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<title>Issue 4132: Throws specifications need to include boolean-testable operations</title>
<meta property="og:title" content="Issue 4132: Throws specifications need to include boolean-testable operations">
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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#New">New</a> status.</em></p>
<h3 id="4132"><a href="lwg-active.html#4132">4132</a>. <i>Throws</i> specifications need to include <code><i>boolean-testable</i></code> operations</h3>
<p><b>Section:</b> 24.2.7.1 <a href="https://wg21.link/associative.reqmts.general">[associative.reqmts.general]</a>, 24.3.11.5 <a href="https://wg21.link/list.ops">[list.ops]</a>, 24.3.10.6 <a href="https://wg21.link/forward.list.ops">[forward.list.ops]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
<b>Submitter:</b> Daniel Krügler <b>Opened:</b> 2024-08-03 <b>Last modified:</b> 2024-08-04</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View other</b> <a href="lwg-index-open.html#associative.reqmts.general">active issues</a> in [associative.reqmts.general].</p>
<p><b>View all other</b> <a href="lwg-index.html#associative.reqmts.general">issues</a> in [associative.reqmts.general].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The standard provides various <i>Throws</i>: elements whose specification refers to boolean-like
expressions, for example 24.3.11.5 <a href="https://wg21.link/list.ops">[list.ops]</a> says:
</p>
<blockquote><p>
-17- <i>Throws</i>: Nothing unless an exception is thrown by <code>*i == value</code> or
<code>pred(*i) != false</code>.
</p></blockquote>
<p>
The expressions <code>*i == value</code> (by [tab:cpp17.equalitycomparable]) and <code>pred(*i)</code>
(by 27.2 <a href="https://wg21.link/algorithms.requirements">[algorithms.requirements]</a>) both require to have a type that models
<code><i>boolean-testable</i></code>, so their actual evaluation potentially requires an additional
conversion to <code>bool</code> (The part "<code>!= false</code>" was not guaranteed to realize that as
pointed out by LWG <a href="lwg-active.html#4127" title="The Standard Library should not use predicates of the form pred(*i) != false (Status: New)">4127</a> and will therefore be removed by that issue).
<p/>
The <code><i>boolean-testable</i></code> requirements (18.5.2 <a href="https://wg21.link/concept.booleantestable">[concept.booleantestable]</a>) doesn't
say anything about potential exceptions thrown by this conversion, so we must assume that this
conversion allows an exception to be thrown.
<p/>
An additional problem arises by the fact that the <code><i>boolean-testable</i></code> requirements
do not require that the implicit and explicit conversion to <code>bool</code> have to behave
consistently in regard to exceptions. 18.4.4 <a href="https://wg21.link/concept.convertible">[concept.convertible]</a> only says
that "The implicit and explicit conversions are required to produce equal results". But
since we don't want to restrict implementations to a specific <code>bool</code> conversion,
the <i>Throws</i> wording should include it in a manner that applies to any form.
<p/>
Besides the obviously necessary conversion to <code>bool</code> we should not restrict
implementations in regard to the potential usage of other <code><i>boolean-testable</i></code>
operations, e.g. negation, which could potentially also throw an exception.
<p/>
Therefore the wording for the affected <i>Throws</i>: elements somehow needs to mention
<code><i>boolean-testable</i></code> operations to cover the effective operation that is
involved here without actually restricting existing implementations unnecessarily.
</p>


<p id="res-4132"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4986" title=" Working Draft, Programming Languages — C++">N4986</a>.
</p>

<ol>
<li><p>Modify 24.2.7.1 <a href="https://wg21.link/associative.reqmts.general">[associative.reqmts.general]</a> as indicated:</p>

<blockquote>
<pre>
a.merge(a2)
</pre>
<blockquote>
<p>
[&hellip;]
<p/>
-116- <i>Throws</i>: Nothing unless the <ins>evaluation of the</ins> comparison object
<ins>including all selected <code><i>boolean-testable</i></code> (18.5.2 <a href="https://wg21.link/concept.booleantestable">[concept.booleantestable]</a>)
operations</ins> throws.
</p>
</blockquote>
</blockquote>
</li>

<li><p>Modify 24.3.10.6 <a href="https://wg21.link/forward.list.ops">[forward.list.ops]</a> as indicated:</p>

<blockquote>
<pre>
size_type remove(const T&amp; value);
template&lt;class Predicate&gt; size_type remove_if(Predicate pred);
</pre>
<blockquote>
<p>
[&hellip;]
<p/>
-15- <i>Throws</i>: Nothing unless an exception is thrown by the equality comparison or the
predicate <ins>evaluation including all selected <code><i>boolean-testable</i></code>
(18.5.2 <a href="https://wg21.link/concept.booleantestable">[concept.booleantestable]</a>) operations</ins>.
</p>
</blockquote>
<p>
[&hellip;]
</p>
<pre>
size_type unique();
template&lt;class BinaryPredicate&gt; size_type unique(BinaryPredicate binary_pred);
</pre>
<blockquote>
<p>
[&hellip;]
<p/>
-22- <i>Throws</i>: Nothing unless an exception is thrown by the predicate
<ins>evaluation including all selected <code><i>boolean-testable</i></code>
(18.5.2 <a href="https://wg21.link/concept.booleantestable">[concept.booleantestable]</a>) operations</ins>.
</p>
</blockquote>
</blockquote>
</li>

<li><p>Modify 24.3.11.5 <a href="https://wg21.link/list.ops">[list.ops]</a> as indicated:</p>

<blockquote class="note">
<p>
[<i>Drafting note</i>: The adjustment of p17 is presented as if issue LWG <a href="lwg-active.html#4127" title="The Standard Library should not use predicates of the form pred(*i) != false (Status: New)">4127</a>
proposed wording has already been applied]
</p>
</blockquote>

<blockquote>
<pre>
size_type remove(const T&amp; value);
template&lt;class Predicate&gt; size_type remove_if(Predicate pred);
</pre>
<blockquote>
<p>
[&hellip;]
<p/>
-17- <i>Throws</i>: Nothing unless an exception is thrown by <code>*i == value</code> or
<code>pred(*i)</code> <ins>including all selected <code><i>boolean-testable</i></code>
(18.5.2 <a href="https://wg21.link/concept.booleantestable">[concept.booleantestable]</a>) operations</ins>.
<p/>
[&hellip;]
</p>
</blockquote>
<pre>
size_type unique();
template&lt;class BinaryPredicate&gt; size_type unique(BinaryPredicate binary_pred);
</pre>
<blockquote>
<p>
[&hellip;]
<p/>
-24- <i>Throws</i>: Nothing unless an exception is thrown by the predicate
<ins>evaluation including all selected <code><i>boolean-testable</i></code>
(18.5.2 <a href="https://wg21.link/concept.booleantestable">[concept.booleantestable]</a>) operations</ins>.
<p/>
[&hellip;]
</p>
</blockquote>
</blockquote>
</li>

</ol>





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