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
60     constexpr __non_propagating_cache(__non_propagating_cache const&) noexcept
61       : __value_(nullopt)
62     { }
63 
64     _LIBCPP_HIDE_FROM_ABI
65     constexpr __non_propagating_cache(__non_propagating_cache&& __other) noexcept
66       : __value_(nullopt)
67     {
68       __other.__value_.reset();
69     }
70 
71     _LIBCPP_HIDE_FROM_ABI
72     constexpr __non_propagating_cache& operator=(__non_propagating_cache const& __other) noexcept {
73       if (this != std::addressof(__other)) {
74         __value_.reset();
75       }
76       return *this;
77     }
78 
79     _LIBCPP_HIDE_FROM_ABI
80     constexpr __non_propagating_cache& operator=(__non_propagating_cache&& __other) noexcept {
81       __value_.reset();
82       __other.__value_.reset();
83       return *this;
84     }
85 
86     _LIBCPP_HIDE_FROM_ABI
87     constexpr _Tp& operator*() { return __value_->__t_; }
88     _LIBCPP_HIDE_FROM_ABI
89     constexpr _Tp const& operator*() const { return __value_->__t_; }
90 
91     _LIBCPP_HIDE_FROM_ABI
92     constexpr bool __has_value() const { return __value_.has_value(); }
93 
94     template<class _Fn>
95     _LIBCPP_HIDE_FROM_ABI
96     constexpr _Tp& __emplace_from(_Fn const& __f) {
97       return __value_.emplace(__from_tag{}, __f).__t_;
98     }
99 
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_;
104     }
105   };
106 
107   struct __empty_cache { };
108 } // namespace ranges
109 
110 #endif // _LIBCPP_STD_VER >= 20
111 
112 _LIBCPP_END_NAMESPACE_STD
113 
114 #endif // _LIBCPP___RANGES_NON_PROPAGATING_CACHE_H
115