r175587 - Add a new 'type_visibility' attribute to allow users to

[Rafael Espíndola]

> Users who want to minimize the set of symbols they export from their
> dynamic libraries usually compile with -fvisibility=hidden.  They then
> find themselves going back and marking certain classes with default
> visibility.  They then find themselves with extra symbols again because
> the inline functions from those class are now default visibility.
> It's all really a huge pain.
>
> The point of type_visibility is that you can give your RTTI symbols
> default visibility, and therefore make dynamic_cast and exceptions
> work across dynamic library boundaries, without affecting any other
> visibility calculations.
>
> libc++ in particular can actually go and mark all of its exported classes
> with type_visibility("default") and visibility("hidden"), and suddenly,
> regardless of compilation mode, RTTI symbols will be properly
> exported and nothing else will.
>
> The only way of achieving that right now is to put DEFAULT on the
> class and then individually mark every single method declaration
> with HIDDEN.

I see. For this use case, can't you give just the typeinfo name
default visibility? That is the only thing whose address (and not just
the contents) is important when implementing dynamic_cast and
exceptions, no?

> You're right that I should make user docs, though.

Thanks!

> John.

Cheers,
Rafael