Modifying an immutable/protected variable of a subclass in a superclass method - matlab

Question

I'm trying to modify an immutable/protected property of a subclass, using a superclass method or an external utility function (I'm trying to use this function in the subclass's constructor, of course).
Example (of what I would like to do):
classdef Sup
methods
function self = setProperties(self, varargin)
% This method sets the properties of the object
% the input arguments come in the form 'propname1', val1, 'propname2', val2, ...
...
end % setProperties
end % methods
end % classdef Sup
classdef sub < Sup
properties (SetAccess = immutable)
prop1
prop2
prop3
end % properties
methods
function self = sub(varargin)
% constructor
self = setProperties(self, varargin)
end % sub
end % methods
end % classdef sub
>> SomeObj = sub('prop1', 1, 'prop2', 10, 'prop3', 100);
This code doesn't work, I get the error message 'You cannot set the read-only property 'prop1' of sub.'
I'm OK with setting sub's properties to be protected, but I wouldn't want them to be public. I'm also OK with the idea that setProperties would be an external utility function (not defined in the superclass), but then again, I'm not able to use setProperties in the constructor of sub.
Would appreciate your help on that.
Thank you,
Avihay

Answer 1

If you want to, you could set the SetAccess attribute of sub to ?Sup - in other words saying that they can only be set by the class Sup.
That seems to do what you're directly asking for - however, I should say that this seems like an unusual pattern to be implementing. I wonder whether a better suggestion might be to examine why you're finding a need to do this, and perhaps redesign your class relationships.
Edit:
If what you need is a general method of conveniently setting properties in a constructor, you could try inheriting your class from the built-in class hgsetget. For example,
classdef sub < hgsetget
properties (SetAccess = immutable)
prop1
prop2
prop3
end % properties
methods
function self = sub(varargin)
set(self,varargin{:})
end
end
end
hgsetget gives you built-in methods set (and get) that can be used with the syntax set(obj, 'myprop', myval, 'myprop2', myval2) and similar (like MATLAB Handle Graphics objects, hence the hg in hgsetget).
That might be more convenient for you. Note, though, that hgsetget is itself a subclass of handle, so you will have to be comfortable with your classes being handle objects. However, if you're considering immutable properties, then you're probably OK with that already.
Edit 2:
Another approach, which would work with value objects, might be the following:
classdef sub
properties (SetAccess = immutable)
prop1
prop2
prop3
end % properties
methods
function self = sub(varargin)
% constructor
props = varargin(1:2:end-1);
vals = varargin(2:2:end);
for i = 1:numel(props)
self.(props{i}) = vals{i};
end
end
end
end
Alternatively, you could make use of inputParser within the constructor, to give you a more flexible range of syntaxes.