MATLAB: Custom error messages with Class Object

classcustomerror handlingerror messagesMATLABoop

Hello everyone,

I have a dynamic properties class I am creating to hold some experimental data. This code will be used by other users so I want to make any error messages associated with it more helpful. When you try to address a property not in the 'properties' list you get an error similar to below:

ex.Test = 1;

The error message I get reads:

"Unrecognized property 'Test' for class 'experimentStructureClass'."

I would like to modify the error message to that it directs people to used the ex.addprop() function to add a property.

Does anyone know how to access the error messages that arise from this?

Thanks a lot,

Michael

Best Answer

Have a look at subsref and subsasgn.

https://www.mathworks.com/help/matlab/matlab_oop/indexed-reference-and-assignment.html#br09ho8

https://www.mathworks.com/help/matlab/matlab_oop/code-patterns-for-subsref-and-subsasgn-methods.html

https://www.mathworks.com/matlabcentral/answers/98647-how-do-i-use-the-subsref-function-to-behave-as-the-builtin-only-for-a-subset-of-subscripted-referenc

This way you can access the error message before it is thrown. A crude example:

classdef Example
    
    properties (SetAccess = private)
        Prop1 = rand
    end
    
    methods
        
        function varargout = subsref(obj,s)
            try
                [varargout{1:nargout}] = builtin('subsref',obj,s);
            catch ME
                error([ME.message ' Use setProp().']);
            end
        end
        
        function obj = subsasgn(obj,s,varargin) 
            try
                obj = builtin('subsasgn',obj,s,varargin{:});
            catch ME
                error([ME.message ' Use setProp().']);
            end
        end
        
    end
    
end

You'd probably want to do a little more work determining what error was thrown before deciding to throw your custom error instead.

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MATLAB: Constant property definition, does this example include a constructor

1. Yes, this is the constructor for the class. The method of the class with the same name as the class is the constructor. Nothing says that the class constructor must accept input arguments. If the class author decided they knew when they implemented the class the exact values they wanted the properties to take on, they could certainly implement a 0-input constructor.

In the version of the example you posted, MATLAB Code Analyzer would (correctly) flag that the input arguments you specified in the definition of the constructor method are not used.

2. From that documentation page:

 Because MyProject is a handle class, you can get the handle to the instance
 that is assigned to the constant property:
p = MyProject.ProjectInfo;

This is an example of the singleton pattern. The Java example on that Wikipedia page has a getInstance method that is used to access the object stored in the INSTANCE property, but the MATLAB example on the documentation page allows direct access to the property ProjectInfo in which the object is stored.

3. You can certainly have Constant and non-Constant properties in the same class.

classdef car < handle
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        numberOfWheels = 4;
    end
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      methods
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                  vehicle.numberOfWheels, ...
                  vehicle.color);
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Cars generally have 4 wheels; anything with a different number we would represent with an instance of a different class. Therefore we make that property Constant. But cars can come in many different colors, and it's not too difficult to change the color, so that needs to be a non-Constant property that can take on different values for different instances.

If we wanted we could even make a landvehicle base class with an Abstract and Constant numberOfWheels property and have the car, motorcycle, tractorTrailer, etc. subclasses each define their own value for their Constant numberOfWheels property (4 for the car subclass, 2 for the motorcycle, 18 for the tractorTrailer, 1 for the unicycle, 0 for the sled, etc.)

To address the last segment of the third part of your question, if you want the square property of your object to always take the current values of the projA and projB properties, you may want to make the square property Dependent. See the "Calculate Data on Demand" section of this documentation page for an example of Dependent properties.

MATLAB: Class properties are being reinitialized each time I call a method

You've shown the class definition, but not your test code. So, the problem appears to be entirely in your imagination:

>> obj=obj.X(20);
>> obj=obj.Y(10);
>> obj.Mul;
   200

However, I suspect that you neglected to assign the output of obj.X() and obj.Y() as above. If you don't want to have to do that, you might consider making this a handle class.

Best Answer

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