Re: [boost] Boost singleton

From: Joaquín Mª López Muñoz (joaquin_at_[hidden])
Date: 2007-05-10 02:05:05

Gregory Dai ha escrito:

> On 5/9/07, Joaquín Mª López Muñoz <joaquin_at_[hidden]> 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