ceff0b7258
As explained in http://eel.is/c++draft/range.nonprop.cache#note-1, we should allow copy and move elision to happen when calling emplace_deref in non-propagating-cache. Before this change, the only way to emplace into the non-propagating-cache was to call `__set(*it)`, which materialized `*it` when binding it to the reference argument of `__set` and disabled move elision. As a fly-by change, this also renames `__set` to `__emplace` for consistency and adds tests for it. Differential Revision: https://reviews.llvm.org/D107932
115 lines
3.8 KiB
C++
115 lines
3.8 KiB
C++
// -*- C++ -*-
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//===----------------------------------------------------------------------===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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#ifndef _LIBCPP___RANGES_NON_PROPAGATING_CACHE_H
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#define _LIBCPP___RANGES_NON_PROPAGATING_CACHE_H
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#include <__config>
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#include <__iterator/concepts.h> // indirectly_readable
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#include <__iterator/iterator_traits.h> // iter_reference_t
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#include <__memory/addressof.h>
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#include <__utility/forward.h>
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#include <concepts> // constructible_from
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#include <optional>
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#include <type_traits>
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#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
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#pragma GCC system_header
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#endif
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_LIBCPP_BEGIN_NAMESPACE_STD
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#if !defined(_LIBCPP_HAS_NO_RANGES)
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namespace ranges {
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// __non_propagating_cache is a helper type that allows storing an optional value in it,
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// but which does not copy the source's value when it is copy constructed/assigned to,
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// and which resets the source's value when it is moved-from.
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//
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// This type is used as an implementation detail of some views that need to cache the
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// result of `begin()` in order to provide an amortized O(1) begin() method. Typically,
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// we don't want to propagate the value of the cache upon copy because the cached iterator
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// may refer to internal details of the source view.
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template<class _Tp>
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requires is_object_v<_Tp>
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class _LIBCPP_TEMPLATE_VIS __non_propagating_cache {
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struct __from_tag { };
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struct __forward_tag { };
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// This helper class is needed to perform copy and move elision when
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// constructing the contained type from an iterator.
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struct __wrapper {
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template<class ..._Args>
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constexpr explicit __wrapper(__forward_tag, _Args&& ...__args) : __t_(_VSTD::forward<_Args>(__args)...) { }
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template<class _Fn>
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constexpr explicit __wrapper(__from_tag, _Fn const& __f) : __t_(__f()) { }
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_Tp __t_;
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};
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optional<__wrapper> __value_ = nullopt;
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public:
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_LIBCPP_HIDE_FROM_ABI __non_propagating_cache() = default;
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_LIBCPP_HIDE_FROM_ABI
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constexpr __non_propagating_cache(__non_propagating_cache const&) noexcept
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: __value_(nullopt)
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{ }
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_LIBCPP_HIDE_FROM_ABI
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constexpr __non_propagating_cache(__non_propagating_cache&& __other) noexcept
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: __value_(nullopt)
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{
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__other.__value_.reset();
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}
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_LIBCPP_HIDE_FROM_ABI
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constexpr __non_propagating_cache& operator=(__non_propagating_cache const& __other) noexcept {
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if (this != _VSTD::addressof(__other)) {
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__value_.reset();
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}
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return *this;
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}
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_LIBCPP_HIDE_FROM_ABI
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constexpr __non_propagating_cache& operator=(__non_propagating_cache&& __other) noexcept {
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__value_.reset();
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__other.__value_.reset();
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return *this;
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}
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_LIBCPP_HIDE_FROM_ABI
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constexpr _Tp& operator*() { return __value_->__t_; }
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_LIBCPP_HIDE_FROM_ABI
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constexpr _Tp const& operator*() const { return __value_->__t_; }
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_LIBCPP_HIDE_FROM_ABI
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constexpr bool __has_value() const { return __value_.has_value(); }
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template<class _Fn>
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_LIBCPP_HIDE_FROM_ABI
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constexpr _Tp& __emplace_from(_Fn const& __f) {
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return __value_.emplace(__from_tag{}, __f).__t_;
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}
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template<class ..._Args>
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_LIBCPP_HIDE_FROM_ABI
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constexpr _Tp& __emplace(_Args&& ...__args) {
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return __value_.emplace(__forward_tag{}, _VSTD::forward<_Args>(__args)...).__t_;
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}
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};
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struct __empty_cache { };
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} // namespace ranges
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#endif // _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_RANGES)
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_LIBCPP_END_NAMESPACE_STD
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#endif // _LIBCPP___RANGES_NON_PROPAGATING_CACHE_H
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