Re: Confused about a thread-safe singleton example.
Alan Johnson wrote:
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
Explain why these are problems. Where are the *hidden* dependencies?
If persistent global state is needed then why enforce a policy that
dictates you don't have persistent global state? How are you to remove
coupling between objects that use each other? Is there some extra form
of coupling you think exists here?
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
It's a minor point that can be fixed through the use of policies. See
Modern C++ Design by Alexandrescu.
= the whole cluster of problems around thread-safe lazy initialization
Yes, threads introduce problems.
- someday you will want more than one (you always do)
You don't use singleton because you think you'll only want one. You use
singleton because there can be only one.
- typically unable to derive from the singleton class (without heroic
effort)
Who says?
You would typically be better off just declaring a global
variable and using that instead.
Once again, this does not protect against the creation of another
variable of that type.