Re: Confused about a thread-safe singleton example.
anon wrote:
jason.cipriani@gmail.com wrote:
On Dec 3, 1:47 am, Alan Johnson <aw...@yahoo.com> wrote:
[lots of cutting]
1. Don't use singletons. Ever. Pretty much all of the value of
the GoF Design Patterns book is negated by the fact that they
chose to legitimize Singleton as a design pattern. Singleton is
just a fancy name for global variable. We should call it the
Global Variable (anti)Pattern.
greatly simplifies a lot of application code. And you are correct,
in this case it basically is, in fact, a global variable.
Both of you are not correct. The singleton is not a global variable.
TheClass* instance = NULL; TheClass * GetInstance() { if ( NULL ==
instance ) { instance = new TheClass; } return instance; }
In this case, the instance is created when the function GetInstance()
gets called the first time, while global variables gets created
before entering main()
That still has all the negative effects of a global variable. To name a few
= hidden dependencies in functions that use it
= persistent global state
- tight coupling between anything that uses it
It also brings its own set of new problems.
= multiple responsibilities in one class (responsible for limiting the
number of instances and for whatever the class is normally supposed to do)
= the whole cluster of problems around thread-safe lazy initialization
- someday you will want more than one (you always do)
- typically unable to derive from the singleton class (without heroic
effort)
You would typically be better off just declaring a global
variable and using that instead.
--
Alan Johnson