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    CWG Issue 2772</TITLE>
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<P><EM>This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21
  Core Issues List revision 118b.
  See http://www.open-std.org/jtc1/sc22/wg21/ for the official
  list.</EM></P>
<P>2025-09-28</P>
<HR>
<A NAME="2772"></A><H4>2772.
  
Missing Annex C entry for linkage effects of <I>linkage-specification</I>
</H4>
<B>Section: </B>C.6.4&#160; [<A href="https://wg21.link/diff.cpp03.dcl.dcl">diff.cpp03.dcl.dcl</A>]
 &#160;&#160;&#160;

 <B>Status: </B>CD7
 &#160;&#160;&#160;

 <B>Submitter: </B>Hubert Tong
 &#160;&#160;&#160;

 <B>Date: </B>2023-07-15<BR>


<P>[Accepted as a DR at the November, 2023 meeting.]</P>



<P>With C++11, anonymous namespaces changed from external linkage
(with a unique namespace name) to internal linkage.  That implies
that <TT>extern "C"</TT>, which affects names with external linkage
only, no longer has an effect inside anonymous namespaces.</P>

<P>However, a corresponding Annex C entry is missing.</P>

<P><B>Proposed resolution (approved by CWG 2023-09-15):</B></P>

<P>Add a new paragraph in C.6.4 [<A href="https://wg21.link/diff.cpp03.dcl.dcl">diff.cpp03.dcl.dcl</A>] as follows:</P>

<BLOCKQUOTE class="ins">

Affected subclause: 9.12 [<A href="https://wg21.link/dcl.link">dcl.link</A>]<BR>
Change: Names declared in an anonymous namespace changed from external
linkage to internal linkage; language linkage applies to names with
external linkage only.<BR>
Rationale: Alignment with user expectations.<BR>
Effect on original feature: Valid C++ 2003 code may violate the one-definition rule (6.3 [<A href="https://wg21.link/basic.def.odr">basic.def.odr</A>]) in this revision of C++. For example:
<PRE>
  namespace { extern "C" { extern int x; } }  // <SPAN CLASS="cmnt">#1, previously external linkage and C language linkage, now internal linkage and C++ language linkage</SPAN>
  namespace A { extern "C" int x = 42; }      // <SPAN CLASS="cmnt">#2, external linkage and C language linkage</SPAN>
  int main(void) { return x; }
</PRE>
This code is valid in C++ 2003, but #2 is not a definition for #1 in
this revision of C++, violating the one-definition rule.
</BLOCKQUOTE>

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