Re: Confused about a thread-safe singleton example.

James Kanze <>
Wed, 3 Dec 2008 14:40:38 -0800 (PST)
On Dec 3, 6:23 pm, Noah Roberts <> wrote: wrote:

On Dec 3, 5:56 am, Alan Johnson <> wrote:


Yet more interesting is a little pattern I found that I'm sure
others have as well, the use of pimpl to implement singleton.
 The object uses the handle/body (pimpl) idiom but the impl is
actually a singleton. Clients who create instances of the
class itself have no knowledge that they're all working with
the same 'object'.

That's interesting. That's the pattern I used before I read the
GoF. I adopted the GoF pattern because it seems worthwhile to
follow a commonly accepted model, but I'll have to admit that
having to constantly write those calls to instance() is
gradually getting on my nerves. On the other hand, the earlier
pattern (which I called the Module pattern, because the class in
fact acted like a module) does sort of create the impression
that you have several independent instances; sometimes, this is
even what you want (where the sharing is an implementation
detail, for example), but most of the time, it seems a little
like confuscation.

Lately, I've been toying with the idea of using the singleton
idiom for the (private) data, but making all of the functions
static. It's just an idea, though; I've not actually tried it
out to see how it works in practice.

 This is something you could do, in theory, with global
variables but ICK!

The problem with singleton is that people abuse it, as can be
seen by the fact that very few participating in this
discussion seem to understand its intended purpose: enforcing
a creational policy.

If all you've got is a hammer, everything looks like a nail.
The singleton is probably the easiest pattern to understand, so
some people just stop there, and use it for everything. It's
appropriate when it's appropriate, but only when it's

James Kanze (GABI Software)
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