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
 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 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.
 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 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
 56public:
 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
 96struct __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