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<title>Issue 3484: Should &lt;stddef.h&gt; declare ::nullptr_t?</title>
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<h3 id="3484"><a href="lwg-active.html#3484">3484</a>. Should <code>&lt;stddef.h&gt;</code> declare <code>::nullptr_t</code>?</h3>
<p><b>Section:</b> 17.15.7 <a href="https://wg21.link/support.c.headers.other">[support.c.headers.other]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Thomas K&ouml;ppe <b>Opened:</b> 2020-09-06 <b>Last modified:</b> 2022-09-18</p>
<p><b>Priority: </b>3
</p>
<p><b>View other</b> <a href="lwg-index-open.html#support.c.headers.other">active issues</a> in [support.c.headers.other].</p>
<p><b>View all other</b> <a href="lwg-index.html#support.c.headers.other">issues</a> in [support.c.headers.other].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
From this <a href="https://github.com/cplusplus/draft/issues/4144">editorial issue request</a>: 
<p/>
The header <code>&lt;stddef.h&gt;</code> is currently specified in 17.15.7 <a href="https://wg21.link/support.c.headers.other">[support.c.headers.other]</a> 
to not declare a global name corresponding to <code>std::byte</code>, but no similar exclusion exists for 
<code>std::nullptr_t</code>.
<p/>
Is an oversight or intentional? There does not seem to be an interoperability reason to provide a global 
namespace name <code>::nullptr_t</code>, since this construction would be meaningless in C and thus the name 
would not be encountered in code that is both valid C and C++.
<p/>
For lack of justification, I would like to propose to require normatively that no name 
<code>::nullptr_t</code> be declared by <code>&lt;stddef.h&gt;</code>.
<p/>
Additional notes: The proposing paper <a href="https://wg21.link/n2431">N2431</a> mentions only an addition 
of "<code>nullptr_t</code>" to <code>&lt;cstddef&gt;</code> and does not discuss the impact on <code>&lt;stddef.h&gt;</code>. 
By omission this means the default rules for <code>&lt;stddef.h&gt;</code> apply and the global name should exist, 
but this does not provide us with a positive signal of intention.
<p/>
I also realize that this is a rather obscure point, and that vendors are already shipping <code>::nullptr_t</code>, 
so I am also happy to drop this issue as not being worth the churn and the increase in implementation complexity 
(since removals don't generally simplify implementations). I would welcome a bit of discussion, though.
</p>

<p><i>[2020-09-29; Priority to P3 after reflector discussions]</i></p>


<p><i>[2022-09-18; Daniel comments]</i></p>

<p>
See also <a href="lwg-defects.html#3782" title="Should &lt;math.h&gt; declare ::lerp? (Status: C++23)">3782</a><sup><a href="https://cplusplus.github.io/LWG/issue3782" title="Latest snapshot">(i)</a></sup>, which points out that <code>nullptr_t</code> has become a part of the C standard library 
as of C23 (see <a href="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3042.htm">WG14-N3042</a> and 
<a href="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3048.htm">WG14-N3048</a>).
</p>


<p id="res-3484"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4910" title=" Working Draft, Standard for Programming Language C++">N4910</a>.
</p>

<ol>
<li><p>Modify 17.15.7 <a href="https://wg21.link/support.c.headers.other">[support.c.headers.other]</a> as indicated:</p>

<blockquote>
<p>
-1- Every C header other than <code>&lt;complex.h&gt;</code> (17.15.2 <a href="https://wg21.link/complex.h.syn">[complex.h.syn]</a>), 
<code>&lt;iso646.h&gt;</code> (17.15.3 <a href="https://wg21.link/iso646.h.syn">[iso646.h.syn]</a>), <code>&lt;stdalign.h&gt;</code> 
(17.15.4 <a href="https://wg21.link/stdalign.h.syn">[stdalign.h.syn]</a>), <code>&lt;stdatomic.h&gt;</code> (32.5.12 <a href="https://wg21.link/stdatomic.h.syn">[stdatomic.h.syn]</a>), 
<code>&lt;stdbool.h&gt;</code> (17.15.5 <a href="https://wg21.link/stdbool.h.syn">[stdbool.h.syn]</a>), and <code>&lt;tgmath.h&gt;</code> (17.15.6 <a href="https://wg21.link/tgmath.h.syn">[tgmath.h.syn]</a>), 
each of which has a name of the form 
<code>&lt;<i>name</i>.h&gt;</code>, behaves as if each name placed in the standard library namespace by the 
corresponding <code>&lt;c<i>name</i>&gt;</code> header is placed within the global namespace scope, except 
for the functions described in 29.7.6 <a href="https://wg21.link/sf.cmath">[sf.cmath]</a>, the declaration of <code>std::byte</code> <ins>and
<code>std::nullptr_t</code></ins> (17.2.1 <a href="https://wg21.link/cstddef.syn">[cstddef.syn]</a>), and the functions and function templates 
described in 17.2.5 <a href="https://wg21.link/support.types.byteops">[support.types.byteops]</a>. It is unspecified whether these names are first
declared or defined within namespace scope (6.4.6 <a href="https://wg21.link/basic.scope.namespace">[basic.scope.namespace]</a>) of the namespace 
<code>std</code> and are then injected into the global namespace scope by explicit <i>using-declaration</i>s 
(9.10 <a href="https://wg21.link/namespace.udecl">[namespace.udecl]</a>).
</p>
</blockquote>
</li>

<li><p>In [diff.cpp??] [Editorial note: new compatibility section to be created for C++20], add a new entry:</p>

<blockquote>
<p>
<ins><b>Affected subclause:</b> 17.15.7 <a href="https://wg21.link/support.c.headers.other">[support.c.headers.other]</a><br/>
<b>Change:</b> Removal of name <code>::nullptr_t</code> from header <code>&lt;stddef.h&gt;</code>.<br/>
<b>Rationale:</b> <code>std::nullptr_t</code> is a C++-only feature that is not useful for interoperability with ISO C.<br/>
<b>Effect on original feature:</b> Valid C++20 code may fail to compile in this International Standard. Occurrences 
of <code>#include &lt;stddef.h&gt;</code> and <code>nullptr_t</code> should be replaced with 
<code>#include &lt;cstddef&gt;</code> and <code>std::nullptr_t</code>.</ins>
</p>
</blockquote>
</li>
</ol>





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