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: 2010-05-19 09:55:31

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.  I mark classes with __declspec(dllexport).  Is it that VC's linker notices that there are no symbols in the DLL for a header-only class and complains?

> If you don't decorate my_exception at all, VC++ is happy, but
> GCC shared libraries compiled with -fvisibility=hidden can't
> successfully throw exceptions up to callers outside the shared
> library. The only way around that I can see is to use a
> separate macro that is only defined for GCC.

Given what you've asserted, I think BOOST_SYMBOL_VISIBLE is reasonable.

_____
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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