When subclassing "double" with new properties in MATLAB, is there an easy way to access th-CodePudding

Say I have a class subclassing double, and I want to add a string (Similar to the 'extendDouble' in the documentation). Is there an easy way to access the actual numeric value without the extra properties, particular for reassigning? Or if I want to change the value, will I have to recreate the value as a new member of the class with the new value and the same string?

e.g.

classdef myDouble < double

properties
    string
end

methods 
    function obj = myDouble(s)
        % Construct object (simplified)
        obj.string = s;
    end
end

end

----------

x = myDouble(2,'string')
x = 
    2 string
    
x = 3
x = 
    3 string

CodePudding user response：

Short answer: NO. There is no easy way to access a single member of a class when the class contains more than one member. You'll always have to let MATLAB know which part of the class you want to manipulate.

You have multiple questions in your post but let's tackle the most interesting one first:

% you'd like to instanciate a new class this way (fine)
x = myDouble(2,'string')
x = 
    2 string
% then you'd like to easily refer to the only numeric part of your class
% for assignment => This can NEVER work in MATLAB.
x = 3
x = 
    3 string

This can never work in MATLAB because of how the interpreter works. Consider the following statements:

% direct assignment
(1) dummy = 3
% indexed assignments
(2) dummy(1) = 3
(3) dummy{1} = 3
(4) dummy.somefieldname = 3

You would like the simplicity of the first statement for assignment, but this is the one we cannot achieve. The statement 2, 3 and 4 are all possible with some fiddling with subasgn and subsref.

The main difference between (1) and [2,3,4] is this:

Direct assignment:

In MATLAB, when you execute a direct assignment to a simple variable name (without indexing with () or {} or a field name) like dummy=3, MATLAB does not check the type of dummy beforehand, in fact it does not even check whether the variable dummy exists at all. No, with this kind of assignment, MATLAB goes the quickest way, it immediately create a new variable dummy and assign it the type and value accordingly. If a variable dummy existed before, too bad for it, that one is lost forever (and a lot of MATLAB users have had their fingers bitten once or twice by this behavior actually as it is an easy mistake to overwrite a variable and MATLAB will not raise any warning or complaint)

Indexed assignments:

In all the other cases, something different happens. When you execute dummy(1)=3, you are not telling MATLAB "create a new dummy variable with that value", you are telling MATLAB, "find the existing dummy variable, find the existing subindex I am telling you, then assign the value to that specific subindex". MATLAB will happlily go on, if it finds everything it does the sub-assignment, or it might complains/error about any kind of misassignment (wrong index, type mismatch, indices length mismatch...).

To find the subindex, MATLAB will call the subassgn method of dummy. If dummy is a built-in class, the subassgn method is also built in and usually under the hood, if dummy is a custom class, then you can write your own subassgn and have full control on how MATLAB will treat the assignment. You can check for the type of the input and decide to apply to this field or another if it's more suitable. You can even do some range check and reject the assignment altogether if it is not suitable, or just assign a default value. You have full control, MATLAB will not force you to anything in your own subassgn.

The problem is, to trigger MATLAB to relinquish control and give the hand to your own subassgn, you have to use an indexed assignment (like [2,3 or 4] above). You cannot do that with type (1) assignment.

Other considerations: You also ask if you can change the numeric part of the class without creating a new object. The answer to that is no as well. This is because of the way value classes work in matlab. There could be a long explanation of what happens under the hood, but the best example is from the MATLAB example you referenced yourself. If we look at the class definition of ExtendDouble, then observe the custom subassgn method which will perform the change of numeric value, what happens there is:

obj = ExtendDouble(b,obj.DataString);

So even Mathworks, to change the numeric value of their extended double class, have to recreate a brand new one (with a new numeric value b, and transfering the old string value obj.DataString).