[Bug c++/55655] New: cannot export specialized template

[Bug c++/55655] New: cannot export specialized template

[mw_triad at users dot sourceforge.net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55655

             Bug #: 55655
           Summary: cannot export specialized template
    Classification: Unclassified
           Product: gcc
           Version: 4.7.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: mw_triad@users.sourceforge.net


Let us say I have a generic template class, e.g. my_smart_ptr. Let us also say
I have a class 'foo', 'clas foo : public my_smart_ptr<bar>' that is part of
some library public API. And last, let us say that 'foo' needs to specialize
'my_smart_ptr<bar>'.

As far as I can tell, it is impossible to export this instantiation with proper
symbol visibility in this configuration. I do NOT want to make all possible
instantiations of my_smart_ptr have default visibility (someone, somewhere may
have an instantiation that should have hidden visibility).

I need to do two things, but they are mutually contradictory:
1. I need to explicitly instantiate the template in order to give it default
visibility. However, if I do this first, it prevents specialization. (I also
get compiler errors for my specific use case, as generic instantiation is not
possible.)
2. I need to declare specialization prior to instantiation. However, if I do
this before instantiation, I cannot set default visibility on the instantiation
and I get link errors.

I think there should be a way to do this... Right now I am forced to not export
the template, requiring each user to use their own instantiation, which creates
a fragile ABI.

[Bug c++/55655] cannot export specialized template

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55655

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |WAITING
   Last reconfirmed|                            |2012-12-11
     Ever Confirmed|0                           |1

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-12-11 20:48:56 UTC ---
(In reply to comment #0)
> Let us say I have a generic template class, e.g. my_smart_ptr. Let us also say
> I have a class 'foo', 'clas foo : public my_smart_ptr<bar>' that is part of
> some library public API. And last, let us say that 'foo' needs to specialize
> 'my_smart_ptr<bar>'.

Please provide some code instead of saying it.  It's much easier to talk about
concrete examples, with the compiler commands used.

[Bug c++/55655] cannot export specialized template

[mw_triad at users dot sourceforge.net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55655

--- Comment #2 from Matthew Woehlke <mw_triad at users dot sourceforge.net> 2012-12-11 21:06:26 UTC ---
Created attachment 28930
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=28930
example code

Here's a simplified concrete example. Since we are talking about correct
visibility, -fvisibility=hidden or #pragma equivalent is implied; otherwise,
compile flags do not (AFAIK) matter.

[Bug c++/55655] cannot export specialized template

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55655

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-12-11 21:49:13 UTC ---
Which symbols are hidden which you don't want to be?

What do you mean "instantiation failed"? You get a warning but it works.

[Bug c++/55655] cannot export specialized template

[mw_triad at users dot sourceforge.net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55655

--- Comment #4 from Matthew Woehlke <mw_triad at users dot sourceforge.net> 2012-12-11 23:34:21 UTC ---
(In reply to comment #3)
> Which symbols are hidden which you don't want to be?

ptr<object>

> What do you mean "instantiation failed"? You get a warning but it works.

It fails if you haven't declared the specialization, which is necessary to get
the correct visibility of ptr<object>. With the specialization declare,
instantiation "succeeds" but with the wrong visibility.