Re: [boost] [config] Request for BOOST_SYMBOL_IMPORT_VISIBLE ?

Subject: Re: [boost] [config] Request for BOOST_SYMBOL_IMPORT_VISIBLE ?
From: John Maddock (boost.regex_at_[hidden])
Date: 2011-02-08 03:45:28

>> Is this really just limited to exception classes?  I got the impression
>> from the GCC docs that the issue might extend to:
>>
>> * Classes with vtables that are used in dynamic_cast's or with typeid.
>> * Classes with static data members.
>
> From my reading of it, the "re-exporting" is limited to exception
> classes;
> albeit, the article could be clearer.  This is because (again, according
> to the article) the catch handler looks for the exception class typeinfo
> in the dso that threw it, and not in the dso that originally defined the
> exception, hence the need to "re-export" the exception from all dsos that
> potentially throw it.
>
> With respect to the category of classes that you mention the
> "re-exporting" issue might extend to, I think the article is just advising
> to just export these classes, or their static data members in the first
> place, not "re-export" them from every dso that they appear in.

Hmmm, it's a bit vague in it's wording isn't it?

> BUT ...., some simple tests I performed seem to only partially validate my
> claim (see attached for the tests).
>
> All tests run on: OSX 10.5, gcc version 4.0.1
>
> With respect to the dynamic_cast/typeid category, my tests revealed no
> need for "re-exporting", even when I compiled with "-O3" flag.
>
> With respect to template classes with static data members, my tests did
> reveal a need for "re-exporting".
>
> Admittedly, it would be nice if we can get some clarification from the gcc
> team regarding this.
>
>> I still think that re-exporting "just enough and no more" is error
>> prone, and we should just follow the gcc guidelines and re-export
>> everything that's imported - I rather got the impression that gcc will
>> only actually generate re-export data for vtables anyway?
>
> True, it may be error prone.  Sorry, I can't seem to find the gcc
> guideline you're referring to, can you point it out?

The usual http://gcc.gnu.org/wiki/Visibility article, under "Step by Step 
Guide", they recommend:

#define FOX_HELPER_DLL_IMPORT __attribute__ ((visibility("default")))

HTH, John.