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<title>Issue 4294: bitset(const CharT*) constructor needs to be constrained</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#Ready">Tentatively Ready</a> status.</em></p>
<h3 id="4294"><a href="lwg-active.html#4294">4294</a>. <code class='backtick'>bitset(const CharT*)</code> constructor needs to be constrained</h3>
<p><b>Section:</b> 22.9.2.2 <a href="https://wg21.link/bitset.cons">[bitset.cons]</a> <b>Status:</b> <a href="lwg-active.html#Ready">Tentatively Ready</a>
 <b>Submitter:</b> Jonathan Wakely <b>Opened:</b> 2025-07-12 <b>Last modified:</b> 2025-08-26</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#bitset.cons">issues</a> in [bitset.cons].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#Tentatively Ready">Tentatively Ready</a> status.</p>
<p><b>Discussion:</b></p>
<p>
This code might be ill-formed, with an error outside the immediate context
that users cannot prevent:
<blockquote><pre><code>
#include &lt;bitset&gt;
struct NonTrivial { ~NonTrivial() { } };
static_assert( ! std::is_constructible_v&lt;std::bitset&lt;1&gt;, NonTrivial*&gt; );
</code></pre></blockquote>
</p>
<p>
The problem is that the <code class='backtick'>bitset(const CharT*)</code> constructor tries to instantiate
<code>basic_string_view&lt;NonTrivial&gt;</code> to find its <code class='backtick'>size_type</code>,
and that instantiation might ill-formed, e.g. if <code class='backtick'>std::basic_string_view</code> or
<code class='backtick'>std::char_traits</code> has a static assert enforcing the requirement for their
character type to be sufficiently char-like. 27.1 <a href="https://wg21.link/strings.general">[strings.general]</a>
defines a char-like type as "any non-array trivially copyable standard-layout
(6.9.1 <a href="https://wg21.link/basic.types.general">[basic.types.general]</a>) type <code class='backtick'>T</code>
where <code>is_trivially_default_constructible_v&lt;T&gt;</code> is <code class='backtick'>true</code>."
</p>

<p><i>[2025-08-21; Reflector poll]</i></p>

<p>
Set status to Tentatively Ready after five votes in favour during reflector poll.
</p>



<p id="res-4294"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N5008" title=" Working Draft, Programming Languages — C++">N5008</a>.
</p>

<ol>

<li><p>Modify 22.9.2.2 <a href="https://wg21.link/bitset.cons">[bitset.cons]</a> as indicated:</p>

<blockquote>
<pre><code>
template&lt;class charT&gt;
  constexpr explicit bitset(
    const charT* str,
    typename basic_string_view&lt;charT&gt;::size_type n = basic_string_view&lt;charT&gt;::npos,
    charT zero = charT(’0’),
    charT one = charT(’1’));
</code></pre>
<blockquote>
<p>
<ins>-?- <i>Constraints</i>:</ins>
<ul>
<li>
<ins><code>is_array_v&lt;charT&gt;</code> is <code class='backtick'>false</code>,</ins>
</li>
<li>
<ins><code>is_trivially_copyable_v&lt;charT&gt;</code> is <code class='backtick'>true</code>,</ins>
</li>
<li>
<ins><code>is_standard_layout_v&lt;charT&gt;</code> is <code class='backtick'>true</code>, and</ins>
</li>
<li>
<ins><code>is_trivially_default_constructible_v&lt;charT&gt;</code> is <code class='backtick'>true</code>.</ins>
</li>
</ul>
</p>
<p>-8- <i>Effects</i>: As if by:
<blockquote>
<pre><code>bitset(n == basic_string_view&lt;charT&gt;::npos
          ? basic_string_view&lt;charT&gt;(str)
          : basic_string_view&lt;charT&gt;(str, n),
       0, n, zero, one)
</code></pre>
</blockquote>
</p>
</blockquote>
</blockquote>
</li>
</ol>







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