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<title>Issue 199: What does allocate(0) return?</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#TC1">TC1</a> status.</em></p>
<h3 id="199"><a href="lwg-defects.html#199">199</a>. What does <code>allocate(0)</code> return?</h3>
<p><b>Section:</b> 16.4.4.6 <a href="https://wg21.link/allocator.requirements">[allocator.requirements]</a> <b>Status:</b> <a href="lwg-active.html#TC1">TC1</a>
 <b>Submitter:</b> Matt Austern <b>Opened:</b> 1999-11-19 <b>Last modified:</b> 2016-01-28</p>
<p><b>Priority: </b>Not Prioritized
</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#TC1">TC1</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Suppose that <code>A</code> is a class that conforms to the
Allocator requirements of Table 32, and <code>a</code> is an
object of class <code>A</code>  What should be the return
value of <code>a.allocate(0)</code>?  Three reasonable
possibilities: forbid the argument <code>0</code>, return
a null pointer, or require that the return value be a
unique non-null pointer.
</p>


<p id="res-199"><b>Proposed resolution:</b></p>
<p>
Add a note to the <code>allocate</code> row of Table 32:
&quot;[<i>Note:</i> If <code>n == 0</code>, the return value is unspecified. <i>--end note</i>]&quot;</p>


<p><b>Rationale:</b></p>
<p>A key to understanding this issue is that the ultimate use of
allocate() is to construct an iterator, and that iterator for zero
length sequences must be the container's past-the-end
representation.  Since this already implies special case code, it
would be over-specification to mandate the return value.
</p>




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