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Subject: Re: [boost] [Boost.utility]
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2010-01-26 09:40:15

Jeff Flinn wrote:
> I've always been mystified as to why a singleton instance is accessed
> via pointer rather than by reference. Singleton models a
> one-and-only-one relationship a pointer models a zero-or-one
> relationship.

I've often thought the same. However, I've always thought the better interface was via a smart pointer. Upon construction, the smart pointer would access the Singleton instance and cache it in a data member. Subsequent access via operator ->() or operator *() would use the data member, thus avoiding the overhead of the instance accessor.

> That said, in my experience singletons are
> generally used as a crutch to access global state.

They do access global state, but there often is global state in an application. If Singletons provide the only means to access state, they formalize that access. Singletons can also, using policies, control lifetime, post-destruction behavior, etc. Consequently, Singletons are more than a crutch, though often overused, abused, or misused.

Rob Stewart robert.stewart_at_[hidden]
Software Engineer, Core Software using std::disclaimer;
Susquehanna International Group, LLP

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