Andrey Semashev <andrey.semashev <at> gmail.com> writes:
Hi,
I've created the ticket #3658 [1] on the flyweights library. The ticket is about Boost.Flyweight depending on the static initialization/destruction order, which can lead to application crashes in some cases.
The library does provide certain tools to work around the problem [2], but these tools are only usable in the most obvious cases and do not solve the problem in its root.
Hi Andrey, Singletons (which is what Boost.Flyweight holder finally boil down to, see http://tinyurl.com/y996eqr ) are notoriously hard to get right, as explained for instance in Alexandrescu's "Modern C++ Design", and in the end one has to settle for some compromise. Boost.Flyweight uses holders in such a way that it guarantees initialization before main() begins, which for the most part avoids problems with lazy initialization in multithreaded environments.
What I have proposed in the ticket is to implement reference counting in the flyweights, so that the flyweight value is only destroyed when no flyweight handles refer to it. AFAIK, Boost.Flyweight already has reference counting, so the work is only to shift value owning from the holder to the flyweights.
I'm not sure what's the exact proposal you have. Take into account that refcounts applies to flyweight values, while holders (as defined in Boost.Flyweight) deal with initializing/destroying the flyweight value *factory*, that is, the object that holds the values. This is basically the implementation of static_holder, the default holder for Boost.Flyweight: template<typename C> struct static_holder_class { static C& get() { static C c; return c; } }; Would you mind sketching what replacement for static_holder_class you have in mind? Thank you, Joaquín M López Muñoz Telefónica, Investigación y Desarrollo PS: There's an additional approach that can solve the issue at hand, namely implementing a holder that never destroys the held object, but this is something I'd like to discuss with you after your original refcount proposal is dealt with.