Re: Anonymous namespace and internal linkage

James Kanze <>
Fri, 6 Feb 2009 07:40:37 CST
On Feb 6, 9:33 am, Anand Hariharan <>

This code fails during linking:

namespace {
void FuncA()
/*extern*/ void FuncB(void);

void FuncB() {}

int main()

Is it to be expected?

Isn't 'extern' qualifier meaningless here?

It should be.

It turns out that the linker does not have a problem when the
'extern' is uncommented.

That sounds like an error.

Technically, of course, you have undefined behavior (both ways).
But the FuncB called in FuncA is a function in the anonymous
namespace; the FuncB you define isn't, so it isn't the same

(FWIW: both Sun CC and g++ give a link error in both cases, with
or without the extern. So does the version of VC++ that I have
here: the one from Visual Studios 8.)

FWIW, here is the diagnostic, copy-pasted:

--- BEGIN ---
C:\temp>cl foo.cpp
Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 15.00.30729.01
for 80x86
Copyright (C) Microsoft Corporation. All rights reserved.

Microsoft (R) Incremental Linker Version 9.00.30729.01
Copyright (C) Microsoft Corporation. All rights reserved.

foo.obj : error LNK2019: unresolved external symbol "void __cdecl FuncB
(void)" (
?FuncB@@YAXXZ) referenced in function "void __cdecl `anonymous
(void)" (?FuncA@?A0xd21949b8@@YAXXZ)
foo.exe : fatal error LNK1120: 1 unresolved externals
--- END ---

That's interesting. When I try this with VC++, I get:

     952@~/tmp/news (21): cl -Dextern= $VCPPFLAGS
     Microsoft (R) 32-bit C/C++ Optimizing Compiler Version
14.00.50727.762 for 80x86
     anon.obj : error LNK2019: unresolved external symbol "void __cdecl
`anonymous na
mespace'::funcB(void)" (?funcB@?A0xb676e5d1@@YAXXZ) referenced in
function "void
  __cdecl `anonymous namespace'::funcA(void)" (?funcA@?

  (I've just cited the relevant lines: the version and the error
  message.) And this whether or not the "extern" is present.
  Note the difference in the error message, however---it clearly
  says that the missing function is in the anonymous namespace.
  The error messages of g++ and Sun CC are equally clear in this
  respect (for g++, at least under Linux---under Solaris, the Sun
  linker is used, and it doesn't know how to demangle the g++
  names, so the anonymous bit must be deduced by recognizing the
  mangling pattern).

  Since the version of the compiler you're using seems more
  recent than mine, I'd say that there is a regression error if
  it accepts the code, either with or without the "extern"; you
  should probably report it.

James Kanze (GABI Software)
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