c++ array tips - internal linkage and arrays

[ben2ong2]

(Newsgroups: comp.lang.c++.moderated, 7 May 99)


MEYERS: Scott Meyers

>>> In fact, the only way I can think of to give a char array
>>> internal linkage is to put it in an unnamed namespace or
>>> make it file static.

HENRIKSSON: Dag Henriksson <dag.henriksson@quidsoft.se>

>> Strictly speaking, to put it in an unnamed namespace doesn't
>> give the char array internal linkage. But it can not be seen
>> from any other translation unit, so the effect is the same.

CLAMAGE: Steve Clamage <clamage@eng.sun.com>

>It's not quite the same:
>
>static const char* s1 = "hello";
>namespace {
>  const char* s2 = "hello";
>}
>template<const char* S> class C { ... };
>
>C<s1> c1; // error: s1 must have external linkage
>C<s2> c2; // ok
>

---------------------------------------------------------

NARAN: Siemel Naran

>>>> extern const char s[]="hello"; // the 'extern' is necessary

MEYERS: Scott Meyers

>>> I'd expect it to have external linkage here, too:
>>> Is there a special linkage rule for char arrays?

BARFURTH: Joerg Barfurth <jbarfurth@vossnet.de>

>> In a way. The way arrays are special is about cv-qualification.
>> See the note at the end of [8.3.4]/p.1.

CLAMAGE: Steve Clamage <clamage@eng.sun.com>

>The C++ committee is currently discussing that note (prompted
>by the postings in this thread).
>
>Here's the last part of that paragraph, including an example,
>and the note in question:
>------------------------------------------------------
>Any type of the form "cv-qualifier-seq array of N T" is adjusted to
>"array of N cv-qualifier-seq T," and similarly for "array of unknown
>bound of T."[Example:
>    typedef int A[5], AA[2][3];
>    typedef const A CA; // type is "array of 5 const int"
>    typedef const AA CAA; // type is "array of 2 array of 3 const int"
>-end example]
>[Note: an "array of N cv-qualifier-seq T" has cv-qualified type; such
>an array has internal linkage unless explicitly declared extern
>(7.1.5.1) and must be initialized as specified in 8.5. ]
>------------------------------------------------------
>
>The note appears to contradict the sentence and example that
>immediately precede it, although some have claimed there is
>no contradiction.
>
>The note does contradict section 3.9.3 paragraph 2 which says
>explicity that cv-qualifiers do not apply to the array type,
>but only to the element type. Since a note is non-normative,
>this paragraph must take precedence.
>
>Finally, if the note is correct, the declaration
>    volatile char s[]="hello";
>would have internal linkage. That is clearly wrong, so at a
>minimum the note must be modified to say "const-qualified"
>instead of cv-qualified.
>
>But I think the whole note is wrong and should be deleted.
>If you want an array to have internal linkage, you can declare
>it static. I don't see any reason to make a special rule.
>
>If a special rule is needed, putting it in a note doesn't work.
>A note or example can only be used clarify or expand on the
>normative (official, non-note, non-example) text. No text other
>than the note currently says that
>    const char s[]="hello";
>has internal linkage.
>
>For the reasons I have listed, I think that const arrays have
>external linkage unless declared static.