How to initialize optional struct/class input variables in the "arguments" block?

I defined a function in the following form

function [obj] = func(obj, aa, bb, cc, dd)
    arguments
        obj;
        aa;
        bb = 'bb';
        cc = obj.c;
        dd.dd1 = ones(obj.dd1);
        dd.dd2 = 2;
    end
end 

where "dd" can be a struct/class type optional input variable. By executing the following command

obj = func(obj, aa, bb, cc, dd)

I ran into the following error.

Error using xxx func
Invalid argument at position 5. Function requires 2 to 4 positional input(s).

How can I fix the error?

Answer 1

[MathWorks Support Team]

The abovementioned error was caused by the invalid use of the “arguments” block (i.e., declare validation argument validation). To properly define “dd” where the fields “dd1” and “dd2” are initialized with the default values, please refer to the updated example of “func” as shown below.

function [obj] = func(obj, aa, bb, cc, dd)
    arguments
        obj;
        aa;
        bb = 'bb';
        cc = obj.c;
        dd = struct('dd1', ones(obj.dd1), 'dd2', 2);
    end
end 

In the event where no optional argument “dd” is passed into “func” as shown in the example below

obj = func(obj, aa)

“dd” will be initialized by the default value “dd = struct('dd1', ones(obj.dd1), 'dd2', 2).”  This is in agreement with the format of the “arguments” block as shown below.

arguments
    argName1 (dimensions) class {validators} = defaultValue    ...
    argNameN ...
end

If you elect to pass “dd” when calling the function, the passed variable's value will be used instead of the default value initialized within the "arguments" block. 

For more information and examples of the “arguments” block (i.e., declare validation argument validation), please refer to the documentation below:

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