Isn't a class with all static members/methods a kind of singleton design pattern? Is there any disadvantage in particular of having such classes? A detailed explanation would help.
+11
A:
Robert C. Martin wrote an article some times ago about the differences between the mono state pattern and the singleton pattern.
Luc Hermitte
2009-04-06 09:21:16
Based on the article, the only material differences that I could see were that in Java, destruction of a Monostate is guaranteed but it cannot be passed between JVMs -- are there any differences relevant to C++?
j_random_hacker
2009-04-06 11:08:39
It's exactly the same as with other variables. If it is dynamically allocated, it should be deallocated with a compatible deallocator. If it is in the static storage (see GOTW/XC++, IIRC), it will be destroyed and the end of the program, but there is no way to implement dependencies.
Luc Hermitte
2009-04-06 12:10:03
Regarding the dependencies aspect, A.Alexandrescu's loki (from MC++D) singletons, and ACE singletons permit to specify dependencies. BTW, they are both non intrusive singletons.
Luc Hermitte
2009-04-06 12:16:24
May you comment the -1 ?
Luc Hermitte
2009-04-07 07:48:03
No -1 from me... In fact I'll give you a +1 for the informative comments :)
j_random_hacker
2009-04-07 12:00:07
Of course. :) I meant to address my question to the one who vote the answer down.
Luc Hermitte
2009-04-07 14:39:52
+9
A:
This kind of class is known as a monostate - it is somewhat different from a singleton.
Why use a monostate rather than a singleton? In their original paper on the pattern, Bell & Crawford suggest three reasonns (paraphrased by me):
- More natural access syntax
- singleton lacks a name
- easier to inherit from
I must admit, I don't find any of these particularly compelling. On the other hand, the monostate is definitely no worse than the singleton.
anon
2009-04-06 09:30:06
Different yes, but doesn't it achieve exactly the same thing? I can't see how this has any advantages or disadvantages over the "traditional" singleton, in C++ at least.
j_random_hacker
2009-04-06 11:06:07
There is the difference that space will not be allocated for a singleton unless it is used, but in practice a singleton object that is never used seems like a rare case. Does anyone have a common use-case where this would be important?
j_random_hacker
2009-04-06 11:10:13
I also tend to think that the lazy-construction feature of typical singletons is useless in most cases -- those where we just need a global variable. However, in library case, we may need an on-the-fly construction in order to simplify the end-user's (developer actually) code.
Luc Hermitte
2009-04-06 12:15:33
A:
For a singleton all constructors have to be private, so that you can access only through a function. But you're pretty close to it.
DaClown
2009-04-06 09:34:15
If you really think that you should read about singletons again. Private constructors and one or more static members only accessilbe through a public member function is the very definition of a singleton
DaClown
2009-04-06 13:37:21
If this answer is so wrong that it deserves 5 downvotes, could you leave a comment why? Something like 'wrong wrong wrong' is not helpful and I'm still not convinced that I'm wrong.
DaClown
2009-04-07 11:16:40
+1
A:
class with all static members/methods a kind of singleton design pattern
Class - not pattern. When we talk about classes we can say class implements pattern.
Static functions - is not member functions, they are similar on global functions. Maybe you don't need any class?
Quote from wikipedia:
In software engineering, the singleton pattern is a design pattern that is used to restrict instantiation of a class to one object.
By this definition your implementation is not singleton implementation - you don't use common idea One (or several in extended definition) instance of class.
But sometimes (not always) usage of class with all static functions and singleton pattern - not have meaningful difference.
bb
2009-04-06 10:04:30
+1
A:
Consider a family of Logging classes. They all implement "LogMessage(message, file, line_number). Some send messages to stderr, some send email to a set of developers, some increment the count of the particular message in a message-frequency table, some route to /dev/null. At runtime, the program checks its argument vector, registry, or environment variables for which Logging technique to use and instantiates the Logging Singleton with an object from a suitable class, possibly loading an end-user-supplied DLL to do so. That functionality is tough to duplicate with a pure static Singleton.
Thomas L Holaday
2009-04-06 12:35:21