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

Subject: Re: [boost] [config] GCC symbol visibility across shared libraries
From: John Maddock (boost.regex_at_[hidden])
Date: 2010-05-19 11:56:58

> I've got a working patch set lying around. Please find it attached.
> Real life got in the way (as usual)...

>Your patch has been VERY helpful, and I've used it as the starting
>point! Thanks!

FYI, there is an open ticket on this: 
https://svn.boost.org/trac/boost/ticket/2114 which still requires some work 
(mostly tests and updated docs) before rolling out.

> 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. 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.

Yeh, that's a tricky one, 'cos VC++ will warn almost whatever you do.  I 
*think* the correct thing to do for header only classes is to mark them as 
__declspec(dllimport) - that way if you contain or inherit from them in an 
exported class you won't get more warnings - the main culprit here is 
noncopyable which is unusable without copious warnings from VC++.  Oh and I 
think I've had warnings when containing a shared_ptr in an exported class 
too... in fact come to think of it if I write an exported class like this:

class BOOST_SYMBOL_EXPORT foo
{
private:
    some_boost_type member;
};

Won't I have issues unless "some_boost_type" is declared visible/exported? 
Bare in mind that this could be practically any template declared in Boost 
:-(

Just thinking out loud yours, John.