libcxx/include/__ranges/non_propagating_cache.h

   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