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<title>P0332r0 : Relaxed Incomplete Multidimensional Array Type Declaration</title>
<meta name="date" content="2016-05-27" />
<meta name="author" content="H. Carter Edwards" />
<meta name="author" content="Bryce Lelbach" />
<meta name="author" content="Christian Trott" />
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<div class="document" id="p0332r0-relaxed-incomplete-multidimensional-array-type-declaration">
<h1 class="title">P0332r0 : Relaxed Incomplete Multidimensional Array Type Declaration</h1>
<table class="docinfo" frame="void" rules="none">
<col class="docinfo-name" />
<col class="docinfo-content" />
<tbody valign="top">
<tr class="field"><th class="docinfo-name">Project:</th><td class="field-body">ISO JTC1/SC22/WG21: Programming Language C++</td>
</tr>
<tr class="field"><th class="docinfo-name">Number:</th><td class="field-body">P0332r0</td>
</tr>
<tr><th class="docinfo-name">Date:</th>
<td>2016-05-27</td></tr>
<tr class="field"><th class="docinfo-name">Reply-to:</th><td class="field-body"><a class="reference external" href="mailto:hcedwar&#64;sandia.gov">hcedwar&#64;sandia.gov</a>, <a class="reference external" href="mailto:balelbach&#64;lbl.gov">balelbach&#64;lbl.gov</a></td>
</tr>
<tr><th class="docinfo-name">Author:</th>
<td>H. Carter Edwards</td></tr>
<tr><th class="docinfo-name">Contact:</th>
<td><a class="first last reference external" href="mailto:hcedwar&#64;sandia.gov">hcedwar&#64;sandia.gov</a></td></tr>
<tr><th class="docinfo-name">Author:</th>
<td>Bryce Lelbach</td></tr>
<tr><th class="docinfo-name">Contact:</th>
<td><a class="first last reference external" href="mailto:balelbach&#64;lbl.gov">balelbach&#64;lbl.gov</a></td></tr>
<tr><th class="docinfo-name">Author:</th>
<td>Christian Trott</td></tr>
<tr><th class="docinfo-name">Contact:</th>
<td><a class="first last reference external" href="mailto:crtrott&#64;sandia.gov">crtrott&#64;sandia.gov</a></td></tr>
<tr><th class="docinfo-name">Author:</th>
<td>Mauro Bianco</td></tr>
<tr><th class="docinfo-name">Contact:</th>
<td><a class="first last reference external" href="mailto:mbianco&#64;cscs.ch">mbianco&#64;cscs.ch</a></td></tr>
<tr><th class="docinfo-name">Author:</th>
<td>Robin Maffeo</td></tr>
<tr><th class="docinfo-name">Contact:</th>
<td><a class="first last reference external" href="mailto:Robin.Maffeo&#64;amd.com">Robin.Maffeo&#64;amd.com</a></td></tr>
<tr><th class="docinfo-name">Author:</th>
<td>Ben Sander</td></tr>
<tr><th class="docinfo-name">Contact:</th>
<td><a class="first last reference external" href="mailto:ben.sander&#64;amd.com">ben.sander&#64;amd.com</a></td></tr>
<tr class="field"><th class="docinfo-name">Audience:</th><td class="field-body">Library Evolution Working Group (LEWG)</td>
</tr>
<tr class="field"><th class="docinfo-name">URL:</th><td class="field-body"><a class="reference external" href="https://github.com/kokkos/array_ref/blob/master/proposals/P0332.rst">https://github.com/kokkos/array_ref/blob/master/proposals/P0332.rst</a></td>
</tr>
</tbody>
</table>
<table border="1" class="docutils">
<colgroup>
<col width="16%" />
<col width="84%" />
</colgroup>
<tbody valign="top">
<tr><td colspan="2"><strong>Revision History</strong></td>
</tr>
<tr><td>P0009r0</td>
<td>Original multidimensional array reference paper with
motivation, specification, and examples.</td>
</tr>
<tr><td>P0009r1</td>
<td>Revised with renaming from <tt class="docutils literal">view</tt> to <tt class="docutils literal">array_ref</tt>
and allow unbounded rank through variadic arguments.</td>
</tr>
<tr><td>P0332r0
<em>(current</em>)</td>
<td>Relaxed array declaration moved from P0009.</td>
</tr>
<tr><td colspan="2"><strong>References</strong></td>
</tr>
<tr><td>P0009r2</td>
<td>Multidimensional array reference specification</td>
</tr>
<tr><td>P0331</td>
<td>Multidimensional array reference motivation and examples</td>
</tr>
</tbody>
</table>
<div class="section" id="motivation">
<h1>Motivation</h1>
<p>The dimensions of multidimensional array reference class <tt class="docutils literal">array_ref</tt>
(P0009r2) are declared through either the data type or array properties
template arguments.</p>
<pre class="code c++ literal-block">
<span class="keyword">template</span><span class="operator">&lt;</span> <span class="keyword">typename</span> <span class="name">DataType</span> <span class="punctuation">,</span> <span class="keyword">typename</span> <span class="name">Properties</span><span class="punctuation">...</span> <span class="operator">&gt;</span>
<span class="keyword">struct</span> <span class="name">array_ref</span> <span class="punctuation">;</span>
</pre>
<!--  -->
<p><tt class="docutils literal">DataType</tt> may be an array type (8.3.4.p3).
For example, <tt class="docutils literal">array_ref&lt;double[ <span class="pre">][3][8]&gt;</span></tt>
declares a rank-three array with a dynamic extent
in dimension coordinate zero.
This compact and intutive syntax is currently limited to a declaring a
single dynamic extent in the array's leading coordinate.
Removing this limitation allows declaration of an arbitrary
number of dynamic extents in any coordinate.</p>
</div>
<div class="section" id="proposal">
<h1>Proposal</h1>
<p>The current array type declarator constraints are defined in in <strong>n4567 8.3.4.p3</strong> as follows.</p>
<blockquote>
<em>When several “array of” specifications are adjacent,
a multidimensional array type is created;
only the first of the constant expressions that
specify the bounds of the arrays may be omitted.
In addition to declarations in which an incomplete
object type is allowed, an array bound may be omitted
in some cases in the declaration of a function parameter (8.3.5).
An array bound may also be omitted when the declarator is
followed by an initializer (8.5).
In this case the bound is calculated from the
number of initial elements (say, N) supplied (8.5.1),
and the type of the identifier of D is “array of N T”.
Furthermore, if there is a preceding declaration
of the entity in the same scope in which the bound was specified,
an omitted array bound is taken to be the same as in that
earlier declaration, and similarly for the definition
of a static data member of a class.</em></blockquote>
<p>The preferred syntax requires a relaxation of
array type declarator constraints defined in <strong>n4567 8.3.4.p3</strong>
exclusively for an incomplete object type.
The following wording change is proposed.</p>
<blockquote>
<em>When several “array of” specifications are adjacent,
a multidimensional array type is created.
The first of the constant expressions that
specify the bounds of the array may be omitted
in some cases in the declaration of a function parameter (8.3.5).
The first array bound may also be omitted when the declarator is
followed by an initializer (8.5).
In this case the first bound is calculated from the
number of initial elements (say, N) supplied (8.5.1),
and the type of the identifier of D is “array of N T”.
Furthermore, if there is a preceding declaration
of the entity in the same scope in which the first bound was specified,
an omitted array first bound is taken to be the same as in that
earlier declaration, and similarly for the definition
of a static data member of a class.</em>
<strong>In declarations in which an incomplete object type is allowed
any of the constant expressions that specify the bounds of the array
may be ommitted.</strong></blockquote>
<p><strong>This minor language specification change has been implemented with
a trivial (one line) patch to Clang and was permissible in gcc prior to
version 5.</strong></p>
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