MATLAB: How to write ‘setter’ methods for properties with unknown names

MATLABooprecursion

The following class cannot be initialized, as any call to the constructor gives the error:

>> MyClass('a',2)

Maximum recursion limit of 500 reached. Use set(0,'RecursionLimit',N) to change the limit. Be aware that exceeding your available

stack space can crash MATLAB and/or your computer.

Error in MyClass/MyClass/@(o,p)setProp(o,propname,p)

classdef MyClass < dynamicprops
    methods
        function obj = MyClass(propname, val)
            P = obj.addprop(propname);
            P.SetMethod = @(o,p)setProp(o,propname,p);
            obj.(propname) = val;
        end
        function setProp(obj, propname, val)
            % do other stuff, like evaluating if val is legitimate
            obj.(propname) = val;
        end
    end
end

Why is this, and what is the best way to write custom 'set' methods for dynamic properties with unknown names?

Best Answer

The infinite recursion is caused by the line 'obj.(propname) = val;' in setProp. This calls the SetMethod for the property 'propname'. When setting the value of a property, MATLAB checks to see if you are currently in the SetMethod for that property in order to avoid infinite recursion. However, since your SetMethod is an anonymous function which calls setProp (and not setProp itself), this assignment is not inside the SetMethod, and the assignment calls the anonymous function.

To avoid this issue, you may use a nested function as shown below:

% start of file MyClass2.m
classdef MyClass2 < dynamicprops
    methods
        function obj = MyClass2(propname, val)
            P = obj.addprop(propname);
            P.SetMethod = propSetFcn(obj, propname);
            obj.(propname) = val;
        end
    end
end
function f = propSetFcn(obj, propname)
    function setProp(obj, val)
        % do other stuff
        obj.(propname) = val;
    end
f = @setProp;
end
% end of file MyClass2.m

Related Solutions

MATLAB: How to call the set-dot method on a subclass object to set a property of the superclass

You cannot define a "set.xyz" method in a subclass to set the "xyz" property of the superclass. This restriction prevents a subclass from overriding a superclass implementation and cannot be bypassed. However, in some situations superclass property needs to grant subclasses the ability to participate in the setting or getting of a property value, like in this case. To do so, a superclass "set" function can call a protected method to allow subclasses to participate in property access while still maintaining some level of control.

So the implementation of the two classes might look like this:

classdef MySuperClass < handle
      properties
          xyz
      end
      methods
        function set.xyz(obj,value)
           disp(['setting xyz in MySuperClass to ', value]);
           obj.xyz = value; % updates the property
           respondToXYZChange(obj) % calls the protected method
        end 
     end 
     methods(Access = protected)
        function respondToXYZChange(obj, value) % no function definition here
        end
     end
end 
classdef MySubClass < MySuperClass
      methods
          function obj=MySubClass()
              disp('!');
          end
          function update(obj)
              disp('updating...');
          end
      end
      methods(Access = protected)
          function respondToXYZChange(obj) % call to the "update" method of subclass
              obj.update();
          end
      end
end
>> m  = MySubClass3;
!
>> m.xyz = 'a'
setting xyz in MySuperClass to a
updating...
m = 
    MySubClass3 with properties:
      xyz: 'a'
>>

Executing the command:

m.xyz = 'a';

calls the superclass "set" method which sets the value of the property "xyz" and then calls the protected "respondToXYZChange" which is defined in the subclass to call the "update" method.

MATLAB: Setter methods for dynamic properties

What about overloading subsasgn

classdef myClass < dynamicprops
    methods
        function obj = myClass()
            P = addprop(obj, 'myProp');
        end
    function A = subsasgn(A, S, B)
      switch S.type
        case '.'
          SetProp(A, S.subs, B);
      end
    end
      
          function SetProp(obj, propname, val)
              obj.(propname) = val;
          end
      end
  end

Best Answer

Related Solutions

MATLAB: How to call the set-dot method on a subclass object to set a property of the superclass

MATLAB: Setter methods for dynamic properties

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