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Merge pull request #154 from steve-downey/rebase_optimize_adaptors
Rebase optimize adaptors paper as published
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‎papers/P2988/Makefile‎

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PYEXECPATH ?= $(shell which python3.12 || which python3.11 || which python3.10)
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PYEXECPATH ?= $(shell which python3.13 || which python3.12 || which python3.11 || which python3.10)
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PYTHON ?= $(shell basename $(PYEXECPATH))
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VENV := .venv/
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SOURCE_VENV := . $(VENV)/bin/activate;

‎papers/P2988/common.tex‎

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\usepackage{color} % define colors for strikeouts and underlines
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\usepackage{amsmath} % additional math symbols
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\usepackage{mathrsfs} % mathscr font
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\usepackage{bm}
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\usepackage[final]{microtype}
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\usepackage[splitindex,original]{imakeidx}
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\usepackage{multicol}
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% pdflang={English}]{hyperref}
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\usepackage{memhfixc} % fix interactions between hyperref and memoir
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\usepackage{environ}
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\usepackage{expl3}
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\usepackage{xparse}
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\usepackage{xstring}

‎papers/P2988/optional_range_optimization.tex‎

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\documentclass[a4paper,10pt,oneside,openany,final,article]{memoir}
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\input{common}
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\settocdepth{chapter}
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\usepackage{minted}
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\usepackage{fontspec}
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\begin{document}
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\title{Optimize for std::optional in range adaptors}
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\author{
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Steve Downey \small<\href{mailto:sdowney@gmail.com}{sdowney@gmail.com}> \\
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Tomasz Kamiński \small<\href{mailto:tomaszkam@gmail.com}{tomaszkam@gmail.com}> \\
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}
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\date{} %unused. Type date explicitly below.
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\maketitle
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\begin{flushright}
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\begin{tabular}{ll}
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Document \#: & P3913R1 \\
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Date: & \today \\
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Project: & Programming Language C++ \\
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Audience: & LWG
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\end{tabular}
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\end{flushright}
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\begin{abstract}
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From PL-011 22.5 [optional] Optimize for std::optional in range adaptors
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The range support was added to the optional, making it usable with range adaptors defined in std::views, however, we have not updated the views specification to handle it optimally when possible. This leads to unnecessary template instantiations.
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\end{abstract}
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\tableofcontents*
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\chapter{Motivation}
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The range support was added to the optional, making it usable with range adaptors defined in std::views, however, we have not updated the views specification to handle it optimally when possible. This leads to unnecessary template instantiations.
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Proposed change:
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Add a special case to recognize optional for adaptors:
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\begin{itemize}
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\item
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views::as_const: should return optional or optional<const U\&> (if T is U\&)
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\item
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views::take(opt, n): empty optional if n is equal to zero, opt otherwise
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\item
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views::drop(opt, n): empty optional if n greater than zero, opt otherwise
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\item
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views::reverse: input unchanged
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\end{itemize}
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\chapter{Design}
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\section{views::as_const}
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Return \tcode{optional}.
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In contrast to \tcode{optional<const T\&>}, \tcode{optional<const T>} is not a view, because it is not assignable. In consequence it should not be returned from \tcode{views::as_const} for \tcode{optional<T>}.
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\section{views::take(opt, n)}
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Empty \tcode{optional} if \tcode{n} is equal to zero, \tcode{optional} otherwise.
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\section{views::drop(opt, n)}
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Empty \tcode{optional} if \tcode{n} greater than zero, \tcode{optional} otherwise.
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\section{views::reverse}
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Input is returned unchanged.
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\chapter{Wording}
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The proposed changes are relative to the current working draft \cite{N5014}.
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\begin{wording}
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\rSec1[ranges.general]{General}
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\rSec2[range.take]{Take view}
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\rSec3[range.take.overview]{Overview}
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\pnum
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\tcode{take_view} produces a view of the first $N$ elements
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from another view, or all the elements if the adapted
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view contains fewer than $N$.
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\pnum
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\indexlibrarymember{take}{views}%
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The name \tcode{views::take} denotes a
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range adaptor object\iref{range.adaptor.object}.
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Let \tcode{E} and \tcode{F} be expressions,
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let \tcode{T} be \tcode{remove_cvref_t<decltype((E))>}, and
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let \tcode{D} be \tcode{range_difference_t<decltype((E))>}.
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If \tcode{decltype((F))} does not model
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\tcode{\libconcept{convertible_to}<D>},
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\tcode{views::take(E, F)} is ill-formed.
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Otherwise, the expression \tcode{views::take(E, F)}
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is expression-equivalent to:
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\begin{itemize}
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\item
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If \tcode{T} is a specialization
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of \tcode{empty_view}\iref{range.empty.view},
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then \tcode{((void)F, \placeholdernc{decay-copy}(E))},
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except that the evaluations of \tcode{E} and \tcode{F}
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are indeterminately sequenced.
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\begin{addedblock}
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\item
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Otherwise, if \tcode{T} is a specialization of \tcode{optional} and \tcode{T} models \tcode{view}, then \tcode{(static_cast<D>(F) == D() ? ((void)E, T()) : \placeholdernc{decay-copy}(E))}.
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\end{addedblock}
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\item
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Otherwise, if \tcode{T} models
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\libconcept{random_access_range} and \libconcept{sized_range}
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and is a specialization of
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\tcode{span}\iref{views.span},
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\tcode{basic_string_view}\iref{string.view}, or
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\tcode{subrange}\iref{range.subrange},
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then
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\tcode{U(ranges::begin(E),
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ranges::be\-gin(E) + std::min<D>(ranges::distance(E), F))},
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except that \tcode{E} is evaluated only once,
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where \tcode{U} is a type determined as follows:
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\begin{itemize}
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\item if \tcode{T} is a specialization of \tcode{span},
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then \tcode{U} is \tcode{span<typename T::element_type>};
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\item otherwise, if \tcode{T} is a specialization of \tcode{basic_string_view},
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then \tcode{U} is \tcode{T};
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\item otherwise, \tcode{T} is a specialization of \tcode{subrange}, and
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\tcode{U} is \tcode{subrange<iterator_t<T>>};
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\end{itemize}
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\item
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otherwise, if \tcode{T} is
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a specialization of \tcode{iota_view}\iref{range.iota.view}
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that models \libconcept{random_access_range} and \libconcept{sized_range},
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then
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\tcode{iota_view(*ranges::begin(E),
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*(ranges::begin(E) + std::\linebreak{}min<D>(ranges::distance(E), F)))},
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except that \tcode{E} is evaluated only once.
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\item
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Otherwise, if \tcode{T} is
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a specialization of \tcode{repeat_view}\iref{range.repeat.view}:
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\begin{itemize}
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\item
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if \tcode{T} models \libconcept{sized_range},
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then
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\begin{codeblock}
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views::repeat(*E.@\exposid{value_}@, std::min<D>(ranges::distance(E), F))
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\end{codeblock}
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except that \tcode{E} is evaluated only once;
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\item
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otherwise, \tcode{views::repeat(*E.\exposid{value_}, static_cast<D>(F))}.
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\end{itemize}
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\item
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Otherwise, \tcode{take_view(E, F)}.
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\end{itemize}
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\rSec2[range.drop]{Drop view}
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\rSec3[range.drop.overview]{Overview}
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\pnum
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\tcode{drop_view} produces a view
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excluding the first $N$ elements from another view, or
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an empty range if the adapted view contains fewer than $N$ elements.
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\pnum
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\indexlibrarymember{drop}{views}%
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The name \tcode{views::drop} denotes
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a range adaptor object\iref{range.adaptor.object}.
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Let \tcode{E} and \tcode{F} be expressions,
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let \tcode{T} be \tcode{remove_cvref_t<decltype((E))>}, and
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let \tcode{D} be \tcode{range_difference_t<decltype((E))>}.
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If \tcode{decltype((F))} does not model
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\tcode{\libconcept{convertible_to}<D>},
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\tcode{views::drop(E, F)} is ill-formed.
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Otherwise, the expression \tcode{views::drop(E, F)}
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is expression-equivalent to:
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\begin{itemize}
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\item
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If \tcode{T} is a specialization of
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\tcode{empty_view}\iref{range.empty.view},
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then \tcode{((void)F, \placeholdernc{decay-copy}(E))},
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except that the evaluations of \tcode{E} and \tcode{F}
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are indeterminately sequenced.
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\begin{addedblock}
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\item
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Otherwise, if \tcode{T} is a specialization of \tcode{optional} and \tcode{T} models \tcode{view}, then \tcode{(static_cast<D>(F) == D() ? \placeholdernc{decay-copy}(E) : ((void)E, T()))}.
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\end{addedblock}
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\item
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Otherwise, if \tcode{T} models
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\libconcept{random_access_range} and \libconcept{sized_range}
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and is
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\begin{itemize}
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\item a specialization of \tcode{span}\iref{views.span},
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\item a specialization of \tcode{basic_string_view}\iref{string.view},
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\item a specialization of \tcode{iota_view}\iref{range.iota.view}, or
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\item a specialization of \tcode{subrange}\iref{range.subrange}
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where \tcode{T::\exposid{StoreSize}} is \tcode{false},
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\end{itemize}
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then \tcode{U(ranges::begin(E) + std::min<D>(ranges::distance(E), F), ranges::end(E))},
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except that \tcode{E} is evaluated only once,
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where \tcode{U} is \tcode{span<typename T::element_type>}
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if \tcode{T} is a specialization of \tcode{span} and \tcode{T} otherwise.
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\item
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Otherwise,
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if \tcode{T} is
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a specialization of \tcode{subrange}
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that models \libconcept{random_access_range} and \libconcept{sized_range},
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then
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\tcode{T(ranges::begin(E) + std::min<D>(ranges::distance(E), F), ranges::\linebreak{}end(E),
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\exposid{to-unsigned-like}(ranges::distance(E) -
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std::min<D>(ranges::distance(E), F)))},
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except that \tcode{E} and \tcode{F} are each evaluated only once.
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\item
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Otherwise, if \tcode{T} is
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a specialization of \tcode{repeat_view}\iref{range.repeat.view}:
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\begin{itemize}
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\item
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if \tcode{T} models \libconcept{sized_range},
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then
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\begin{codeblock}
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views::repeat(*E.@\exposid{value_}@, ranges::distance(E) - std::min<D>(ranges::distance(E), F))
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\end{codeblock}
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except that \tcode{E} is evaluated only once;
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\item
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otherwise, \tcode{((void)F, \placeholdernc{decay-copy}(E))},
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except that the evaluations of \tcode{E} and \tcode{F} are indeterminately sequenced.
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\end{itemize}
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\item
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Otherwise, \tcode{drop_view(E, F)}.
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\end{itemize}
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\rSec2[range.as.const]{As const view}
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\rSec3[range.as.const.overview]{Overview}
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\pnum
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\tcode{as_const_view} presents a view of an underlying sequence as constant.
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That is, the elements of an \tcode{as_const_view} cannot be modified.
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\pnum
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The name \tcode{views::as_const} denotes
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a range adaptor object\iref{range.adaptor.object}.
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Let \tcode{E} be an expression,
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let \tcode{T} be \tcode{decltype((E))}, and
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let \tcode{U} be \tcode{remove_cvref_t<T>}.
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The expression \tcode{views::as_const(E)} is expression-equivalent to:
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\begin{itemize}
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\item
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If \tcode{views::all_t<T>} models \libconcept{constant_range},
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then \tcode{views::all(E)}.
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\item
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Otherwise,
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if \tcode{U} denotes \tcode{empty_view<X>}
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for some type \tcode{X}, then \tcode{auto(views::empty<const X>)}.
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\begin{addedblock}
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\item
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Otherwise, if \tcode{U} denotes \tcode{optional<X\&>} for some type \tcode{X}, then \tcode{optional<const X\&>(E)}.
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\end{addedblock}
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\item
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Otherwise,
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if \tcode{U} denotes \tcode{span<X, Extent>}
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for some type \tcode{X} and some extent \tcode{Extent},
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then \tcode{span<const X, Extent>(E)}.
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\item
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Otherwise,
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if \tcode{U} denotes \tcode{ref_view<X>} for some type \tcode{X} and
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\tcode{const X} models \libconcept{constant_range},
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then \tcode{ref_view(static_cast<const X\&>(E.base()))}.
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\item
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Otherwise,
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if \tcode{E} is an lvalue,
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\tcode{const U} models \libconcept{constant_range}, and
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\tcode{U} does not model \libconcept{view},
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then \tcode{ref_view(static_cast<const U\&>(E))}.
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\item
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Otherwise, \tcode{as_const_view(E)}.
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\end{itemize}
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\rSec2[range.reverse]{Reverse view}
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\rSec3[range.reverse.overview]{Overview}
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\pnum
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\tcode{reverse_view} takes a bidirectional view and produces
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another view that iterates the same elements in reverse order.
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\pnum
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\indexlibrarymember{reverse}{views}%
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The name \tcode{views::reverse} denotes a
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range adaptor object\iref{range.adaptor.object}.
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Given a subexpression \tcode{E}, the expression
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\tcode{views::reverse(E)} is expression-equivalent to:
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\begin{itemize}
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\item
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If the type of \tcode{E} is
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a (possibly cv-qualified) specialization of \tcode{reverse_view},
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then \tcode{E.base()}.
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\begin{addedblock}
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\item
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Otherwise, if \tcode{E} is specialization of \tcode{optional} and \tcode{E} models \tcode{view}, then \tcode{\placeholdernc{decay-copy}(E)}.
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\end{addedblock}
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\item
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Otherwise, if the type of \tcode{E} is \cv{} \tcode{subrange<reverse_iterator<I>, reverse_iterator<I>, K>}
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for some iterator type \tcode{I} and
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value \tcode{K} of type \tcode{subrange_kind},
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\begin{itemize}
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\item
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if \tcode{K} is \tcode{subrange_kind::sized}, then
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\tcode{subrange<I, I, K>(E.end().base(), E.begin().base(), E.size())};
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\item
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otherwise, \tcode{subrange<I, I, K>(E.end().base(), E.begin().base())}.
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\end{itemize}
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However, in either case \tcode{E} is evaluated only once.
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\item
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Otherwise, \tcode{reverse_view\{E\}}.
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\end{itemize}
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\end{wording}
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\renewcommand{\bibname}{References}
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\bibliographystyle{abstract}
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\bibliography{wg21,mybiblio}
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\end{document}

‎papers/P2988/stdtex/layout.tex‎

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%%--------------------------------------------------
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%% set header and footer positions and sizes
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\setheadfoot{\onelineskip}{4\onelineskip}
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\setheadfoot{3\onelineskip}{4\onelineskip}
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\setheaderspaces{*}{2\onelineskip}{*}
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%%--------------------------------------------------

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