Patterns: Local Singleton vs. Global Singleton?
- by Mike Rosenblum
There is a pattern that I use from time to time, but I'm not quite sure what it is called. I was hoping that the SO community could help me out.
The pattern is pretty simple, and consists of two parts:
A singleton factory, which creates objects based on the arguments passed to the factory method.
Objects created by the factory.
So far this is just a standard "singleton" pattern or "factory pattern".
The issue that I'm asking about, however, is that the singleton factory in this case maintains a set of references to every object that it ever creates, held within a dictionary. These references can sometimes be strong references and sometimes weak references, but it can always reference any object that it has ever created.
When receiving a request for a "new" object, the factory first searches the dictionary to see if an object with the required arguments already exits. If it does, it returns that object, if not, it returns a new object and also stores a reference to the new object within the dictionary.
This pattern prevents having duplicative objects representing the same underlying "thing". This is useful where the created objects are relatively expensive. It can also be useful where these objects perform event handling or messaging - having one object per item being represented can prevent multiple messages/events for a single underlying source.
There are probably other reasons to use this pattern, but this is where I've found this useful.
My question is: what to call this?
In a sense, each object is a singleton, at least with respect to the data it contains. Each is unique. But there are multiple instances of this class, however, so it's not at all a true singleton.
In my own personal terminology, I tend to call the factory method a "global singleton". I then call the created objects "local singletons". I sometimes also say that the created objects have "reference equality", meaning that if two variables reference the same data (the same underlying item) then the reference they each hold must be to the same exact object, hence "reference equality".
But these are my own invented terms, and I am not sure that they are good ones.
Is there standard terminology for this concept? And if not, could some naming suggestions be made?
Thanks in advance...
