[llvm-dev] [cfe-dev] [RFC] __attribute__((internal_linkage))

[Martin J. O'Riordan via llvm-dev]

I haven't been able to figure out from this thread why this attribute is necessary at all.

Anonymous or unnamed namespaces were added to C++ for this very purpose, but the ISO C++ Standard does not discuss "linkage" per-se because it is outside the scope of the language specification.  But it does describes it in terms of having a hidden name which is "unique" to the translation unit, and under the "as if" rule, internal linkage is a common convention for achieving this.

This is just Standardese for dealing with what compiler implementers typically handle as "internal linkage" anyway; at the same time, the use of 'static' for this purpose was deprecated.  This is specifically stated in C++98 section 7.3.1.1 and unnamed namespaces are still similarly defined in C++17 (Working Paper) although the specific reference to the deprecation of 'static' for this purpose is now gone.

The closest the Standard gets to talking about linkage is with Linkage Specifications, and even here it tries to avoid to avoid treading on the definitions things like internal versus external linkage.

So I am curious, what does this proposed attribute on namespaces achieve that cannot already be achieved with an anonymous or unnamed namespace?

Thanks,

	Martin O'Riordan - Movidius Ltd.

-----Original Message-----
From: cfe-dev [mailto:cfe-dev-bounces at lists.llvm.org] On Behalf Of Evgenii Stepanov via cfe-dev
Sent: 23 October 2015 22:41
To: Steven Stewart-Gallus <sstewartgallus00 at mylangara.bc.ca>
Cc: Clang Dev <cfe-dev at lists.llvm.org>
Subject: Re: [cfe-dev] [RFC] __attribute__((internal_linkage))

Sounds right. The proposed attribute is a rough equivalent of

static void foo::do_method(void)
{
  // implementation here
}

where "static" does not mean a static class member, but makes the declaration local to the translation unit. C-style "static".

The patch in http://reviews.llvm.org/D13925 (waiting for review, btw!) supports this attribute on classes and namespaces, with this syntax:

namespace __attribute__((internal_linkage)) { }


On Fri, Oct 23, 2015 at 9:03 AM, Steven Stewart-Gallus via cfe-dev <cfe-dev at lists.llvm.org> wrote:
> Hello,
>
> Can I ask for some clarification?
>
> Apparently, C++ doesn't allow to static class methods?
>
> While in C one might write inside a header file:
>
> static inline void mylib_foo_do_method(struct mylib_foo *foo) {
>    // implementation here
> }
>
> In C++ the typical style would be to write something like:
>
> namespace mylib {
>
> void foo::do_method(void)
> {
>   // implementation here
> }
> }
>
> Unfortunately, the C++ case then implies some linkage behaviour that 
> some might not want.
>
> Apparently, one can't do things like:
>
> namespace mylib {
>
> static void foo::do_method(void)
> {
>   // implementation here
> }
> }
>
> Or,
>
> namespace {
> namespace mylib {
>
> static void foo::do_method(void)
> {
>   // implementation here
> }
> }
> }
>
> Also, if you wanted to an attribute to a whole namespace you should do 
> something like the following I think:
>
> namespace mylib {
> [[clang::internal_linkage]];
>
> static void foo::do_method(void)
> {
>   // implementation here
> }
> }
>
>
> Thank you,
> Steven Stewart-Gallus
> _______________________________________________
> cfe-dev mailing list
> cfe-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
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