32

I am trying to create a MATLAB class with a member variable that's being updated as a result of a method invocation, but when I try to change the property within the class it (apperently, from what I understood from MATLAB's memory management) creates a copy of the object and then modifies it, leaving the original object's property untouched.

classdef testprop  
    properties  
        numRequests=0;  
    end  
    methods  
        function Request(this, val)  
            disp(val);  
            this.numRequests=this.numRequests+1;  
        end  
    end  
end

.

>> a=testprop;
>> a.Request(9);
>> a.Request(5);  
>> a.numRequests  

ans = 0
Daniel
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3 Answers3

29

Using a Value (Vanilla) Class

When using a value class you need to tell Matlab to store a modified copy of the object to save the changes in the property value. So,

>> a=testprop
>> a.Request(5); % will NOT change the value of a.numRequests.
5

>> a.Request(5) 
5

>> a.numRequests
ans = 
       0

>> a=a.Request; % However, this will work but as you it makes a copy of variable, a.
5

>> a=a.Request; 
5

>> a.numRequests
ans =
       2

As Kamran notes, this requires changing the definition of function Request to be

function this = Request(this, val)`

Using a Handle Class

If you inherit from the handle class, that is

classdef testprop < handle

then you can write,

>> a.Request(5);
>> a.Request(5);
>> a.numRequests
ans = 
       2

Note that this changes the behavior of the objects, see the documentation to learn the difference between a value class and a handle class.

Cris Luengo
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Azim J
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  • Yes - this demonstrated the issue, although line 8 was meant to be this.numRequests=this.numRequests+1; –  Oct 16 '08 at 16:23
7

You have to remember that syntactically in Matlab, you're probably closer to C, than C++ or Java, at least with respect to objects. So, of you want to change the "contents" of a value object (really just a special struct), you need to return the object from the function.

Azim was correct to point out that if you want Singleton behavior (which, from your code, you seem to), you need to use a "handle" class. Instances of classes that derive from Handle all point to a single instance, and operate only on it.

You can read more about the differences between Value and Handle classes.

Marc
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    Deriving from `handle` does not, by itself, make the Singleton pattern. – Dave Jul 25 '13 at 15:46
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    The link no longer points to what I think was intended. [Here is an updated link.](https://www.mathworks.com/help/matlab/matlab_oop/comparing-handle-and-value-classes.html) – benJephunneh Mar 08 '19 at 04:55
5

I made the class testprop and tried to excute the code which Azim suggested but it did not work. When I executed the following command:

a=a.Request(1)

The following error was generated:

??? Error using ==> Request Too many output arguments.

I think the problem is that we did not determine any output when declaring Request method. So we should change it to:

function this = Request(this, val)

and now: 

>> a = testprop;
>> a = a.Request(1);        
>> a.numRequests

ans = 1
GEOCHET
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Kamran Bigdely
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  • If "testprop" is to be written as a standard/vanilla class, you would need to return the object from "Request". If "testprop" is derived from the handle class, you shouldn't need to return anything. You would simply use the call "a.Request(1);". – gnovice Mar 23 '09 at 13:45
  • yes you're right, but as in the question, Azim has supposed the difiniton of the testprop class is not derived from handle class. so he shoud have modified the method "Request". – Kamran Bigdely Mar 23 '09 at 15:04
  • @Kamran, just checked and you are correct. I must have included an output argument for Request when I was testing the Vanilla case. Thanks for the clarification. – Azim J Mar 23 '09 at 15:35