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<title>Issue 997: "Effects: Equivalent to" is underspecified</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++11">C++11</a> status.</em></p>
<h3 id="997"><a href="lwg-defects.html#997">997</a>. "<i>Effects</i>: Equivalent to" is underspecified</h3>
<p><b>Section:</b> 16.3.2.4 <a href="https://wg21.link/structure.specifications">[structure.specifications]</a> <b>Status:</b> <a href="lwg-active.html#C++11">C++11</a>
 <b>Submitter:</b> Thomas Plum <b>Opened:</b> 2009-03-03 <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#structure.specifications">active issues</a> in [structure.specifications].</p>
<p><b>View all other</b> <a href="lwg-index.html#structure.specifications">issues</a> in [structure.specifications].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++11">C++11</a> status.</p>
<p><b>Discussion:</b></p>
<p><b>Addresses UK-163 [CD1]</b></p>
<p>
Many functions are defined as "Effects: Equivalent to a...", which seems
to also define the preconditions, effects, etc. But this is not made
clear.
</p>

<p>
After studying the occurrences of "Effects: Equivalent to", I agree with
the diagnosis but disagree with the solution.  In 27.4.3.3 <a href="https://wg21.link/string.cons">[string.cons]</a>
we find
</p>

<blockquote>
<p>
14 <i>Effects:</i> If <code>InputIterator</code> is an integral type, equivalent to
<code>basic_string(static_cast&lt;size_type&gt;(begin), static_cast&lt;value_type&gt;(end), a)</code>
</p>
<p>
15 Otherwise constructs a string from the values in the range <code>[begin,
end)</code>, as indicated in the Sequence Requirements table (see 23.1.3).
</p>
</blockquote>

<p>
This would be devishly difficult to re-write with an explicit
"Equivalent to:" clause.  Instead, I propose the following, which will
result in much less editorial re-work.
</p>

<p><i>[
2009-05-09 Alisdair adds:
]</i></p>


<blockquote><p>
This issue is related to <a href="lwg-closed.html#492" title="Invalid iterator arithmetic expressions (Status: NAD)">492</a><sup><a href="https://cplusplus.github.io/LWG/issue492" title="Latest snapshot">(i)</a></sup>.
</p></blockquote>

<p><i>[
Batavia (2009-05):
]</i></p>

<blockquote><p>
We agree with the proposed resolution.
Move to Tentatively Ready.
</p></blockquote>


<p id="res-997"><b>Proposed resolution:</b></p>
<p>
Add a new paragraph after 16.3.2.4 <a href="https://wg21.link/structure.specifications">[structure.specifications]</a>, p3:
</p>

<blockquote>
<p>
-3- Descriptions of function semantics contain the following elements (as appropriate):<sup>154</sup>
</p>

<ul>
<li>
<i>Requires:</i> the preconditions for calling the function
</li>
<li>
<i>Effects:</i> the actions performed by the function
</li>
<li>
<i>Postconditions:</i> the observable results established by the function
</li>
<li>
<i>Returns:</i> a description of the value(s) returned by the function
</li>
<li>
<i>Throws:</i> any exceptions thrown by the function, and the conditions that would cause the exception
</li>
<li>
<i>Complexity:</i> the time and/or space complexity of the function
</li>
<li>
<i>Remarks:</i> additional semantic constraints on the function
</li>
<li>
<i>Error conditions:</i> the error conditions for error codes reported by the function.
</li>
<li>
<i>Notes:</i> non-normative comments about the function
</li>
</ul>

<p><ins>
Whenever the <i>Effects</i> element specifies that the semantics of some
function <code>F</code> are <i>Equivalent to</i> some <i>code-sequence</i>, then
the various elements are interpreted as follows.  If <code>F</code>'s
semantics specifies a <i>Requires</i> element, then that requirement is
logically imposed prior to the <i>equivalent-to</i> semantics.  Then,
the semantics of the <i>code-sequence</i> are determined by the
<i>Requires</i>, <i>Effects</i>, <i>Postconditions</i>, <i>Returns</i>,
<i>Throws</i>, <i>Complexity</i>, <i>Remarks</i>, <i>Error
Conditions</i> and <i>Notes</i> specified for the (one or more) function
invocations contained in the <i>code-sequence</i>. The value returned from
<code>F</code> is specified by <code>F</code>'s <i>Returns</i> element, or
if <code>F</code> has no <i>Returns</i> element, a non-<code>void</code> return from <code>F</code> is specified 
by the <i>Returns</i> elements in <i>code-sequence</i>.  If
<code>F</code>'s semantics contains a <i>Throws</i> (or
<i>Postconditions</i>, or <i>Complexity</i>) element, then that
supersedes any occurrences of that element in the <i>code-sequence</i>.
</ins></p>
</blockquote>






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