Custom Argument Validation Function for NameValueArgs

While working on a project, I found an interesting error when using custom argument validation functions on name-value arguments. I constructed this contrived example class to demonstrate:

classdef Dog
    
    properties
        Name
        HouseNumber = []
    end
    
    methods
        function obj = Dog(Name, NameValueArgs)
            arguments
                Name (1,1) string                
                NameValueArgs.HouseNumber double {Dog.mustBeScalarOrEmpty(NameValueArgs.HouseNumber)}
            end
            
            obj.Name = Name;
            
            if isfield(NameValueArgs, 'HouseNumber')
                obj.HouseNumber = NameValueArgs.HouseNumber;
            end
        end
    end
    
    methods (Static = true)
        function mustBeScalarOrEmpty(var)
            if ~(isscalar(var) || isempty(var))
                error('Not scalar nor empty.');
            end
        end
    end
end
    

When the following is run:

dog1 = Dog("Test",'HouseNumber',23)

This error is produced:

Error using Dog
Invalid name-value argument 'HouseNumber'. Index exceeds the number of array elements (1).

I'm not understanding why this error is being triggered. Is it not valid to call a static method from the class as a custom argument validation function?

Answer 1

[Roshni Garnayak]

Try using ‘varargin’ in place of ‘NameValueArgs’. To validate the properties and assign values to them, use the ‘set’ method.

You can refer to the following link to get an idea of how to use Name-value pair arguments in constructor:

https://www.mathworks.com/help/matlab/ref/matlab.system.constructor.html

For further details on ‘varargin’ and ‘set’ method refer to the following links:

https://www.mathworks.com/help/matlab/ref/varargin.html

https://www.mathworks.com/help/matlab/matlab_oop/property-set-methods.html

[David DeVries]

Thanks for the answer Roshni! I do realize that varargin could be used, but using varargin seems like a work around compared to NameValueArgs. I guess I'm more curious if others have seen this error, and why it exists. I'm especially curious since static class methods can be called as custom validation functions for non-NameValueArgs inputs (e.g. I could use Dog.mustBeScalarOrEmpty after Name (1,1) string and no error would occur).

[Jan Kappen]

@Roshni Garnayak, are you really proposing varargin when the new argument validation features are that awesome?!

@David DeVries, your approach is much better imo. And in R2020b Update 2 it runs well:

>> dog1 = Dog("Test",'HouseNumber',23)

dog1 = 

  Dog with properties:

           Name: "Test"
    HouseNumber: 23

What happens if you move the validation function out of the classdef?

    classdef Dog
    
    properties
        Name
        HouseNumber = []
    end
    
    methods
        function obj = Dog(Name, NameValueArgs)
            arguments
                Name (1,1) string
                NameValueArgs.HouseNumber double {mustBeScalarOrEmpty(NameValueArgs.HouseNumber)}
            end
            
            obj.Name = Name;
            
            if isfield(NameValueArgs, 'HouseNumber')
                obj.HouseNumber = NameValueArgs.HouseNumber;
            end
        end
    end
    
    %     methods (Static = true)
    %         function mustBeScalarOrEmpty(var)
    %             if ~(isscalar(var) || isempty(var))
    %                 error('Not scalar nor empty.');
    %             end
    %         end
    %     end
end

% subfunction outside classdef!
function mustBeScalarOrEmpty(var)
    if ~(isscalar(var) || isempty(var))
        error('Not scalar nor empty.');
    end
end

Maybe you can create a non-static validator that calls the class static one?

[David DeVries]

@Jan Kappen: Glad to hear that the bug was fixed in MATLAB 2020b.

In the meantime I was using another class and defining "mustBeScalarOrEmpty" in a different class as a public static method, and it seemed to work.

In general, defining functions outside of a classdef breaks many object orientated programming rules, so I avoid that where I can.