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From: Deniz Bahadir (deniz.bahadir_at_[hidden])
Date: 2021-02-07 15:36:05

Hi there,

I recently made some interesting observation regarding Boost.Flyweight
and (hidden) symbol visibility and have now some questions regarding this.

Background story:

In our company's code base we have been using Boost.Flyweight for
several years now, compiling it with GCC (and Clang) and it worked just
fine. (Thanks for it!)

However, lately we are trying to apply hidden symbol visibility to our
code base and some of our unit-tests started failing mysteriously. After
some intensive investigation I found out that it seems to be due to how
we used Boost.Flyweight.

We are instantiating it with some custom types, using the default
`boost::flyweights::static_holder` class, and are compiling it into a
shared library A. However, these Boost.Flyweight types are not only used
within that shared library A but also in some other shared library B and
in some unit-test executables which link against shared library A. (And
these unit-tests started failing.)


Reading the documentation of Boost.Flyweight and looking at the header
`boost/flyweight/static_holder.hpp` I realized that we should have used
`boost::flyweights::intermodule_holder` in the first place instead of
`boost::flyweights::static_holder`, because we were using
Boost.Flyweight from different shared libraries.

Still, with non-hidden visibility applied everything always worked
correctly even when using `boost::flyweights::static_holder`.

Looking at the generated symbols in the different shared libraries I
realized that these `static_holder` class types were marked as "unique
global symbol" (an GNU extension to the standard set of ELF symbol
bindings). So, all shared libraries and executables still used the same
single instance of that `static_holder` class (which is the reason why
everything worked).

With hidden symbol visibility applied by default, these symbols are now
marked as local symbols (from the BSS section). So, each shared library
as well as the executables have their own version of the `static_holder`
class types (which would explain why the equality-comparisions between
different Boost.Flyweight variables in our unit-tests now started failing).

If I understand it correctly, using
`boost::flyweights::intermodule_holder` is the right thing to do, in
particular with hidden symbol visibility.


However, apart from longer program startup times, what other
disadvantages might it have to use the `intermodule_holder` instead of
the `static_holder` (assuming it still would be marked as "unique global

And if, as it seems, `static_holder` has some advantages in general,
would it be possible to (maybe conditionally) mark the `static_holder`
(and/or `static_holder_class`) class with `BOOST_SYMBOL_VISIBLE` or
something similar to allow using it even with hidden symbol visibility
from different shared libraries?
- Only applying default visibility to the template instantiations is
ignored by GCC (with warning: "type attributes ignored after type is
already defined [-Wattributes]").
- Besides, the C++ standard does not allow applying C++ attributes to
template instantiations. (There was a proposal for allowing it,, but I do not know what happened to it.)

Would you have any other recommendations or suggestions I did not think

Thanks for any help or opinion,

Dipl.-Inform. Deniz Bahadir
Reuchlinstr. 10 D
10553 Berlin
Phone: +49 - 30 / 577 0004-22
Email: deniz.bahadir_at_[hidden]
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