter_reference_t 16 #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_header 22 #endif 23 24 _LIBCPP_BEGIN_NAMESPACE_STD 25 26 #if _LIBCPP_STD_VER >= 20 27 28 namespace 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 the 34 // 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 iterator 36 // may refer to internal details of the source view. 37 template <class _Tp> 38 requires is_object_v<_Tp> 39 class _LIBCPP_TEMPLATE_VIS __non_propagating_cache { 40 struct __from_tag {}; 41 struct __forward_tag {}; 42 43 // This helper class is needed to perform copy and move elision when 44 // 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 56 public: 57 _LIBCPP_HIDE_FROM_ABI __non_propagating_cache() = default; 58 59 _LIBCPP_HIDE_FROM_ABI constexpr __non_propagating_cache(__non_propagating_cache const&) noexcept 60 : __value_(nullopt) {} 61 62 _LIBCPP_HIDE_FROM_ABI constexpr __non_propagating_cache(__non_propagating_cache&& __other) noexcept 63 : __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 96 struct __empty_cache {}; 97 } // namespace ranges 98 99 #endif // _LIBCPP_STD_VER >= 20 100 101 _LIBCPP_END_NAMESPACE_STD 102 103 #endif // _LIBCPP___RANGES_NON_PROPAGATING_CACHE_H 104