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<html xmlns="http://www.w3.org/1999/xhtml" lang xml:lang>
<head>
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  <meta name="generator" content="mpark/wg21" />
  <meta name="viewport" content="width=device-width, initial-scale=1.0, user-scalable=yes" />
  <meta name="dcterms.date" content="2022-06-15" />
  <title>mdspan: rename pointer and contiguous</title>
  <style>
      code{white-space: pre-wrap;}
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      span.underline{text-decoration: underline;}
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pre > code.sourceCode > span:empty { height: 1.2em; }
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code.sourceCode > span { color: inherit; text-decoration: inherit; }
div.sourceCode { margin: 1em 0; }
pre.sourceCode { margin: 0; }
@media screen {
div.sourceCode { overflow: auto; }
}
@media print {
pre > code.sourceCode { white-space: pre-wrap; }
pre > code.sourceCode > span { text-indent: -5em; padding-left: 5em; }
}
pre.numberSource code
  { counter-reset: source-line 0; }
pre.numberSource code > span
  { position: relative; left: -4em; counter-increment: source-line; }
pre.numberSource code > span > a:first-child::before
  { content: counter(source-line);
    position: relative; left: -1em; text-align: right; vertical-align: baseline;
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    -khtml-user-select: none; -moz-user-select: none;
    -ms-user-select: none; user-select: none;
    padding: 0 4px; width: 4em;
    color: #aaaaaa;
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pre.numberSource { margin-left: 3em; border-left: 1px solid #aaaaaa;  padding-left: 4px; }
div.sourceCode
  {  background-color: #f6f8fa; }
@media screen {
pre > code.sourceCode > span > a:first-child::before { text-decoration: underline; }
}
code span { } /* Normal */
code span.al { color: #ff0000; } /* Alert */
code span.an { } /* Annotation */
code span.at { } /* Attribute */
code span.bn { color: #9f6807; } /* BaseN */
code span.bu { color: #9f6807; } /* BuiltIn */
code span.cf { color: #00607c; } /* ControlFlow */
code span.ch { color: #9f6807; } /* Char */
code span.cn { } /* Constant */
code span.co { color: #008000; font-style: italic; } /* Comment */
code span.cv { color: #008000; font-style: italic; } /* CommentVar */
code span.do { color: #008000; } /* Documentation */
code span.dt { color: #00607c; } /* DataType */
code span.dv { color: #9f6807; } /* DecVal */
code span.er { color: #ff0000; font-weight: bold; } /* Error */
code span.ex { } /* Extension */
code span.fl { color: #9f6807; } /* Float */
code span.fu { } /* Function */
code span.im { } /* Import */
code span.in { color: #008000; } /* Information */
code span.kw { color: #00607c; } /* Keyword */
code span.op { color: #af1915; } /* Operator */
code span.ot { } /* Other */
code span.pp { color: #6f4e37; } /* Preprocessor */
code span.re { } /* RegionMarker */
code span.sc { color: #9f6807; } /* SpecialChar */
code span.ss { color: #9f6807; } /* SpecialString */
code span.st { color: #9f6807; } /* String */
code span.va { } /* Variable */
code span.vs { color: #9f6807; } /* VerbatimString */
code span.wa { color: #008000; font-weight: bold; } /* Warning */
code.diff {color: #898887}
code.diff span.va {color: #6.0e28}
code.diff span.st {color: #bf0303}
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body {
margin: 5em;
font-family: serif;

hyphens: auto;
line-height: 1.35;
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div.wrapper {
max-width: 60em;
margin: auto;
}
ul {
list-style-type: none;
padding-left: 2em;
margin-top: -0.2em;
margin-bottom: -0.2em;
}
a {
text-decoration: none;
color: #4183C4;
}
a.hidden_link {
text-decoration: none;
color: inherit;
}
li {
margin-top: 0.6em;
margin-bottom: 0.6em;
}
h1, h2, h3, h4 {
position: relative;
line-height: 1;
}
a.self-link {
position: absolute;
top: 0;
left: calc(-1 * (3.5rem - 26px));
width: calc(3.5rem - 26px);
height: 2em;
text-align: center;
border: none;
transition: opacity .2s;
opacity: .5;
font-family: sans-serif;
font-weight: normal;
font-size: 83%;
}
a.self-link:hover { opacity: 1; }
a.self-link::before { content: "§"; }
ul > li:before {
content: "\2014";
position: absolute;
margin-left: -1.5em;
}
:target { background-color: #C9FBC9; }
:target .codeblock { background-color: #C9FBC9; }
:target ul { background-color: #C9FBC9; }
.abbr_ref { float: right; }
.folded_abbr_ref { float: right; }
:target .folded_abbr_ref { display: none; }
:target .unfolded_abbr_ref { float: right; display: inherit; }
.unfolded_abbr_ref { display: none; }
.secnum { display: inline-block; min-width: 35pt; }
.header-section-number { display: inline-block; min-width: 35pt; }
.annexnum { display: block; }
div.sourceLinkParent {
float: right;
}
a.sourceLink {
position: absolute;
opacity: 0;
margin-left: 10pt;
}
a.sourceLink:hover {
opacity: 1;
}
a.itemDeclLink {
position: absolute;
font-size: 75%;
text-align: right;
width: 5em;
opacity: 0;
}
a.itemDeclLink:hover { opacity: 1; }
span.marginalizedparent {
position: relative;
left: -5em;
}
li span.marginalizedparent { left: -7em; }
li ul > li span.marginalizedparent { left: -9em; }
li ul > li ul > li span.marginalizedparent { left: -11em; }
li ul > li ul > li ul > li span.marginalizedparent { left: -13em; }
div.footnoteNumberParent {
position: relative;
left: -4.7em;
}
a.marginalized {
position: absolute;
font-size: 75%;
text-align: right;
width: 5em;
}
a.enumerated_item_num {
position: relative;
left: -3.5em;
display: inline-block;
margin-right: -3em;
text-align: right;
width: 3em;
}
div.para { margin-bottom: 0.6em; margin-top: 0.6em; text-align: justify; }
div.section { text-align: justify; }
div.sentence { display: inline; }
span.indexparent {
display: inline;
position: relative;
float: right;
right: -1em;
}
a.index {
position: absolute;
display: none;
}
a.index:before { content: "⟵"; }

a.index:target {
display: inline;
}
.indexitems {
margin-left: 2em;
text-indent: -2em;
}
div.itemdescr {
margin-left: 3em;
}
.bnf {
font-family: serif;
margin-left: 40pt;
margin-top: 0.5em;
margin-bottom: 0.5em;
}
.ncbnf {
font-family: serif;
margin-top: 0.5em;
margin-bottom: 0.5em;
margin-left: 40pt;
}
.ncsimplebnf {
font-family: serif;
font-style: italic;
margin-top: 0.5em;
margin-bottom: 0.5em;
margin-left: 40pt;
background: inherit; 
}
span.textnormal {
font-style: normal;
font-family: serif;
white-space: normal;
display: inline-block;
}
span.rlap {
display: inline-block;
width: 0px;
}
span.descr { font-style: normal; font-family: serif; }
span.grammarterm { font-style: italic; }
span.term { font-style: italic; }
span.terminal { font-family: monospace; font-style: normal; }
span.nonterminal { font-style: italic; }
span.tcode { font-family: monospace; font-style: normal; }
span.textbf { font-weight: bold; }
span.textsc { font-variant: small-caps; }
a.nontermdef { font-style: italic; font-family: serif; }
span.emph { font-style: italic; }
span.techterm { font-style: italic; }
span.mathit { font-style: italic; }
span.mathsf { font-family: sans-serif; }
span.mathrm { font-family: serif; font-style: normal; }
span.textrm { font-family: serif; }
span.textsl { font-style: italic; }
span.mathtt { font-family: monospace; font-style: normal; }
span.mbox { font-family: serif; font-style: normal; }
span.ungap { display: inline-block; width: 2pt; }
span.textit { font-style: italic; }
span.texttt { font-family: monospace; }
span.tcode_in_codeblock { font-family: monospace; font-style: normal; }
span.phantom { color: white; }

span.math { font-style: normal; }
span.mathblock {
display: block;
margin-left: auto;
margin-right: auto;
margin-top: 1.2em;
margin-bottom: 1.2em;
text-align: center;
}
span.mathalpha {
font-style: italic;
}
span.synopsis {
font-weight: bold;
margin-top: 0.5em;
display: block;
}
span.definition {
font-weight: bold;
display: block;
}
.codeblock {
margin-left: 1.2em;
line-height: 127%;
}
.outputblock {
margin-left: 1.2em;
line-height: 127%;
}
div.itemdecl {
margin-top: 2ex;
}
code.itemdeclcode {
white-space: pre;
display: block;
}
span.textsuperscript {
vertical-align: super;
font-size: smaller;
line-height: 0;
}
.footnotenum { vertical-align: super; font-size: smaller; line-height: 0; }
.footnote {
font-size: small;
margin-left: 2em;
margin-right: 2em;
margin-top: 0.6em;
margin-bottom: 0.6em;
}
div.minipage {
display: inline-block;
margin-right: 3em;
}
div.numberedTable {
text-align: center;
margin: 2em;
}
div.figure {
text-align: center;
margin: 2em;
}
table {
border: 1px solid black;
border-collapse: collapse;
margin-left: auto;
margin-right: auto;
margin-top: 0.8em;
text-align: left;
hyphens: none; 
}
td, th {
padding-left: 1em;
padding-right: 1em;
vertical-align: top;
}
td.empty {
padding: 0px;
padding-left: 1px;
}
td.left {
text-align: left;
}
td.right {
text-align: right;
}
td.center {
text-align: center;
}
td.justify {
text-align: justify;
}
td.border {
border-left: 1px solid black;
}
tr.rowsep, td.cline {
border-top: 1px solid black;
}
tr.even, tr.odd {
border-bottom: 1px solid black;
}
tr.capsep {
border-top: 3px solid black;
border-top-style: double;
}
tr.header {
border-bottom: 3px solid black;
border-bottom-style: double;
}
th {
border-bottom: 1px solid black;
}
span.centry {
font-weight: bold;
}
div.table {
display: block;
margin-left: auto;
margin-right: auto;
text-align: center;
width: 90%;
}
span.indented {
display: block;
margin-left: 2em;
margin-bottom: 1em;
margin-top: 1em;
}
ol.enumeratea { list-style-type: none; background: inherit; }
ol.enumerate { list-style-type: none; background: inherit; }

code.sourceCode > span { display: inline; }

div#refs p { padding-left: 32px; text-indent: -32px; }
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<h1 class="title" style="text-align:center"><code>mdspan</code>: rename
<code>pointer</code> and <code>contiguous</code></h1>

<table style="border:none;float:right">
  <tr>
    <td>Document #: </td>
    <td>P2604</td>
  </tr>
  <tr>
    <td>Date: </td>
    <td>2022-06-15</td>
  </tr>
  <tr>
    <td style="vertical-align:top">Project: </td>
    <td>Programming Language C++<br>
      LEWG<br>
    </td>
  </tr>
  <tr>
    <td style="vertical-align:top">Reply-to: </td>
    <td>
      Christian Trott<br>&lt;<a href="mailto:crtrott@sandia.gov" class="email">crtrott@sandia.gov</a>&gt;<br>
    </td>
  </tr>
</table>

</header>
<div style="clear:both">
<div id="TOC" role="doc-toc">
<h1 id="toctitle">Contents</h1>
<ul>
<li><a href="#revision-history"><span class="toc-section-number">1</span> Revision History<span></span></a>
<ul>
<li><a href="#initial-version-2022-06-mailing"><span class="toc-section-number">1.1</span> Initial Version 2022-06
Mailing<span></span></a></li>
</ul></li>
<li><a href="#description"><span class="toc-section-number">2</span>
Description<span></span></a>
<ul>
<li><a href="#the-issue"><span class="toc-section-number">2.1</span> The
Issue<span></span></a>
<ul>
<li><a href="#pointer"><span class="toc-section-number">2.1.1</span>
<code>pointer</code><span></span></a></li>
<li><a href="#contiguous"><span class="toc-section-number">2.1.2</span>
<code>contiguous</code><span></span></a></li>
</ul></li>
</ul></li>
<li><a href="#wording"><span class="toc-section-number">3</span>
Wording<span></span></a>
<ul>
<li><a href="#pointer-to-data_handle_type"><span class="toc-section-number">3.1</span> <code>pointer</code> to
<code>data_handle_type</code><span></span></a></li>
<li><a href="#data-to-data_handle"><span class="toc-section-number">3.2</span> <code>data()</code> to
<code>data_handle()</code><span></span></a></li>
<li><a href="#contiguous-to-exhaustive"><span class="toc-section-number">3.3</span> <code>contiguous</code> to
<code>exhaustive</code><span></span></a></li>
</ul></li>
</ul>
</div>
<h1 data-number="1" id="revision-history"><span class="header-section-number">1</span> Revision History<a href="#revision-history" class="self-link"></a></h1>
<h2 data-number="1.1" id="initial-version-2022-06-mailing"><span class="header-section-number">1.1</span> Initial Version 2022-06
Mailing<a href="#initial-version-2022-06-mailing" class="self-link"></a></h2>
<h1 data-number="2" id="description"><span class="header-section-number">2</span> Description<a href="#description" class="self-link"></a></h1>
<h2 data-number="2.1" id="the-issue"><span class="header-section-number">2.1</span> The Issue<a href="#the-issue" class="self-link"></a></h2>
<p>During LWG review a few members of classes in the <code>mdspan</code>
proposals were identified as problematic, this paper proposes renaming
those members.</p>
<h3 data-number="2.1.1" id="pointer"><span class="header-section-number">2.1.1</span> <code>pointer</code><a href="#pointer" class="self-link"></a></h3>
<p>The <code>pointer</code> type is provided by the <em>accessor
policy</em> of an <code>mdspan</code>. While it actually is
<code>accessor_type::element_type*</code> for the only proposed accessor
policy in <code>P0009</code>, the requirements for <code>pointer</code>
are very loose. It does not need be dereferencable, indexable or most
any other thing one would expect from a C++ <em>pointer</em>. The only
thing happening with an <code>mdspan</code> pointer is that it is handed
together with an offset to the accessor policy’s, <code>access</code>
function, which returns a <em>reference</em>.</p>
<p>To make this clearer we propose to rename <code>pointer</code> to
<code>data_handle_type</code>, and <code>mdspan::data()</code> to
<code>mdspan::data_handle()</code>. <code>SOME_handle_type</code>
combined with a <code>SOME_handle()</code> function to retrieve it has
precedence in the library, with <code>std::thread</code> which has
<code>native_handle_type</code> and <code>native_handle()</code> member
types.</p>
<p>In <code>mdspan</code> <code>pointer</code> is serving as an opaque
handle to data, hence <code>data_handle</code>.</p>
<p><code>pointer</code> has uses in the <em>accessor policy
requirements</em>, the <code>default_accessor</code> class, and the
<code>mdspan</code> class.</p>
<h3 data-number="2.1.2" id="contiguous"><span class="header-section-number">2.1.2</span> <code>contiguous</code><a href="#contiguous" class="self-link"></a></h3>
<p><code>contiguous</code> is used in the <code>mdspan</code> proposal
to indicate that for a given <em>layout mapping</em> <code>map</code>
for all values in the range <span class="math inline">[0,</span>
<code>map.required_span_size()</code><span class="math inline">)</span>
there exists a multidimensional index in the maps index space, that
returns that value when used in its mapping operator.</p>
<p>However, the concern was brought up that these values are not
necessarily return in a linear order. For example a 1D custom mapping
could map the values <code>{0, 1, 2, 3, 4}</code> to the offsets
<code>{4, 1, 3, 0, 2}</code>. This would still be
<code>contiguous</code> in the sense it is used in
<code>mdspan</code>.</p>
<p>To avoid confusing we propose to replace <code>contiguous</code> with
<code>exhaustive</code>, indicating that the entire range of possible
return values of the mapping operator, is in fact used by the
mapping.</p>
<h1 data-number="3" id="wording"><span class="header-section-number">3</span> Wording<a href="#wording" class="self-link"></a></h1>
<p>Since P0009 is under active revision in LWG exact wording is not
proposed, since there is no up-to-date target.</p>
<h2 data-number="3.1" id="pointer-to-data_handle_type"><span class="header-section-number">3.1</span> <code>pointer</code> to
<code>data_handle_type</code><a href="#pointer-to-data_handle_type" class="self-link"></a></h2>
<p>Every use of <code>pointer</code> in the P0009 wording section is to
be replaced by <code>data_handle_type</code>. Note: there is a use of
<code>is_pointer_v</code> and <code>Pointer</code> in the proposal after
merging of P2554, which needs to remain as such (i.e. for the deduction
guide in question, it really is a C++ pointer since we have to use
<code>remove_pointer_t</code> on it.</p>
<h2 data-number="3.2" id="data-to-data_handle"><span class="header-section-number">3.2</span> <code>data()</code> to
<code>data_handle()</code><a href="#data-to-data_handle" class="self-link"></a></h2>
<p>The <code>mdspan::data()</code> function is renamed to
<code>mdspan::data_handle()</code>.</p>
<h2 data-number="3.3" id="contiguous-to-exhaustive"><span class="header-section-number">3.3</span> <code>contiguous</code> to
<code>exhaustive</code><a href="#contiguous-to-exhaustive" class="self-link"></a></h2>
<p>Every use of <code>contiguous</code> in the wording section would be
replaced with <code>exhaustive</code>, which specifically renames the
<code>is_contiguous</code> and <code>is_always_contiguous</code> members
of mappings (layout mapping requirements, <code>layout_left</code>,
<code>layout_right</code> and <code>layout_stride</code>) and the
<code>mdspan</code> class itself. Furthermore, two uses of
<em>contiguous</em> in prose text in notes will be replaced.</p>
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