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cor3ntin committed Nov 27, 2024
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94 changes: 56 additions & 38 deletions src/D2841.tex
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Original file line number Diff line number Diff line change
Expand Up @@ -2142,22 +2142,31 @@ \subsection{\textcolor{noteclr}{3. Wording for variable-template and concept tem

\rSec3[temp.constr.concept]{Concept-dependent constraint}

A \defn{concept-dependent constraint} is formed from a concept-id \tcode{C<A1, ..., An>}
where \tcode{C} names a concept template parameter.
A \defn{concept-dependent constraint} \tcode{CD} is an atomic constraint whose
expression is a concept-id CI whose \grammarterm{concept-name} names a dependent concept named \tcode{C}.

\ednote{replace formed from?}
To determine if \tcode{CD} is satisfied, the parameter mapping and template arguments are first substituted into \tcode{C}. If substitution results in an invalid concept-id in the immediate context of the constraint ([temp.deduct.general]), the constraint is not satisfied.
Otherwise, let \tcode{CI'} be the normal form of the concept-id after substitution of \tcode{C}.

To determine if a concept-dependent constraint is satisfied, template arguments are first substituted into \tcode{C}. If substitution results in an invalid concept-id in the immediate context of the constraint ([temp.deduct.general]), the constraint is not satisfied.
Otherwise, let \tcode{E} be the normal form of the concept-id after substitution of \tcode{C}.
The constraint is satisfied if \tcode{E} is satisfied.
\begin{note}
Normalization of \tcode{CI} may be ill-formed; no diagnostics is required.
\end{note}

To form \tcode{CI''}, each appearance of \tcode{C}'s template parameters in the parameter mappings of the atomic constraints (including concept-dependent constraints) in \tcode{CI'} is substituted with their respective arguments from the
the parameter mapping of \tcode{CD} and the arguments of \tcode{CI}.

%
%Each argument in the parameter mapping of CI'
%is substituted for the respective template parameter
%in the parameter mapping of CD.

The constraint is satisfied if \tcode{CI''} is satisfied.

\begin{note}
Checking whether \tcode{CI''} is satisfied can lead to further
normalization of concept-dependent constraints.
\end{note}

%During the instantiation of a specialization or member function [temp.inst],
%if the argument \tcode{A} corresonding
%to the concept template parameter named by \tcode{C} is not dependent, \tcode{C<Args..>} is substituted by
%the normal form of \tcode{A<Args..>}.
%The substituted normal form of \tcode{C<A1, ..., An>} is the normal form of \tcode{A<A1, ..., An>} where \tcode{A} is the argument
%corresponding to the concept template parameter named by \tcode{C}.

\end{addedblock}

Expand All @@ -2171,16 +2180,16 @@ \subsection{\textcolor{noteclr}{3. Wording for variable-template and concept tem
%the \defn{substitutable concept parameter}{s} of \tcode{E} are the set of concept template parameters named by \tcode{E}
%that are declared in in the enclosing template of \tcode{D} and for which there exist a corresponding set of non-dependent template arguments.
%
Given a concept-id \tcode{C<A$_1$, A$_2$, ..., A$_n$>},
the \defn{substitutable concept parameters} of an expression \tcode{E} that is a subexpression of the \grammarterm{constraint-expression} of \tcode{C} is the set of concept template parameters of \tcode{C} for which there is a corresponding argument in \tcode{C<A$_1$, A$_2$, ..., A$_n$>}.
%Given a concept-id \tcode{C<A$_1$, A$_2$, ..., A$_n$>},
%the \defn{substitutable concept parameters} of an expression \tcode{E} that is a subexpression of the \grammarterm{constraint-expression} of \tcode{C} is the set of concept template parameters of \tcode{C} for which there is a corresponding argument in \tcode{C<A$_1$, A$_2$, ..., A$_n$>}.
%
%The \defn{substitutable concept parameters} of \tcode{E}
%is the set of non-dependent associated concept parameters of \tcode{E}.

The normal form of a concept-dependent constraint \tcode{CD} whose concept-id \tcode{CI} is \tcode{C<A$_1$, A$_2$, ..., A$_n$>} is the normal form of \tcode{CI} after substituting each \tcode{C} and \tcode{A$_1$} that names a substitutable concept parameter by the corresponding template argument
%of \tcode{C<A$_1$, A$_2$, ..., A$_n$>}.
If substitution results in an invalid concept-id, the normal form of \tcode{CD} is \tcode{false}.
If no concept template parameter is substituted, the normal form of \tcode{CD} is \tcode{CD}.
%The normal form of a concept-dependent constraint \tcode{CD} whose concept-id \tcode{CI} is \tcode{C<A$_1$, A$_2$, ..., A$_n$>} is the normal form of \tcode{CI} after substituting each \tcode{C} and \tcode{A$_1$} that names a substitutable concept parameter by the corresponding template argument
%%of \tcode{C<A$_1$, A$_2$, ..., A$_n$>}.
%If substitution results in an invalid concept-id, the normal form of \tcode{CD} is \tcode{false}.
%If no concept template parameter is substituted, the normal form of \tcode{CD} is \tcode{CD}.

\end{addedblock}

Expand All @@ -2204,22 +2213,29 @@ \subsection{\textcolor{noteclr}{3. Wording for variable-template and concept tem
is the conjunction of
the normal forms of \tcode{E1} and \tcode{E2}.

\added{For a concept-id \tcode{CI} of the form \tcode{C<A$_1$, A$_2$, ..., A$_n$>}}
\begin{itemize}
\begin{addedblock}
%\item When \tcode{C} names a substitutable concept parameter, the normal form of a concept-id \tcode{C<A$_1$, A$_2$, ..., A$_n$>} is
%the normal form of \tcode{C'<A$_1$, A$_2$, ..., A$_n$>} after substituting \tcode{C} by the corresponding template argument \tcode{C'}.
\item When \tcode{C} or any \tcode{A$_{0 \le i < n}$} name a concept template parameter, the normal form of \tcode{CI} is the normal form of a concept-dependent constraint whose concept-id is \tcode{CI}.

\end{addedblock}
\item
\added{Otherwise}, \changed{The}{the} normal form of \tcode{CI}

\changed{The normal form of}{For} a concept-id \tcode{C<A$_1$, A$_2$, ..., A$_n$>} \added{termed \added{CI}:}
\begin{removedblock}
is the normal form of the \grammarterm{constraint-expression} of \tcode{C},
after substituting \tcode{A$_1$, A$_2$, ..., A$_n$} for
\tcode{C}{'s} respective template parameters in the
parameter mappings in each atomic constraint.
If any such substitution results in an invalid type or expression,
the program is ill-formed; no diagnostic is required.
\tcode{C}{'s} respective template parameters in the
parameter mappings in each atomic constraint.
If any such substitution results in an invalid type or expression,
the program is ill-formed; no diagnostic is required.
\end{removedblock}
\begin{addedblock}
\begin{itemize}
\item If \tcode{C} names a dependent concept, the normal form of \tcode{CI} is a concept dependent constraint whose concept-id is \tcode{CI}.

\item To form \tcode{CE}, any non-dependent concept template argument A$_i$
is substituted into the constraint-expression of \tcode{C}.
If any such substitution results in an invalid concept-id the program is ill-formed; no diagnostic is required.

The normal form of \tcode{CI} is the result of substituting, in the normal \tcode{N} of \tcode{CE}, appearances of \tcode{C}'s template parameters in the parameter mappings of the atomic constraints in N with their respective arguments from \tcode{C}. If any such substitution results in an invalid type or expression, the program is ill-formed; no diagnostic is required.
\end{itemize}
\end{addedblock}
%\begin{addedblock}
%\begin{itemize}
%\item Substituting each substitutable concept parameter in each concept-id \tcode{CI} of each non-atomic subexpression of the \grammarterm{constraint-expression} of \tcode{C} by the corresponding template argument of \tcode{C}. If substitution \tcode{CI} results in an invalid concept-id, the normal form of \tcode{CI} is \tcode{false}.
Expand All @@ -2232,9 +2248,6 @@ \subsection{\textcolor{noteclr}{3. Wording for variable-template and concept tem
%\end{itemize}
%\end{addedblock}

\end{itemize}


\begin{example}
\begin{codeblock}
template<typename T> concept A = T::value || true;
Expand Down Expand Up @@ -2281,16 +2294,21 @@ \subsection{\textcolor{noteclr}{3. Wording for variable-template and concept tem

\begin{addedblock}
\item
For an expression \tcode{F} of the form \tcode{( E \grammarterm{Op} ... )}
For an expression \tcode{F} of the form \tcode{( E \grammarterm{Op} ... )},

let \tcode{E'} be the normal form of \tcode{E}.
\begin{itemize}
\item

If \tcode{E} names one or more unexpanded substitutable concept template parameter pack $P_k$ which do not denote a pack, let $N_k$ be the number of arguments specified for each pack $P_k$.
If \tcode{E} names one or more unexpanded concept template parameter pack $P_k$ which has corresponding template arguments in the parameter mapping of any
atomic constraint (including context-dependent constraint)
of \tcode{E'}, let $N_k$ be the number of arguments specified for each pack $P_k$.
If the value of each $N_k$ differs, the program is ill-formed.

Otherwise, the normal form of \tcode{F} is
the normal form of $E_0$ \tcode{Op} $E_1$ \tcode{Op} ... \tcode{Op} $E_{N_0}$.
Within $E_{0 \le i < {N_0}}$, $P_k$ designates the $i^\text{th}$ pack element of $P_k$.
the normal form of $E_0$ \tcode{Op} $E_1$ \tcode{Op} ... \tcode{Op} $E_{N_0}$ after substituting in $E_{0 \le i < {N_0}}$ each respective $i_th$ non-dependent concept argument of each $P_k$ .
If any such substitution results in an invalid type or expression, the program is ill-formed; no diagnostic is required.
%Within $E_{0 \le i < {N_0}}$, $P_k$ designates the $i^\text{th}$ pack element of $P_k$.



Expand All @@ -2316,7 +2334,7 @@ \subsection{\textcolor{noteclr}{3. Wording for variable-template and concept tem
\item Otherwise
%(if \tcode{E} does not contain an unexpanded parameter pack that denotes a substitutable concept template parameter pack),
the normal form of \tcode{( E Op ... )} is a fold expanded constraint \iref{temp.constr.fold}
whose constraint is the normal form of \tcode{E} and
whose constraint is \tcode{E'} and
whose \grammarterm{fold-operator} is \tcode{Op}.
\end{itemize}
\end{addedblock}
Expand Down
27 changes: 17 additions & 10 deletions src/D2846.tex
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Expand Up @@ -5,7 +5,7 @@
\documentclass{wg21}

\title{\tcode{reserve\_hint}: Eagerly reserving memory for not-quite-sized lazy ranges}
\docnumber{P2846R4}
\docnumber{D2846R5}
\audience{LEWG}
\author{Corentin Jabot}{[email protected]}

Expand All @@ -19,6 +19,12 @@ \section{Abstract}

\section{Revisions}

\subsection{R5}

\begin{itemize}
\item Apply some wording feedback following LEWG review, remove the nothrow_move_constructible requirement from vector's constructor complexity requirements, fix feature test macro.
\end{itemize}

\subsection{R4}

\begin{itemize}
Expand Down Expand Up @@ -214,7 +220,7 @@ \subsection{\tcode{vector} constructors}
\textbf{This is the option implemented in R3, following LEWG guidance}

Tomasz also realized that types that may throw on move might also change the behavior of existing code.
To avoid that we restrict the optimization guarantee in vector to types that are nothrow move-constructible.
To avoid that we restrict the optimization guarantee in vector to types that are nothrow move-constructible. however, LEWG voted not to add this requirement (Wroclaw).

\item Allowing implementations to do so without requiring it.

Expand All @@ -232,7 +238,7 @@ \subsection{\tcode{std::vector} \tcode{insert} and \tcode{append} methods}

\subsection{\tcode{string}}

\tcode{string} doesn't currently mandate complexity guarantees so we have ekected not to modify the specification,
\tcode{string} doesn't currently mandate complexity guarantees so we have elected not to modify the specification,
and instead preferring a blanket recommended practice that \tcode{reserve_hint} should be used to reduce reallocations.

\section{Benchmarks}
Expand All @@ -247,11 +253,11 @@ \section{Benchmarks}

The graph shows that, for a forward range, reserving is consistently about twice as fast. This is consistent with the fact the range is traversed twice.

\includegraphics[width=\textwidth]{D2846_bench1.png}
% \includegraphics[width=\textwidth]{D2846_bench1.png}

If the input range is non-forward, there is less difference on small datasets:

\includegraphics[width=\textwidth]{D2846_bench2.png}
% \includegraphics[width=\textwidth]{D2846_bench2.png}

But as the data gets bigger and the number of allocations increases being able to reserve memory gets more noticeably impactful (performance delta of 5-10\%).

Expand Down Expand Up @@ -292,10 +298,10 @@ \section{Naming}

\section{Wording}

\ednote{Add the macro \tcode{__cpp_lib_ranges_zip} to \tcode{<version>} and \tcode{<ranges>}}
\ednote{Add the macro \tcode{__cpp_lib_ranges_reserve_hint} to \tcode{<version>} and \tcode{<ranges>}}
\begin{addedblock}
\begin{codeblock}
#define __cpp_lib_ranges_zip 2026XX (**placeholder**)
#define __cpp_lib_ranges_reserve_hint 2024XX (**placeholder**)
\end{codeblock}
\end{addedblock}

Expand Down Expand Up @@ -326,8 +332,9 @@ \section{Wording}
\begin{addedblock}
Performs no reallocations if:
\begin{itemize}
\item \tcode{R} models \tcode{ranges::\libconcept{sized_range}},
\item \tcode{R} models \tcode{ranges::\libconcept{approximately_sized_range}}, \tcode{is_nothrow_move_constructible_v<T>} is \tcode{true}, and \tcode{ranges::distance(rg) <= ranges::reserve_hint(rg)} is \tcode{true}, or
\item \tcode{R} models \tcode{ranges::\libconcept{sized_range}}, or
\item \tcode{R} models \tcode{ranges::\libconcept{approximately_sized_range}}, %\tcode{is_nothrow_move_constructible_v<T>} is \tcode{true},
and \tcode{ranges::distance(rg) <= ranges::reserve_hint(rg)} is \tcode{true}, or
\item \tcode{R} models \tcode{ranges::\libconcept{forward_range}}.
\end{itemize}
\end{addedblock}
Expand Down Expand Up @@ -392,7 +399,7 @@ \section{Wording}
\begin{addedblock}
For the declarations taking a range \tcode{R}, performs at most one reallocation if:
\begin{itemize}
\item \tcode{R} models \tcode{ranges::\libconcept{sized_range}},
\item \tcode{R} models \tcode{ranges::\libconcept{sized_range}}, or
\item \tcode{R} models \tcode{ranges::\libconcept{approximately_sized_range}} and \tcode{ranges::distance(rg) <= ranges::reserve_hint(rg)} is \tcode{true}, or
\item \tcode{R} models \tcode{ranges::\libconcept{forward_range}}.
\end{itemize}
Expand Down

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