Re: [boost] [config] GCC symbol visibility across shared libraries

Subject: Re: [boost] [config] GCC symbol visibility across shared libraries
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2010-05-20 07:23:46

Beman Dawes wrote:
> On Wed, May 19, 2010 at 9:55 AM, Stewart, Robert
> <Robert.Stewart_at_[hidden]> wrote:
> > Beman Dawes wrote:
> >> On Thu, May 6, 2010 at 12:42 PM, Jürgen Hunold
> >> <juergen.hunold_at_[hidden]> wrote:
> >>
> >> > The patch introduces two Boost.Config macros
> >> > - BOOST_SYMBOL_EXPORT
> >> > - BOOST_SYMBOL_IMPORT
> >>
> >> AFAICT, a third macro is needed for GCC. I've named it
> >> BOOST_SYMBOL_VISIBLE and define it as
> >> __attribute__((visibility("default")))
> >>
> >> The need for this macro arises in header only classes like this:
> >>
> >>     class BOOST_SYMBOL_VISIBLE my_exception
> >>        : public std::runtime_error { ... };
> >>
> >> If you use BOOST_SYMBOL_EXPORT, VC++ will warn because
> >> __declspec(dllexport) isn't valid in this context.
> >
> > I don't understand the problem in that case.
>
> At the least, you this warning:
>
> c:\boost\trunk\boost\system\system_error.hpp(26) : warning C4275: non
> dll-interface class 'std::runtime_error' used as base for
> dll-interface class 'boost::system::system_error'

I hate that warning!

> But for one of the Boost.System tests, VC++ gets so confused that it
> fails to inline the functions defined in the class, and you get a
> bunch of unresolved externals.

Wow!

Let me get this straight, then.  Since the class is header only, it needn't be exported from a DLL, hence using dllexport is wrongheaded for MSVC.  However, GCC's -fvisibility=hidden interferes because of the need to explicitly mark exception types as visible.  Have I got things right so far?

Is there any other case in which a symbol must be marked visible explicitly where BOOST_SYMBOL_EXPORT is not appropriate?  If not, BOOST_SYMBOL_VISIBLE is the wrong name.  It should be BOOST_VISIBLE_EXCEPTION_TYPE or something like that to be clear that it is for use with exception types only.

_____
Rob Stewart                           robert.stewart_at_[hidden]
Software Engineer, Core Software      using std::disclaimer;
Susquehanna International Group, LLP  http://www.sig.com

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