Symbol visibility and namespace

[DÃniel Varga <vargad88 at gmail dot com>]

Hi,

I'm experimenting with C++ symbol visibility on Linux and gcc. It
seems that the preferred way is to use -fvisibility=hidden, and export
used symbols one-by-one according to Visibility gcc wiki page
(http://gcc.gnu.org/wiki/Visibility).
My problem is that *many* libraries does not handle this well, they
forget to explicitly export symbols, which is a serious problem. After
several fixed bugs even some parts of boost may still be affected. Of
course those bugs should be fixed, but till that I would like to use a
"safe" way to hide as much as symbols as possible.

I came up with a solution: I place all the symbols in a namespace and
I use symbol hide attribute on that and export the public interface,
this way only my symbols can be affected.

The problem is that I got a warning message when I compile something
against that library for every class that I haven't exported and I use
in the application as class field.

    namespace MyDSO __attribute__ ((visibility ("hidden"))) {
      struct Foo {
        void bar() __attribute__ ((visibility ("default"))) {}
      };
    }

    struct Bar {
      MyDSO::Foo foo;
    };

    int main() {}

The warning message can be reproduced in this small example, but of
course the namespace should be in a library the other class in the
application.

    $ gcc-4.7.1 namespace.cpp -o namespace
    namespace.cpp:7:8: warning: âBarâ declared with greater visibility
than the type of its field âBar::fooâ [-Wattributes]

As I understand symbol visibility, hiding namespace should have quite
similar effect to using -fvisibility=hidden, but I never got similar
warnings using the latter. I see that when I pass -fvisibility=hidden
to the application the class in the application will also be hidden,
so I won't get a warning. But when I don't pass the option none of the
symbols in the headers will seem hidden to the compiler, so I won't
get a warning again.

What is the propose of this warning message? Is it a serious problem?
In which situations can this cause any problem? How hiding namespace
is different to fvisibility=hidden?

Thanks,
Dani