Use of Persistent Variables in Class Methods Producing Incorrect Result.

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I am attempting to make use of a persistent variable within a class method. However, invoking the method from different instances is producing incorrect results. Here is a simple case of what I mean:
CLASS:
classdef MYCLASS < handle
properties(GetAccess = 'private', SetAccess = 'private')
m_Count
end
methods(Access = 'public')
%constructor
function obj = MYCLASS
obj.m_Count = 0;
end
function out = DO_SOMETHING_MOUSE(obj)
persistent n
if isempty(n)
n = 0;
end
n = n+1;
out = n;
end
function out = DO_SOMETHING_CAT(obj)
obj.m_Count = obj.m_Count + 1;
out = obj.m_Count;
end
end
end
MATLAB FILE:
clear all
clear classes
clc
Instance_A = MYCLASS;
Instance_B = MYCLASS;
n = Instance_A.DO_SOMETHING_MOUSE
n = Instance_B.DO_SOMETHING_MOUSE
n = Instance_A.DO_SOMETHING_CAT
n = Instance_B.DO_SOMETHING_CAT
COMMAND WINDOW RESULTS:
a =
1
b =
2
a =
1
b =
1
>>
Here you can see that we call the DO_SOMETHING_MOUSE method from two different instances of the class, yet, the value of the persistent variable "n" was somehow shared between instances.
The result we should have gotten is that which was illustrated by the call to the DO_SOMETHING_CAT method. Each instance had it's own value correctly stored.
What's wrong?

Accepted Answer

Srijith Vijay
Srijith Vijay on 4 Sep 2017
This is the expected behavior. Persistent variables are the same as global variables expect for the fact that these variables can be accessed only within the function in which they are created. These variables retain their value till they are cleared from the memory.
When you created two different instances of the class, two independent instances were created with their own properties, whereas only one instance of the persistent variable was created in the memory. Therefore both objects access the same memory for the persistent variable
  4 Comments
Thomas Michiels
Thomas Michiels on 1 Jul 2019
i know this is an old thread, but i do feel like i need to comment on this for future readers.
The Singleton pattern is NOT a good usecase for persistent data. It is better to use a constant property with an instance of a handleclass assigned to it for this usecase.
Persistent data is used where you would use simple static data inside your class, think of it like a protected static variable in java, whereas the constant property is more a public static final in java.

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More Answers (1)

per isakson
per isakson on 23 May 2015
Edited: per isakson on 23 May 2015
I reproduced the behavior with R2013a and it's not what I would have expected. However, whether it's "incorrect results" depends on what the documentation says.
It resembles a case, which is discussed at UndocumentedMatlab, Handle object as default class property value. See especially the comment by David Foti at April 10, 2015 at 2:42 pm.

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