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<title>Issue 1173: "Equivalence" wishy-washiness</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#NAD">NAD</a> status.</em></p>
<h3 id="1173"><a href="lwg-closed.html#1173">1173</a>. "Equivalence" wishy-washiness</h3>
<p><b>Section:</b> 16 <a href="https://wg21.link/library">[library]</a> <b>Status:</b> <a href="lwg-active.html#NAD">NAD</a>
 <b>Submitter:</b> David Abrahams <b>Opened:</b> 2009-07-14 <b>Last modified:</b> 2016-11-21</p>
<p><b>Priority: </b>3
</p>
<p><b>View other</b> <a href="lwg-index-open.html#library">active issues</a> in [library].</p>
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<p><b>Discussion:</b></p>
<p>
Issue: The <code>CopyConstructible</code> requirements are wishy-washy.  It requires
that the copy is "equivalent" to the original, but "equivalent" is never
defined.
</p>
<p>
I believe this to be an example of a more general lack of rigor around
copy and assignment, although I haven't done the research to dig up all
the instances.
</p>
<p>
It's a problem because if you don't know what <code>CopyConstructible</code> means,
you also don't know what it means to copy a pair of <code>CopyConstructible</code>
types.  It doesn't prevent us from writing code, but it is a hole in our
ability to understand the meaning of copy.
</p>
<p>
Furthermore, I'm pretty sure that vector's copy constructor doesn't
require the elements to be <code>EqualityComparable</code>, so that table is actually
referring to some ill-defined notion of equivalence when it uses ==.
</p>

<p><i>[
2009 Santa Cruz:
]</i></p>


<blockquote><p>
Move to "Open". Dave is right that this is a big issue. Paper D2987
("Defining Move Special Member Functions", Bjarne Stroustrup and
Lawrence Crowl) touches on this but does not solve it. This issue is
discussed in Elements of Programming.
</p></blockquote>


<p><i>[
2010 Rapperswil:
]</i></p>


<blockquote><p>
This issue is quite vague, so it is difficult to know if and when it has been resolved.
John Lakos wrote a paper covering this area a while back, and there is a real interest 
in providing some sort of clean-up in the future. We need a more clearly draughted 
issues with an addressable set of concerns, ideally with a paper proposing a resolution, 
but for a future revision of the standard. Move to Tentatively NAD Future.
</p></blockquote>

<p><i>[
Moved to NAD Future at 2010-11 Batavia
]</i></p>


<p><i>[2016-11-12, Issaquah]</i></p>

<p>Sat PM: Status: NAD</p>
<p>It was said in 2010: "We need a more clearly draughted issue with an addressable 
set of concerns, ideally with a paper proposing a resolution, 
but for a future revision of the standard."</p>


<p id="res-1173"><b>Proposed resolution:</b></p>





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