Re: Single Object Instance Techniques

From:

"Phlip" <phlipcpp@yahoo.com>

Newsgroups:

comp.lang.c++

Date:

Fri, 16 Mar 2007 20:01:38 -0700

Message-ID:

<pSIKh.9627$P47.2163@newssvr22.news.prodigy.net>

JohnQ wrote:

Why would anyone write:

class SomeThing // class littered with non-domain single-instancing code
:(
{
private:
SomeThing();
static SomeThing* pInstance_;

I don't have the book Design Patterns to hand. Under Singleton, does it show
the above code, or this correct code below?

SomeThing* GetSomeThing() // single-instancing accessor function
{
static SomeThing* thing = new SomeThing();
return thing;
}

If you must Singleton, do it like that.

However, Singleton is a very abusable pattern when it enables rampant
coupling. The "instantiate once" motivation is not the same as the
"available to everyone like a big fat global" motivation. The latter is an
antipattern!

... 'SingleInstancer' should be a template class.

Such a template is often the result of a junior effort with Singleton. I
don't think it solves the problem how to prevent secondary instantiation of
the templated type.

Furtherless, if you have unit tests, you _need_ secondary instatiation, so
you can fake objects for your tests!

Singletons should be treated with high suspicion.

--
Phlip
http://www.greencheese.us/ZeekLand <-- NOT a blog!!!