calling one method from a Static method

Hello,

Here is the class definition:

classdef first < handle
    methods (Static)
        function hello
            obj_first.bye
        end
    end
    methods
        function bye(obj)
            disp('that works')
        end
    end
end

Here are my commands:

>> obj_first=first;
>> obj_first.bye
that works
>> obj_first.hello
Undefined variable "obj_first" or class "obj_first.bye".

Error in first.hello (line 4)
              obj_first.bye

>>

Can you please help me?

Thank you very much

Answer 1

[Guillaume]

Static methods apply to the class itself rather than instances (objects) of the class.You normally call such a method with classname.methodname, (i.e. in your case with first.hello). Matlab allows to call static methods from instances just so you don't have to determine the class of the object yourself, but other than determining which class the method belongs to, the object itself plays no role in the invocation of a static method. It is not passed to the method.

I don't understand why you thought your code would work. There's no obj_first in the scope of the static method, and even if somehow you made it able to access the variable from another workspace, there's no guarantee that such a variable exists.

More importantly, I don't understand what you're trying to achieve with your static method here. Can you explain what problem you're trying to solve?

One simple way to fix your code is to change the static method, so it takes an input:

function hello(some_obj)
   some_obj.bye;
end

and you would then call it with:

obj_first.hello(obj_first)

But the whole lot would be pointless. You may as wall call the object method directly.

Another way to solve this, would be to create a singleton obj_first within the class but I've no idea if that's what you meant to do:

classdef first < handle
    methods (Static)
        function hello
            obj_first = getsingleton();
            obj_first.bye();
        end

         function obj_first = getsingleton()
           persistent obj_first;
           if isempty(obj_first)
              obj_first = first();
           end
        end
      end

      methods
          function bye(obj)
              disp('that works')
          end
      end
  end
>>obj_first = first.getsingleton;
>>obj2 = first();
>>first.hello();

[Guillaume]

Michaël's comment as an answer moved here:

Thank you for your reply!

The only syntax I used to use for events/listeners in class in that described at http://www.mathworks.com/help/matlab/matlab_oop/events-and-listeners--syntax-and-techniques.html

In this example, the listener callback function belongs to a static method, which prevents from directly calling other (instanced) non-static method.

[Guillaume]

Michaël's comment as an answer moved here:

Oops, using your code, I get this error:

>> obj_first = first.getsingleton;
Error: File: first.m Line: 8 Column: 23
The PERSISTENT declaration must precede any use of the variable obj_first.

[Guillaume]

Please use comments rather than answers.

Sorry I should have tested the code. I just learnt something new, you can't use a persistent variable as a return variable. To fix the persistent error, simply use a different variable name for the return variable and assign to it at the end of the function:

        function o = getsingleton()
           persistent obj_first;
           if isempty(obj_first)
              obj_first = first();
           end
           o = obj_first;
        end

However, now that I know more about what you're doing, you're not interested in a singleton, so the above is the wrong approach anyway.

If you want your callback method to use some properties of instances of the class in which it is declared then that callback cannot be static. It has to be a normal method of the class and the callback will receive three arguments as I've shown you in your other question: the class object, the event source, and the event argument. E.g.:

classdef RespondToToggle < handle
   properties
       id;  %for demo 
   end
   methods
      function obj = RespondToToggle(toggle_button_obj)
         obj.id = randi([1 1e6]); %for demo
         addlistener(toggle_button_obj,'ToggledState',@obj.handleEvnt);
      end        
      function handleEvnt(obj, src,~)
         disp(['in handleEvnt of ', num2str(obj.id)]);
         if src.State
            disp('ToggledState is true')          
         else
            disp('ToggledState is false')
         end
      end
   end
end
>> t  = ToggleButton;
>> rtt1 = RespondToToggle(t)
  rtt1 = 
       RespondToToggle with properties:
         id: 704033
>> rt2 = RespondToToggle(t)
      rtt2 = 
       RespondToToggle with properties:
         id: 16187
>> t.OnStateChange(true)
in handleEvnt of 16187
ToggledState is true
in handleEvnt of 704033
ToggledState is true

[Michaël]

Thank you!

Can you please have a look at http://www.mathworks.com/matlabcentral/answers/273854-calling-one-method-from-another-method ?

There, I don't use Static methods.