Why I cannot change the class property in this case?

Below is the example code of OBJECTO.m

classdef OBJECTO
    properties (Access = private)
        pos {mustbevector}
    end
    methods (Access = public)
        function obj = OBJECTO(vec)
            % constructor
            if nargin < 1
                obj.pos = [0, 0, 0];
            else
                obj.pos = vec
            end
        end
        function obj = setpos(vec)
            % set position
            obj.pos = vec
        end
        function vec = getpos(obj)
            % get position
            vec = obj.pos
            disp(vec)
        end
    end
end

and I made .m file to execute above. (remove ; for the debugging)

function ans = myfunc()
    myobject1 = OBJECTO()
    
    myobject1.setpos([100, 0, 0])
    
    tmp = getpos(myobject1)
end

and in consol, typed the above function

myfunc()

the result says

ans =
       OBJECTO with properties
       
            pos = [0, 0, 0]
            
ans =
       OBJECTO with properties
       
            pos = [100, 0, 0]

tmp =
       0      0      0

that means the instance of OBJECTO (I mean 'myobject1' in the above) value is not changened

I have changed the property pos from 'private' to 'public' but same thing occured

why this happens?

Answer 1

[Matt J]

You called setpos() without returning anything. So, you need to have,

myobject1 = myobject1.setpos([100, 0, 0])

Or, inherit the class from handle.

[NoYeah]

so if I want to modify some properties of a class, then I have to write handler twice?

Why the matlab choose the most uncomfortable way

[Walter Roberson]

Why would you write the handler twice?

MATLAB has two types of classes: value objects, and handle objects.

Value objects work like typical MATLAB numeric arrays, where operations on the object do not change the object unless you assign the new value over top of old one. Just like

A = 3;
update_me(A)
A

A = 3

function update_me(X)
  X = X + 1;
end

This does not change the value of A and most people would not expect it to update the value of A.

Handle objects are more like passing around pointers to heap objects, where changes inside the class methods do change everyone's view of the object.

You should implement which-ever of the two makes sense in your situation. You do not need to implement both.