2 //===----------------------------------------------------------------------===//
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
8 //===----------------------------------------------------------------------===//
9 #ifndef _LIBCPP___RANGES_NON_PROPAGATING_CACHE_H
10 #define _LIBCPP___RANGES_NON_PROPAGATING_CACHE_H
13 #include <__iterator/concepts.h> // indirectly_readable
14 #include <__iterator/iterator_traits.h> // iter_reference_t
15 #include <__memory/addressof.h>
16 #include <__utility/forward.h>
17 #include <concepts> // constructible_from
19 #include <type_traits>
21 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
22 # pragma GCC system_header
25 _LIBCPP_BEGIN_NAMESPACE_STD
27 #if _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
30 // __non_propagating_cache is a helper type that allows storing an optional value in it,
31 // but which does not copy the source's value when it is copy constructed/assigned to,
32 // and which resets the source's value when it is moved-from.
34 // This type is used as an implementation detail of some views that need to cache the
35 // result of `begin()` in order to provide an amortized O(1) begin() method. Typically,
36 // we don't want to propagate the value of the cache upon copy because the cached iterator
37 // may refer to internal details of the source view.
39 requires is_object_v<_Tp>
40 class _LIBCPP_TEMPLATE_VIS __non_propagating_cache {
41 struct __from_tag { };
42 struct __forward_tag { };
44 // This helper class is needed to perform copy and move elision when
45 // constructing the contained type from an iterator.
47 template<class ..._Args>
48 constexpr explicit __wrapper(__forward_tag, _Args&& ...__args) : __t_(std::forward<_Args>(__args)...) { }
50 constexpr explicit __wrapper(__from_tag, _Fn const& __f) : __t_(__f()) { }
54 optional<__wrapper> __value_ = nullopt;
57 _LIBCPP_HIDE_FROM_ABI __non_propagating_cache() = default;
60 constexpr __non_propagating_cache(__non_propagating_cache const&) noexcept
65 constexpr __non_propagating_cache(__non_propagating_cache&& __other) noexcept
68 __other.__value_.reset();
72 constexpr __non_propagating_cache& operator=(__non_propagating_cache const& __other) noexcept {
73 if (this != std::addressof(__other)) {
80 constexpr __non_propagating_cache& operator=(__non_propagating_cache&& __other) noexcept {
82 __other.__value_.reset();
87 constexpr _Tp& operator*() { return __value_->__t_; }
89 constexpr _Tp const& operator*() const { return __value_->__t_; }
92 constexpr bool __has_value() const { return __value_.has_value(); }
96 constexpr _Tp& __emplace_from(_Fn const& __f) {
97 return __value_.emplace(__from_tag{}, __f).__t_;
100 template<class ..._Args>
101 _LIBCPP_HIDE_FROM_ABI
102 constexpr _Tp& __emplace(_Args&& ...__args) {
103 return __value_.emplace(__forward_tag{}, std::forward<_Args>(__args)...).__t_;
107 struct __empty_cache { };
108 } // namespace ranges
110 #endif // _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
112 _LIBCPP_END_NAMESPACE_STD
114 #endif // _LIBCPP___RANGES_NON_PROPAGATING_CACHE_H