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<title>Issue 402: wrong new expression in [some_]allocator::construct</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#CD1">CD1</a> status.</em></p>
<h3 id="402"><a href="lwg-defects.html#402">402</a>. wrong new expression in [some_]allocator::construct</h3>
<p><b>Section:</b> 16.4.4.6 <a href="https://wg21.link/allocator.requirements">[allocator.requirements]</a>, 20.2.10.2 <a href="https://wg21.link/allocator.members">[allocator.members]</a> <b>Status:</b> <a href="lwg-active.html#CD1">CD1</a>
 <b>Submitter:</b> Markus Mauhart <b>Opened:</b> 2003-02-27 <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#CD1">CD1</a> status.</p>
<p><b>Discussion:</b></p>
<p>
This applies to the new expression that is contained in both par12 of
20.2.10.2 <a href="https://wg21.link/allocator.members">[allocator.members]</a> and in par2 (table 32) of  [default.con.req].
I think this new expression is wrong, involving unintended side
effects.
</p>


<p>20.2.10.2 <a href="https://wg21.link/allocator.members">[allocator.members]</a>  contains the following 3 lines:</p>

<pre>
  11 Returns: the largest value N for which the call allocate(N,0) might succeed.
     void construct(pointer p, const_reference val);
  12 Returns: new((void *) p) T( val)
</pre>


<p> [default.con.req] in table 32 has the following line:</p>
<pre>
  a.construct(p,t)   Effect: new((void*)p) T(t)
</pre>

<p>
.... with the prerequisits coming from the preceding two paragraphs,
especially from table 31:
</p>

<pre>
  alloc&lt;T&gt;             a     ;// an allocator for T
  alloc&lt;T&gt;::pointer    p     ;// random access iterator
                              // (may be different from T*)
  alloc&lt;T&gt;::reference  r = *p;// T&amp;
  T const&amp;             t     ;
</pre>

<p>
Cause of using "new" but not "::new", any existing "T::operator new"
function will hide the global placement new function. When there is no
"T::operator new" with adequate signature,
every_alloc&lt;T&gt;::construct(..) is ill-formed, and most
std::container&lt;T,every_alloc&lt;T&gt;&gt; use it; a workaround
would be adding placement new and delete functions with adequate
signature and semantic to class T, but class T might come from another
party. Maybe even worse is the case when T has placement new and
delete functions with adequate signature but with "unknown" semantic:
I dont like to speculate about it, but whoever implements
any_container&lt;T,any_alloc&gt; and wants to use construct(..)
probably must think about it.
</p>


<p id="res-402"><b>Proposed resolution:</b></p>
<p>
Replace "new" with "::new" in both cases.
</p>







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