Function Argument Validation: Comparing property size to a variable

I would like to ensure that a property of a class is of a certain size, defined by another property of the class which is set in the constructor. I can implement this by using a private property and set and get methods as shown in the code example below:

classdef TestClassI < handle

    properties(SetAccess=immutable)
        matrixSize single
    end
    properties(Dependent)
        matrix single {mustBeNumeric}
    end
    properties(Access=private, Hidden=true)
        matrix_
    end

    methods
        function obj = TestClassI(matrixSize)
            obj.matrixSize = matrixSize;
        end
        function matrix = get.matrix(obj)
            matrix = obj.matrix_;
        end
        function set.matrix(obj, val)
            validateattributes(val, {'numeric'}, {'size', obj.matrixSize});
            obj.matrix_ = val;
        end
    end
    
end

However, this is pretty cumbersome. What I'd prefer to do is something like:

classdef TestClassII < handle
    properties(SetAccess=immutable)
        matrixSize single
    end
    properties
        matrix(10,1) single {mustBeNumeric}
    end
    methods
        function obj = TestClassII(matrixSize)
            obj.matrixSize = matrixSize;
        end
    end
end

But, where (10,1) is not hard-coded but given by matrixSize. Is there a way to do this?

Answer 1

[Matt J]

No, you can't do that, because the object has to be able to instantiate properties before the constructor is called.Therefore, property validation steps won't be able to use data that is only brought into existence later by the constructor.

My recommendation would be to overload subsasgn(). There, you can define any object-dependent validation steps that you want.

Or, if you really need the validation to be done even internally within the classdef, you could use matlab.mixin.indexing.RedefinesDot, though in my experience, that route can be more delicate.

[Bradley Treeby]

> the object has to be able to instantiate properties before the constructor is called

Got it, that makes sense.

Note to my future self, it's also possible to do the same thing as I originally posted but without an internal variable. The MATLAB code analyzer warns against a set method accessing another property, but I think it's safe in this scenario, as matrixSize is declared as immutable.

classdef TestClassIII < handle
    properties(SetAccess=immutable)
        matrixSize single
    end
    properties
        matrix single {mustBeNumeric}
    end
    methods
        function obj = TestClassIII(matrixSize)
            obj.matrixSize = matrixSize;
        end
        function set.matrix(obj, val)
            validateattributes(val, {'numeric'}, {'size', obj.matrixSize});
            obj.matrix = val;
        end
    end
end

Answer 2

[Shubham]

Hi Bradley,

Yes, there is a way to achieve this without the use of set and get methods. One possible solution is to use the validateattributes function in the constructor to check that the matrix property has the correct size. Here's an example implementation:

classdef TestClassII < handle
    properties(SetAccess=immutable)
        matrixSize single
    end
    properties
        matrix single {mustBeNumeric}
    end
    methods
        function obj = TestClassII(matrixSize)
            obj.matrixSize = matrixSize;
            defaultSize = [matrixSize, 1];
            defaultMat = zeros(defaultSize, 'single');
            validateattributes(defaultMat, {'numeric'}, {'size', defaultSize});
            obj.matrix = defaultMat;
        end
    end
end

In this implementation, we initialize the matrix property using a default size of [matrixSize, 1] and fill it with zeros using the zeros function. Then we use validateattributes to check that the size of the default matrix is correct.

Note that validateattributes raises an error if the validation fails, so we only set obj.matrix if the validation succeeds.

[Bradley Treeby]

Thanks! But this doesn't validate the size of obj.matrix if the values are changed outside the constructor right?