Controlling the Number of Instances
You can limit the number of instances of a class that can exist at any one time. For example, a singleton class can have only one instance and provides a way to access this instance. You can create a singleton class using these elements:
- A persistent variable to contain the instance
- A sealed class (Sealed attribute set to true) to prevent subclassing
- A private constructor (Access attribute set to private)
- A static method to return the handle to the instance, if it exists, or to create the instance when needed.
Implementing a Singleton Class
The following skeletal class definition shows how you can approach the implementation of a class that allows you to create only one instance at a time:
classdef (Sealed) SingleInstance < handle methods (Access = private) function obj = SingleInstance end end methods (Static) function singleObj = getInstance persistent localObj if isempty(localObj) || ~isvalid(localObj) localObj = SingleInstance; end singleObj = localObj; end end end
The getInstance static method returns a handle to the object created, which the class stores in a persistent variable. getInstance creates an instance only the first time called in a session or when the object becomes invalid. For example:
sobj = SingleInstance.getInstance sobj = SingleInstance handle with no properties. Methods, Events, Superclasses
As long as sobj exists as a valid handle, calling getInstance returns a handle to the same object. If you delete sobj, then calling getInstance creates an object and returns the handle.
delete(sobj) isvalid(sobj) ans = 0 sobj = SingleInstance.getInstance; isvalid(sobj) ans = 1