Custom error messages with Class Object

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

[Ameer Hamza]

Tommy, Good example. This is exactly what the OP asked to do. I suggest adding this as an answer to this question.

[Tommy]

Absolutely - good call.

Answer 1

[Tommy]

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.

[Michael Savage]

This is great starting point for me, thank you very much!

Answer 2

[Anmol Dhiman]

Hi Michael,

The above functionality can be achieved using Exception Handling. You can modify the error displayed by catching the exception in try block and rewriting your own error message in the catch block.

For more information on how to do the above, you can refer to below documentation page.

https://www.mathworks.com/help/matlab/matlab_prog/respond-to-an-exception.html

If you face any issues in implementing it, let me know.

Thanks,

Anmol

[Michael Savage]

Thank you for your response, I have used error handling code before in scripts and functions. I just did not know how to implement it into a Class Object. Obviously, the above answer has pointed me in the right direction.