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// -*- C++ -*-
//===----------------------------------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
#ifndef _LIBCPP___RANGES_NON_PROPAGATING_CACHE_H
#define _LIBCPP___RANGES_NON_PROPAGATING_CACHE_H
#include <__config>
#include <__iterator/concepts.h> // indirectly_readable
#include <__iterator/iterator_traits.h> // iter_reference_t
#include <__memory/addressof.h>
#include <__utility/forward.h>
#include <optional>
#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
# pragma GCC system_header
#endif
_LIBCPP_BEGIN_NAMESPACE_STD
#if _LIBCPP_STD_VER >= 20
namespace ranges {
// __non_propagating_cache is a helper type that allows storing an optional value in it,
// but which does not copy the source's value when it is copy constructed/assigned to,
// and which resets the source's value when it is moved-from.
//
// This type is used as an implementation detail of some views that need to cache the
// result of `begin()` in order to provide an amortized O(1) begin() method. Typically,
// we don't want to propagate the value of the cache upon copy because the cached iterator
// may refer to internal details of the source view.
template<class _Tp>
requires is_object_v<_Tp>
class _LIBCPP_TEMPLATE_VIS __non_propagating_cache {
struct __from_tag { };
struct __forward_tag { };
// This helper class is needed to perform copy and move elision when
// constructing the contained type from an iterator.
struct __wrapper {
template <class... _Args>
_LIBCPP_HIDE_FROM_ABI constexpr explicit __wrapper(__forward_tag, _Args&&... __args)
: __t_(std::forward<_Args>(__args)...) {}
template <class _Fn>
_LIBCPP_HIDE_FROM_ABI constexpr explicit __wrapper(__from_tag, _Fn const& __f) : __t_(__f()) {}
_Tp __t_;
};
optional<__wrapper> __value_ = nullopt;
public:
_LIBCPP_HIDE_FROM_ABI __non_propagating_cache() = default;
_LIBCPP_HIDE_FROM_ABI
constexpr __non_propagating_cache(__non_propagating_cache const&) noexcept
: __value_(nullopt)
{ }
_LIBCPP_HIDE_FROM_ABI
constexpr __non_propagating_cache(__non_propagating_cache&& __other) noexcept
: __value_(nullopt)
{
__other.__value_.reset();
}
_LIBCPP_HIDE_FROM_ABI
constexpr __non_propagating_cache& operator=(__non_propagating_cache const& __other) noexcept {
if (this != std::addressof(__other)) {
__value_.reset();
}
return *this;
}
_LIBCPP_HIDE_FROM_ABI
constexpr __non_propagating_cache& operator=(__non_propagating_cache&& __other) noexcept {
__value_.reset();
__other.__value_.reset();
return *this;
}
_LIBCPP_HIDE_FROM_ABI
constexpr _Tp& operator*() { return __value_->__t_; }
_LIBCPP_HIDE_FROM_ABI
constexpr _Tp const& operator*() const { return __value_->__t_; }
_LIBCPP_HIDE_FROM_ABI
constexpr bool __has_value() const { return __value_.has_value(); }
template<class _Fn>
_LIBCPP_HIDE_FROM_ABI
constexpr _Tp& __emplace_from(_Fn const& __f) {
return __value_.emplace(__from_tag{}, __f).__t_;
}
template<class ..._Args>
_LIBCPP_HIDE_FROM_ABI
constexpr _Tp& __emplace(_Args&& ...__args) {
return __value_.emplace(__forward_tag{}, std::forward<_Args>(__args)...).__t_;
}
};
struct __empty_cache { };
} // namespace ranges
#endif // _LIBCPP_STD_VER >= 20
_LIBCPP_END_NAMESPACE_STD
#endif // _LIBCPP___RANGES_NON_PROPAGATING_CACHE_H
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