Can NOT access properties when creating a new class by inheriting the gpib class

Hi,

I'm having a difficulty of accessing properties of my own class, inherited from the gpib class.

The purpose is to create a specific class for one lab instrument. the sample code is like:

classdef OTF920 < gpib
  %UNTITLED Summary of this class goes here
  %   Detailed explanation goes here

    properties (Access = protected)
        VENDOR
        BOARDINDEX
        PRIMARYADDRESS
    end

    properties (Access = public)
        Wavelength
        Attenuation
    end

    methods
        function obj = OTF920(in_VENDOR, in_BOARDINDEX, in_PRIMARYADDRESS)
            obj = obj@gpib(in_VENDOR, in_BOARDINDEX, in_PRIMARYADDRESS);
            obj.VENDOR = in_VENDOR;
            obj.BOARDINDEX = in_BOARDINDEX;
            obj.PRIMARYADDRESS = in_PRIMARYADDRESS;
        end

        function obj = changeWL(obj, in_Wavelength)
            obj.Wavelength = in_Wavelength;
            disp(['Wavelength changed to ', num2str(obj.Wavelength), ' nm.']);
        end
    end

end

when creating an object such as

a = OTF920('ni',1,2)

I could not access any of the properties by using dot index. However, I can only access BoardIndex of the gpib class by using a code like

get(a, 'BoardIndex')

What should I do if I want to access the properties of the superclass 'gpib' and the subclass OTF920? Did I do anything wrong?

Thank you so much!

BR, Deming

[Adam]

I don't know anything about gpib so I don't know what its properties are and what their access specifiers are etc, but it seems a bit odd to be passing constructor arguments to the base class and then also storing them in your derived class. What does the base class do with these properties?

Also, if there some reason why the properties are 'protected'? Are you intending further subclasses?

I assume since you are subclassing it that gpib is not a Sealed class.

[Deming Kong]

Hi Adam,

Thank you for your reply. The superclass construction function requires at least 3 inputs, as the vendor, gpib_boardindex, and gpib_address. I simply passby these inputs to store them in the user-defined subclass for testing. Sorry I did not mention this. ;-)

The problem is, I should be able to access property Wavelength, as well as Attenuation. But currently I can not. ;-(

BR, Deming

Answer 1

[Sean de Wolski]

Looking through the metaclass stack for gpib ( >>?gpib or >>metaclass(gpibobj) and looking at the metaclasses' superclasses) it doesn't look like BoardIndex is actually a property of the class.

It appears that the superclass for gpib, instrument, implements a subsref that overloads the dot indexing to grab these other fields and make them look like properties.

edit instrument.subsref

That explains what's going on. How to fix it? What exactly are you trying to change about those properties? I'd think you could add properties to your class, make them dependent, and set them to be whatever you want in the getter.

This seems like a worthwhile enhancement request so I'd recommend contacting Tech Support as well.

[Deming Kong]

Hi Sean,

Thank you for your answer! ;-)

I've also found the problem also. It seems there's a subref defined along with the instrument class. When I want to access the Wavelength property of the drived class OTF920, it gives an error:

Error using instrument/subsref (line 113)
Not enough input arguments.

However, I found the comment in the subref function

%   OBJ.PROPERTY returns the property value of PROPERTY for instrument
%   object OBJ.
%
%   Supported syntax for instrument objects:
%
%   Dot Notation:                  Equivalent Get Notation:
%   =============                  ========================
%   obj.Tag                        get(obj,'Tag')
%   obj(1).Tag                     get(obj(1),'Tag')
%   obj(1:4).Tag                   get(obj(1:4), 'Tag')
%   obj(1)

It seems it allows the dot index to get the properties. So it really confuses me a lot. Then I tried

properties gpib

It returns

No properties for class gpib.

I also tried the same thing with the derived class OTF920

properties OTF920

It returns the two properties in the OTF920 class

Properties for class OTF920:

      Wavelength
      Attenuation

which, I can not access... even with

get(a,'Wavelength')

It still generates an error

Error using instrument/get (line 81)
The name 'Wavelength' is not an accessible property for an instance of class 'GpibNI'.

I'll also try to contact Tech Support. But thank you so much for your answer!

BR, Deming

[Deming Kong]

Sorry I did not make it clear.

I can only give value to the properties of OTF920 by implementing the set functions.

function obj = set.Wavelength(obj, value)
            if isnumeric(value) && value>0
                obj.Wavelength = value;
            else
                error('Wavelength must be a number')
            end
end

However, still no luck in accessing them. And I want to change these properties from time to time...

Thank you again for your help!

BR, Deming