SIngleton implementation without persistent variables

Hi,

I have a handle class with a singleton implementation that uses persistent variables.

classdef MyHandler < handle

   properties
       ...
       ...
       ...
   end
   
   methods(Access=protected)
       function obj = MyHandler()
           %Constructor logic
       end
   end
    
   methods(Static)
       function inst = instance()
           persistent obj;
           if isempty(obj)
               obj = MyPackage.MyHandler();
           end
           inst = obj;
       end
   end
    
end

Is there a way to implement the same functionality without usage of persistent variables ? I am trying to generate C code from a system object that uses this handle class.

Answer 1

[Malay Agarwal]

Hi @Rohan,

You can create a singleton class without using persistent variables by defining the instance as a constant property. I have attached an example class to the answer. You can verify that the class indeed behaves like a singleton:

% Create two objects
obj = singleton.getInstance

obj = 
  singleton with properties:

    x: 5
    y: "Hello"

obj1 = singleton.getInstance

obj1 = 
  singleton with properties:

    x: 5
    y: "Hello"

% The two objects refer to the same handle
obj == obj1

ans = logical
   1

% Changing non-constant properties in one object reflects changes in both the objects
obj.x = 2;
obj, obj1

obj = 
  singleton with properties:

    x: 2
    y: "Hello"

obj1 = 
  singleton with properties:

    x: 2
    y: "Hello"

If you right click the class file and click on "Check Code Generation Readiness", you'll see that the tool does not report any issues:

Please refer to the following resources for more information:

• Constant properties documentation - https://www.mathworks.com/help/releases/R2022b/matlab/matlab_oop/properties-with-constant-values.html.
• Guidelines to make code generation work for classes - https://www.mathworks.com/help/releases/R2022b/simulink/ug/how-working-with-matlab-classes-is-different-for-code-generation.html.

Hope this helps!

[Rohan]

Thanks for your answer Malay !

[Xiangrui Li]

Neat implementation!

I do see one culprit. If I try to access the help information, like

help singleton % or
doc singleton

the instance will be initialized. This is can be easirly verified if you add

disp('Initializing') 

in the actual constructor.

Any way to avoid this behavior?