'When constructing an instance of class 'storage', the constructor must preserve the class of the returned object."

This class displays the correct answer followed by the error message 'When constructing an instance of class 'storage', the constructor

must preserve the class of the returned object." I would like to return the stringLength given a stringInput not necessarily using the display function. (Other community responses related to this topic have not been clear to me.) Thank you!

classdef storage
    properties 
        stringInput
    end
    methods
        function stringLength = storage(stringInput)
            stringLength = strlength(stringInput)*2;
            disp(num2str(stringLength));
        end
    end
end

I tested as follows:

>> stringInput = 'Hello world!';

>> stringLength = storage(stringInput)

24

When constructing an instance of class 'storage', the constructor

must preserve the class of the returned object.

Answer 1

[Walter Roberson]

function stringLength = storage(stringInput) 

is your constructor. It must return the constructed object.

In the simple case of a non-derived class, the object just pops into existence in the variable named on the left side of the = of the constructor method. In this case, that means that inside the storage method, the variable stringLength will hold the constructed object of class storage . You can then potentially modify that variable stringLength but when you do so you must do so in a way that the class storage is preserved.

It looks to me as if you are trying to create a constructor that stores the input character vector and immediately return... twice its length? (Perhaps representing number of bytes of storage ?) You cannot do that in one shot: you need to return the object and then you take the strlength of the stored object as a separate operation. You will want to use a different method for that, perhaps named stringLength

classdef storage
    properties 
        stringInput
    end
    methods
        function obj = storage(stringInput)
           obj.stringInput = stringInput;
        end
        function len = stringLength(obj)
            len = strlength(obj.stringInput)*2;
        end
    end
end

test with

obj = storage('Hello world!')
obj.stringLength

[Dora Velcsov]

Thank you very much. It works now, of course. I am just getting started with OOP, and I understand the concept now. Thanks again!