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<title>Issue 2466: allocator_traits::max_size() default behavior is incorrect</title>
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<p><em>This page is a snapshot from the LWG issues list, see the <a href="lwg-active.html">Library Active Issues List</a> for more information and the meaning of <a href="lwg-active.html#C++17">C++17</a> status.</em></p>
<h3 id="2466"><a href="lwg-defects.html#2466">2466</a>. <code>allocator_traits::max_size()</code> default behavior is incorrect</h3>
<p><b>Section:</b> 16.4.4.6 <a href="https://wg21.link/allocator.requirements">[allocator.requirements]</a>, 20.2.9.3 <a href="https://wg21.link/allocator.traits.members">[allocator.traits.members]</a> <b>Status:</b> <a href="lwg-active.html#C++17">C++17</a>
 <b>Submitter:</b> Howard Hinnant <b>Opened:</b> 2015-01-17 <b>Last modified:</b> 2017-07-30</p>
<p><b>Priority: </b>3
</p>
<p><b>View other</b> <a href="lwg-index-open.html#allocator.requirements">active issues</a> in [allocator.requirements].</p>
<p><b>View all other</b> <a href="lwg-index.html#allocator.requirements">issues</a> in [allocator.requirements].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++17">C++17</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Table 28 &mdash; "Allocator requirements" says that default behavior for <code>a.max_size()</code> is 
<code>numeric_limits&lt;size_type&gt;::max()</code>. And this is consistent with the matching statement 
for <code>allocator_traits</code> in 20.2.9.3 <a href="https://wg21.link/allocator.traits.members">[allocator.traits.members]</a>/p7:
</p>
<blockquote>
<pre>
static size_type max_size(const Alloc&amp; a) noexcept;
</pre>
<blockquote>
<p>
<i>Returns</i>: <code>a.max_size()</code> if that expression is well-formed; otherwise, 
<code>numeric_limits&lt;size_type&gt;::max()</code>.
</p>
</blockquote>
</blockquote>
<p>
However, when allocating memory, an allocator must allocate <code>n*sizeof(value_type)</code> bytes, for example:
</p>
<blockquote>
<pre>
value_type*
allocate(std::size_t n)
{
  return static_cast&lt;value_type*&gt;(::operator new (n * sizeof(value_type)));
}
</pre>
</blockquote>
<p>
When <code>n == numeric_limits&lt;size_type&gt;::max(), n * sizeof(value_type)</code> is <em>guaranteed</em> 
to overflow except when <code>sizeof(value_type) == 1</code>.
<p/>
A more useful default would be <code>numeric_limits&lt;size_type&gt;::max() / sizeof(value_type)</code>.
</p>

<p><i>[2015-05, Lenexa]</i></p>

<p>
Marshall: Is this the right solution?<br/>
PJP: I think it's gilding the lily.<br/>
STL: I think this is right, and it doesn't interact with the incomplete container stuff because it's in a member function.<br/>
Marshall: Objections to this?<br/>
STL: Spaces around binary operators.<br/>
Hwrd: It's completely wrong without spaces.<br/>
Marshall: All in favor of Ready?<br/>
Lots. 
</p>


<p id="res-2466"><b>Proposed resolution:</b></p>
<p>This wording is relative to N4296.</p>

<ol>
<li><p>Change 16.4.4.6 <a href="https://wg21.link/allocator.requirements">[allocator.requirements]</a>, Table 28 &mdash; "Allocator requirements", as indicated:</p>
<blockquote>
<table border="1">
<caption>Table 28 &mdash; Allocator requirements</caption>
<tr>
<th>Expression</th>
<th>Return type</th>
<th>Assertion&#47;note<br/>pre-&#47;post-condition</th>
<th>Default</th>
</tr>

<tr>
<td colspan="4" align="center">
<code>&hellip;</code>
</td>
</tr>

<tr>
<td>
<code>a.max_size()</code>
</td>
<td>
<code>X::size_type</code>
</td>
<td>
the largest value that can<br/>
meaningfully be passed to<br/>
<code>X::allocate()</code>
</td>
<td>
<code>numeric_limits&lt;size_type&gt;::max()<ins>/sizeof(value_type)</ins></code>
</td>
</tr>

<tr>
<td colspan="4" align="center">
<code>&hellip;</code>
</td>
</tr>

</table>
</blockquote>

</li>

<li><p>Change 20.2.9.3 <a href="https://wg21.link/allocator.traits.members">[allocator.traits.members]</a>/p7 as indicated:</p>

<blockquote>
<pre>
static size_type max_size(const Alloc&amp; a) noexcept;
</pre>
<blockquote>
<p>
<i>Returns</i>: <code>a.max_size()</code> if that expression is well-formed; otherwise, 
<code>numeric_limits&lt;size_type&gt;::max()<ins>/sizeof(value_type)</ins></code>.
</p>
</blockquote>
</blockquote>

</li>
</ol>





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