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[2026-03 LWG Motion 23] P3982R2 Split strided_slice into extent_slice and range_slice for C++26 #8909

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[2026-03 LWG Motion 23] P3982R2 Split strided_slice into extent_slice and range_slice for C++26 #8909

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P3982R2 Split strided_slice into extent_slice and range_slice for C++…
eisenwave Apr 2, 2026
f5ba5a4
P3982R2 Split strided_slice into extent_slice and range_slice for C++…
eisenwave Apr 2, 2026
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218 changes: 152 additions & 66 deletions source/containers.tex
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Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -21280,7 +21280,9 @@

// \ref{mdspan.sub}, \tcode{submdspan} creation
template<class OffsetType, class LengthType, class StrideType>
struct strided_slice;
struct extent_slice;
template<class FirstType, class LastType, class StrideType = std::constant_wrapper<1zu>>
struct range_slice;

template<class LayoutMapping>
struct submdspan_mapping_result;
Expand Down Expand Up @@ -25543,11 +25545,17 @@
\item
\tcode{is_convertible_v<$S$, IndexType>} is \tcode{true};
\item
$S$ is a specialization of \tcode{strided_slice} and
$S$ is a specialization of \tcode{extent_slice} and
\tcode{is_convertible_v<$X$, IndexType>} is \tcode{true} for $X$ denoting
\tcode{$S$::offset_type},
\tcode{$S$::extent_type}, and
\tcode{$S$::stride_type}; or
\tcode{$S$::stride_type};
\item
$S$ is a specialization of \tcode{range_slice} and
\tcode{is_convertible_v<X, IndexType>} is \tcode{true} for $X$ denoting type of
\tcode{$S$::first},
\tcode{$S$::last}, and
\tcode{$S$::stride} members; or
\item
all of the following hold:
\begin{itemize}
Expand Down Expand Up @@ -25580,7 +25588,7 @@
\item
$S$ is a canonical \tcode{submdspan} index type for \tcode{IndexType}; or
\item
$S$ is a specialization of \tcode{strided_slice}
$S$ is a specialization of \tcode{extent_slice}
where all of the following hold:
\begin{itemize}
\item
Expand All @@ -25596,7 +25604,7 @@
\pnum
A type \tcode{S} is a \defnadj{collapsing}{slice type} if
it is neither \tcode{full_extent_t} nor
a specialization of \tcode{strided_slice}.
a specialization of \tcode{extent_slice}.
\begin{note}
Each collapsing slice type in \tcode{submdspan_mapping}'s parameter pack
of slice specifier types
Expand All @@ -25607,7 +25615,7 @@
A type \tcode{S} is a \defnadj{unit-stride}{slice type} if
\begin{itemize}
\item
\tcode{S} is a specialization of \tcode{strided_slice}
\tcode{S} is a specialization of \tcode{extent_slice}
where \tcode{S::stride_type} is a specialization of \tcode{constant_wrapper} and
\tcode{S::stride_type::value} is equal to 1, or
\item
Expand All @@ -25624,6 +25632,10 @@
\item
\range{$0$}{e.extent($k$)},
if \tcode{S} is \tcode{full_extent_t};
\item
\range{E::index_type(s.offset)}{E::index_type(s.offset)},
if \tcode{S} is a specialization of \tcode{extent_slice} and
\tcode{E::index_type(s.extent)} is zero; otherwise
\item
\range{E::index_type(s.offset)}{E::index_type(s.offset + s.extent)},
if \tcode{S} is a specialization of \tcode{stri\-ded_slice}; otherwise
Expand All @@ -25640,19 +25652,33 @@
either $x$ is equal to \tcode{dynamic_extent}; or
\begin{itemize}
\item
if \tcode{S} is a specialization of \tcode{strided_slice}:
if \tcode{S} is a specialization of \tcode{extent_slice}:
\begin{itemize}
\item
if \tcode{S::offset_type} is a specialization of \tcode{constant_wrapper},
then \tcode{S::offset_type::value} is less than or equal to $x$;
$o$ is less than or equal to $x$;
\item
if \tcode{S::offset_type} is a specialization of \tcode{constant_wrapper},
then \tcode{S::extent_type::value} is less than or equal to $x$; and
$e$ is less than or equal to $x$;
\item
if both \tcode{S::offset_type} and \tcode{S::extent_type}
are specializations of \tcode{constant_wrapper},
then \tcode{S::offset_type::value + S::extent_type::value}
is less than or equal to $x$; or
if $e$ is greater than one,
then $t$ is greater than zero,
\item
if $e$ is greater than zero,
then $o + 1 + (e - 1) * t$ is less than or equal to $x$
\end{itemize}
where
\begin{itemize}
\item
$o$ is value of \tcode{S::offset_type::value}
if \tcode{S::offset_type} is a specialization of \tcode{constant_wrapper} and
\tcode{0} otherwise,
\item
$e$ is value of \tcode{S::extent_type::value}
if \tcode{S::extend_type} is a specialization of \tcode{constant_wrapper} and
\tcode{0} otherwise,
\item
$t$ is value of \tcode{S::stride_type::value}
if \tcode{S::stride_type} is a specialization of \tcode{constant_wrapper} and
\tcode{1} otherwise.
\end{itemize}
\item
if $S$ is a specialization of \tcode{constant_wrapper},
Expand All @@ -25674,26 +25700,27 @@
contains the \tcode{submdspan} slice range of \tcode{s}
for the $k^\text{th}$ extent of \tcode{e}; and
\item
if \tcode{S} is a specialization of \tcode{strided_slice}, then:
if \tcode{S} is a specialization of \tcode{extent_slice}, then:
\begin{itemize}
\item \tcode{s.extent} is greater than or equal to zero, and
\item either \tcode{s.extent} equals zero or \tcode{s.stride} is greater than zero.
\item either \tcode{s.extent} is less than two or \tcode{s.stride} is greater than zero.
\end{itemize}
\end{itemize}

\rSec4[mdspan.sub.strided.slice]{\tcode{strided_slice}}
\rSec4[mdspan.sub.range.slices]{\tcode{Range slices}}

\pnum
\tcode{strided_slice} represents a set of
\tcode{extent_slice} and \tcode{range_slice} represent a set of
\tcode{extent} regularly spaced integer indices.
The indices start at \tcode{offset}, and
The indices start at \tcode{offset} and \tcode{first} respectively, and
increase by increments of \tcode{stride}.

\indexlibraryglobal{strided_slice}%
\indexlibraryglobal{extent_slice}%
\indexlibraryglobal{range_slice}%
\begin{codeblock}
namespace std {
template<class OffsetType, class ExtentType, class StrideType>
struct strided_slice {
struct extent_slice {
using offset_type = OffsetType;
using extent_type = ExtentType;
using stride_type = StrideType;
Expand All @@ -25702,21 +25729,31 @@
[[no_unique_address]] extent_type extent{};
[[no_unique_address]] stride_type stride{};
};

template<class FirstType, class LastType, class StrideType = std::constant_wrapper<1zu>>
struct range_slice {
[[no_unique_address]] FirstType first{};
[[no_unique_address]] LastType last{};
[[no_unique_address]] StrideType stride{};
};
}
\end{codeblock}

\pnum
\tcode{strided_slice} has the data members and special members specified above.
It has no base classes or members other than those specified.
\tcode{extent_slice} and \tcode{range_slice}
have the data members and special members specified above.
They have no base classes or members other than those specified.

\pnum
\mandates
\tcode{OffsetType}, \tcode{ExtentType}, and \tcode{StrideType}
\tcode{OffsetType}, \tcode{ExtentType},
\tcode{FirstType}, \tcode{LastType}, and \tcode{StrideType}
are signed or unsigned integer types, or
model \exposconcept{integral-constant-like}.
\begin{note}
\tcode{strided_slice\{.offset = 1, .extent = 10, .stride = 3\}}
indicates the indices \tcode{1}, \tcode{4}, \tcode{7}, and \tcode{10}.
Both \tcode{extent_slice\{.offset = 1, .extent = 4, .stride = 3\}} and
\tcode{range_slice\{.first = 1, .last = 11, .stride = 3\}}
indicate the indices \tcode{1}, \tcode{4}, \tcode{7}, and \tcode{10}.
Indices are selected from the half-open interval \range{1}{1 + 10}.
\end{note}

Expand Down Expand Up @@ -25756,14 +25793,26 @@

\begin{itemdecl}
template<class T>
concept @\defexposconcept{is-strided-slice}@ = @\seebelow;@
concept @\defexposconcept{is-extent-slice}@ = @\seebelow;@
\end{itemdecl}

\begin{itemdescr}
\pnum
The concept \tcode{\exposconcept{is-strided-slice}<T>}
The concept \tcode{\exposconcept{is-extent-slice}<T>}
is satisfied and modeled if and only
if \tcode{T} is a specialization of \tcode{strided_slice}.
if \tcode{T} is a specialization of \tcode{extent_slice}.
\end{itemdescr}

\begin{itemdecl}
template<class T>
concept @\defexposconcept{is-range-slice}@ = @\seebelow;@
\end{itemdecl}

\begin{itemdescr}
\pnum
The concept \tcode{\exposconcept{is-range-slice}<T>}
is satisfied and modeled if and only
if \tcode{T} is a specialization of \tcode{range_slice}.
\end{itemdescr}

\indexlibraryglobal{\exposid{canonical-index}}%
Expand Down Expand Up @@ -25796,6 +25845,53 @@
\end{itemize}
\end{itemdescr}

\begin{itemdecl}
template<class IndexType, class OffsetType, class SpanType, class... StrideTypes>
constexpr auto @\exposid{canonical-range-slice}@(OffsetType offset, SpanType span, StrideTypes... strides);
\end{itemdecl}

\begin{itemdescr}
\pnum
Let:
\begin{itemize}
\item
\tcode{StrideType} denote
\tcode{constant_wrapper<IndexType(1)>}
if \tcode{StrideTypes} is an empty pack or
\tcode{SpanType} denotes \tcode{constant_wrapper<IndexType(0)>}, otherwise
\tcode{StrideTypes...[0]};
\item
\tcode{stride} be
\tcode{StrideType()} if \tcode{StrideType}
is a specialization of \tcode{constant_wrapper}, otherwise
\tcode{IndexType(1)} if \tcode{span == 0} is \tcode{true}, otherwise
\tcode{strides...[0]};
\item
\exposid{extent-value} be \tcode{1 + (span - 1) / stride}
if \tcode{span != 0} is \tcode{true}, and
\tcode{0} otherwise;
\item
\tcode{extent} be \tcode{cw<IndexType(\exposid{extent-value})>}
if both \tcode{SpanType} and \tcode{StrideType}
are specializations of \tcode{constant_wrapper}, and
\tcode{IndexType(\exposid{extent-value})} otherwise.
\end{itemize}

\pnum
\mandates
\tcode{sizeof...(StrideTypes) <= 1} is \tcode{true}, and
if \tcode{StrideType} is a specialization of \tcode{con\-stant_wrapper},
then \tcode{StrideType::value > 0} is \tcode{true}.

\pnum
\expects
\tcode{stride > 0} is \tcode{true}.

\pnum
\returns
\tcode{extent_slice\{ .offset = offset, .extent = extent, .stride = stride \};}
\end{itemdescr}

\indexlibraryglobal{\exposid{canonical-slice}}
\begin{itemdecl}
template<class IndexType, class S>
Expand All @@ -25815,31 +25911,28 @@
return static_cast<full_extent_t>(std::move(s));
} else if constexpr (is_convertible_v<S, IndexType>) {
return @\exposid{canonical-index}@<IndexType>(std::move(s));
} else if constexpr (@\exposconcept{is-strided-slice}@<S>) {
} else if constexpr (@\exposconcept{is-extent-slice}@<S>) {
auto c_extent = @\exposid{canonical-index}@<IndexType>(std::move(s.extent));
auto c_offset = @\exposid{canonical-index}@<IndexType>(std::move(s.offset));
if constexpr (is_same_v<decltype(c_extent), constant_wrapper<IndexType(0)>>) {
return strided_slice{
.offset = c_offset,
.extent = c_extent,
.stride = cw<IndexType(1)>
};
} else {
return strided_slice{
.offset = c_offset,
.extent = c_extent,
.stride = @\exposid{canonical-index}@<IndexType>(std::move(s.stride))
};
}
return extent_slice{
.offset = @\exposid{canonical-index}@<IndexType>(std::move(s.extent)),
.extent = @\exposid{canonical-index}@<IndexType>(std::move(s.offset)),
.stride = @\exposid{canonical-index}@<IndexType>(std::move(s.stride))
};
} else if constexpr (@\exposid{is-range-slice}@<S>) {
auto c_first = canonical-index<IndexType>(std::move(s.first));
auto c_last = canonical-index<IndexType>(std::move(s.last));
return @\exposid{canonical-slice-range}@<IndexType>(
c_first,
@\exposid{canonical-index}@<IndexType>(c_last - c_first),
@\exposid{canonical-index}@<IndexType>(std::move(s.stride)));
} else {
auto [s_first, s_last] = std::move(s);
auto c_first = @\exposid{canonical-index}@<IndexType>(std::move(s_first));
auto c_last = @\exposid{canonical-index}@<IndexType>(std::move(s_last));
return strided_slice{
.offset = c_first,
.extent = @\exposid{canonical-index}@<IndexType>(c_last - c_first),
.stride = cw<IndexType(1)>
};
return @\exposid{canonical-slice-range}@<IndexType>(
c_first,
@\exposid{canonical-index}@<IndexType>(c_last - c_first));
}
\end{codeblock}
\end{itemdescr}
Expand Down Expand Up @@ -25929,7 +26022,7 @@
\tcode{\placeholder{MAP_RANK}(slices, Extents::rank())}; and

\item
for each rank index $k$ of \tcode{Extents} such that
for each rank index $k$ of \tcode{extents<IndexType, Extents...>} such that
the type of \tcode{slices...[$k$]} is not a collapsing slice type,
\tcode{SubExt\-ents::static_extent(\placeholder{MAP_RANK}(slices, $k$))}
equals the following, where $\Sigma_k$
Expand All @@ -25938,20 +26031,13 @@
\begin{itemize}
\item
\tcode{Extents::static_extent($k$)}
if $\Sigma_k$ denotes the \tcode{full_extent_t};
otherwise

\item
\tcode{0},
if $\Sigma_k$ is a specialization of \tcode{strided_slice} and
\tcode{$\Sigma_k$::extent_type} denotes \tcode{constant_wrapper<IndexType(0)>};
if $\Sigma_k$ denotes \tcode{full_extent_t};
otherwise

\item
\tcode{1 + (($\Sigma_k$::extent_type::value - 1) / $\Sigma_k$::stride_type::value)},
if $\Sigma_k$ is a specialization of \tcode{strided_slice} whose
\tcode{extent_type} and \tcode{stride_type}
denote specializations of \tcode{constant_wrapper};
\tcode{$\Sigma_k$::extent_type::value}
if $\Sigma_k$ is a specialization of \tcode{extent_slice} whose
\tcode{extent_type} denotes a specialization of \tcode{constant_wrapper};

\item
otherwise,
Expand All @@ -25969,8 +26055,8 @@
where $\sigma_k$ denotes \tcode{slices...[$k$]}:
\begin{itemize}
\item
\tcode{$\sigma_k$.extent == 0 ? 0 : 1 + ($\sigma_k$.extent - 1) / $\sigma_k$.stride}
if the type of $\sigma_k$ is a specialization of \tcode{strided_slice},
\tcode{$\sigma_k$.extent}
if the type of $\sigma_k$ is a specialization of \tcode{extent_slice},
\item
otherwise,
$U - L$, where \range{$L$}{$U$} is the \tcode{submdspan} slice range
Expand Down Expand Up @@ -26008,7 +26094,7 @@
\item \tcode{IT},
\item \tcode{full_extent_t},
\item a specialization of \tcode{constant_wrapper}, or
\item a specialization of \tcode{strided_slice}.
\item a specialization of \tcode{extent_slice}.
\end{itemize}
\end{itemize}

Expand Down Expand Up @@ -26133,8 +26219,8 @@
\begin{itemize}
\item
\tcode{stride(k) * s.stride}
if the type of \tcode{s} is a specialization of \tcode{strided_slice} and
\tcode{s.stride < s.ex\-tent} is \tcode{true},
if the type of \tcode{s} is a specialization of \tcode{extent_slice} and
\tcode{s.extent > 1} is \tcode{true},
where \tcode{s} is \tcode{slices...[$k$]};
\item
otherwise, \tcode{stride($k$)}.
Expand Down
2 changes: 1 addition & 1 deletion source/support.tex
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -852,7 +852,7 @@
#define @\defnlibxname{cpp_lib_string_subview}@ 202506L // also in \libheader{string}, \libheader{string_view}
#define @\defnlibxname{cpp_lib_string_udls}@ 201304L // also in \libheader{string}
#define @\defnlibxname{cpp_lib_string_view}@ 202403L // also in \libheader{string}, \libheader{string_view}
#define @\defnlibxname{cpp_lib_submdspan}@ 202511L // freestanding, also in \libheader{mdspan}
#define @\defnlibxname{cpp_lib_submdspan}@ 202603L // freestanding, also in \libheader{mdspan}
#define @\defnlibxname{cpp_lib_syncbuf}@ 201803L // also in \libheader{iosfwd}, \libheader{syncstream}
#define @\defnlibxname{cpp_lib_task}@ 202506L // also in \libheader{execution}
#define @\defnlibxname{cpp_lib_text_encoding}@ 202306L // also in \libheader{text_encoding}
Expand Down
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