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<title>Impact of Language Changes&nbsp; on LWG Schedule</title>
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<p>Doc. no.&nbsp;&nbsp; WG21/N2004=06-0074<br>
Date:&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
<!--webbot bot="Timestamp" s-type="EDITED" s-format="%Y-%m-%d" startspan -->2006-04-11<!--webbot bot="Timestamp" endspan i-checksum="12261" --><br>
Project:&nbsp;&nbsp;&nbsp;&nbsp; Programming Language C++<br>
Reply to:&nbsp;&nbsp; Beman Dawes &lt;<a href="mailto:bdawes@acm.org">bdawes@acm.org</a>&gt;</p>
<h1>Impact of Language Changes&nbsp;on LWG Schedule</h1>
<p>The Library Working Group in Berlin reviewed all pending language change 
proposals from the standpoint of their potential impact on the LWG's ability to 
stick to the proposed schedule for the next standard. <b>The relative merits 
of proposals were not considered - only the impact on schedule was discussed.
</b> </p>
<p>Two questions were asked of each potential language change:</p>
<ul>
  <li>Would the proposed language change require or strongly suggest changes to the Standard Library 
  interface or specifications?<br>
&nbsp;</li>
  <li>Even if not absolutely required, would the LWG like to utilize the proposed feature to improve the library 
  interface or specifications?</li>
</ul>
<p>If the answer to either of these questions was &quot;yes&quot;, further questions were 
considered:</p>
<ul>
  <li>How time-consuming do the potential library changes appear to be? A change 
  might be considered time consuming for several reasons.&nbsp;It might impact 
  large portions of the library, be technically difficult, or have a steep 
  learning curve for LWG members.<br>
&nbsp;</li>
  <li>Must a potential library change be tested with a real compiler? Some 
  interface changes are complex enough that the LWG won't be able to tell if 
  they work unless tested with a real compiler implementation. Testing 
  may also be required to ensure that interface changes do not unexpectedly break 
  backward compatibility for user programs. (The LWG is willing to break 
  backward compatibility after due consideration, but does not want to 
  inadvertently break backward compatibility.)</li>
</ul>
<p>If the answer to these questions was &quot;yes&quot;, the proposed change has a 
possible impact on schedule.</p>
<h2>Proposed language changes with possible impact on C++ 0x schedule</h2>
<h3>Very high impact changes</h3>
<p>The following proposed changes will cause the C++0x schedule to slip if the 
committee does not <a href="#commit">commit</a> to them by the end of the October, 2006 
meeting, or if compilers implementing the language features do not become 
available to LWG members soon (say within the next year).</p>
<ul>
  <li>R-value Reference.</li>
  <li>Concepts.</li>
  <li>Contract Programming.</li>
</ul>
<h3>High impact changes</h3>
<p>The following proposed changes will cause the C++0x schedule to slip if the 
committee does not <a href="#commit">commit</a> to them by the April, 2007 meeting. Although 
availability of test compilers is less critical, compilers supporting the bulk 
of the features are needed over the next 18 months.</p>
<ul>
  <li>Auto and decltype.</li>
  <li>Lambda <span style="background-color: #FFFFFF">expressions and closures</span>.</li>
  <li>Variatic templates.</li>
  <li>Template aliases (needed for TR2).</li>
  <li>Generalized constant expressions.</li>
  <li>New for-loop.</li>
  <li>Uniform Calling Syntax (Re-opening public interfaces).</li>
  <li>Extensible literals.</li>
</ul>
<h3><a name="commit">What</a> does it mean for the committee to &quot;commit&quot; to a 
proposal?</h3>
<p>The LWG doesn't have a single litmus test for the committee committing to a 
proposal. For a few, it might be just a report from the EWG that a proposal is 
likely to be brought forward. Quite a few members would like to see a proposal 
forwarded from EWG to CWG first. Some would like a report that CWG expects to 
have a formal motion ready soon. One or two LWG members want to see a proposal 
actually voted into the WP. Regardless of the details, LWG members do not want 
to spend serious effort on library changes, only to have a feature then not make 
it into the language.</p>
<h3>Disclamer</h3>
<p>This paper was prepared on behalf of the LWG. All errors and omissions, 
however, are my own.&nbsp; --BGD</p>
<hr>
<p>&nbsp;</p>

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