Gregory Dai ha escrito:
On 5/9/07, Joaquín Mª López Muñoz <joaquin@tid.es> wrote:
Hello Greg, I think there's a potential problem with your static_instantiation policy [...] You can solve this by combining a Meyers singleton with some static initialization forcing technique: [...] Isn't this kind of equivalent to the local_static_instantiation (the second policy)? In general, we use local static storage to avoid static class member construction, don't we?
These policies work differently: local_static_instantiation creates the singleton the first time make() is called, be it in initialization time or after main() has begun; while static_instantiation guarantees that the creation always happens in initialization time.
In other words, shall we say that the static_instantiation (instantiation policy) is there for illustration purpose only, not for applications in production.
I'd say there is an important use case for static_instantiation which local_static_instantiation does not cover: if no threads are launched before main() begins, which is an extremely common situation, then the automatic creation of the singleton in initialization time is naturally thread safe, and so singleton<...>::instance need not be mutex-protected even if in a multithreaded app.. Joaquín M López Muñoz Telefónica, Investigación y Desarrollo