How to obtain static member variables in MATLAB classes?

Question

Is there a way to define static member variables in MATLAB classes?

This doesn\'t work:

classdef A

    properties ( Static )
        m = 0;
    end
         


        

Answer 1

Another workaround to get something like static properties is to use the fact that initialisation code for member variables is only executed once when the class file is loaded. That means, if you have a definition like

classdef foo
    properties
        stuff = some_function()
    end
end

then some_function is invoked only once, and if it returns an object of class type, this will be shared by all instances. I've added a sample implementation that shows how that can be used:

classdef ClassWithStaticMembers
    properties
        classvars = StaticVarContainer('foo', 0, 'bar', 2);
        othervar
    end
    methods
        function obj=ClassWithStaticMembers(var)
            obj.othervar = var;
        end
    end 
end

classdef StaticVarContainer < dynamicprops
    methods
        function obj=StaticVarContainer(varargin)
            for i=1:2:numel(varargin)
                obj.addprop(varargin{i});
                obj.(varargin{i}) = varargin{i+1};
            end
        end
    end
end

If you run this sample code

obj1 = ClassWithStaticMembers(3);
obj2 = ClassWithStaticMembers(5);
obj1.classvars.foo = [2,3];

obj1.othervar
obj1.classvars

obj2.othervar
obj2.classvars

you'll see, that classvars is indeed shared. I think this solution is much nicer than using persistent variables in functions, since you can reuse the StaticVarContainer as often as you want, it's easier to use, and furthermore, you directly see the initialisation of the static variables in the properties section.

To get the result, that is desired in the OP's question (i.e. implementing an object counter) the shared property can be made Constant, so that it can be referenced without an instance at hand:

classdef ClassWithCounter
    properties (Constant)
        static = StaticVarContainer('counter', 0);
    end
    methods
        function obj=ClassWithCounter()
            obj.static.counter = obj.static.counter + 1;
        end
    end 
end

clear all
obj1 = ClassWithCounter();
obj2 = ClassWithCounter();
obj3 = ClassWithCounter();

ClassWithCounter.static.counter

Note, that the Constant attribute only means that, e.g. obj1.static cannot be changed, but it does not affect obj1.static.counter which is not constant, and can be set to heart's desire.