This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 116a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-12-19

2772. Missing Annex C entry for linkage effects of linkage-specification

[Accepted as a DR at the November, 2023 meeting.]

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

However, a corresponding Annex C entry is missing.

Proposed resolution (approved by CWG 2023-09-15):

Add a new paragraph in C.6.4 [diff.cpp03.dcl.dcl] as follows:

Affected subclause: 9.11 [dcl.link]
Change: Names declared in an anonymous namespace changed from external linkage to internal linkage; language linkage applies to names with external linkage only.
Rationale: Alignment with user expectations.
Effect on original feature: Valid C++ 2003 code may violate the one-definition rule (6.3 [basic.def.odr]) in this revision of C++. For example:

  namespace { extern "C" { extern int x; } }  // #1, previously external linkage and C language linkage, now internal linkage and C++ language linkage
  namespace A { extern "C" int x = 42; }      // #2, external linkage and C language linkage
  int main(void) { return x; }

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.