On Thu, May 20, 2010 at 7:23 AM, Stewart, Robert <Robert.Stewart@sig.com> wrote:
Beman Dawes wrote:
On Wed, May 19, 2010 at 9:55 AM, Stewart, Robert <Robert.Stewart@sig.com> wrote:
Beman Dawes wrote:
On Thu, May 6, 2010 at 12:42 PM, Jürgen Hunold <juergen.hunold@ivembh.de> 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!
Yeah, this is really bad. My guess is that this is an example of the case a couple of folks mentioned could require decorating members with declspec. Much better to not decorate at all for VC++ IMO.
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?
Yes, but it isn't just exception types.
Is there any other case in which a symbol must be marked visible explicitly where BOOST_SYMBOL_EXPORT is not appropriate?
IIUC, what is actually at stake is whether RTTI info is visible. So it apparently also impacts things like static members of a class template and certain dynamic_casts. See http://gcc.gnu.org/wiki/Visibility, particularly the section "Problems with C++ exceptions (please read!)". But be aware that I'm just feeling my way through this - I'm not primarily a UNIX, Posix, or GCC programmer so don't have even superficial knowledge of how shared libraries work in that environment.
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.
Apparently the problem is wider that just exception types:-) I'm not wedded to BOOST_SYMBOL_VISIBLE. It seemed to work with BOOST_SYMBOL_EXPORT/IMPORT, but I'm not wedded to those either. But they seem appropriate, and I didn't see any reason to deviate from Jürgen's patch. Thanks for the comments, --Beman