Using builtin assignment for classes that overload the assignment operator '='

Hi,

I'm trying to find out if it is possible to recover the functionality of '=' for properties in a class that have their own set method.

For instance, this piece of code:

classdef myClass < hgsetget
    properties
       myProperty
    end
    methods
       function set.myProperty(obj,value)
          mySetFunction(obj,value);
       end
       function mySetFunction(obj,value)
           obj.myProperty = value;
       end
    end
end

generates a recursion error if we try to set the property myProperty by any value.

Is there a way to avoid this effect? If there were a builtin function that represents the effect of the "=" I could use it inside mySetFunction, but I do not know if this is possible.

In other words, I know that inside the set.myProperty function, a command

obj.myProperty = value;

would call the builtin function corresponding to "=", but I do not know how to force this behaviour explicitely in any function that is not a set.<property>

Best, Daniel

Answer 1

[Philip Caplan]

You are correct, your code does throw an error about recursion limits. However, I believe the operator you are interested in overloading is the subscripted assignment operator, "subsasgn". This will allow you to do:

obj.myProperty = value;

and call you own set function without receiving an error about the recursion limit. Here is what the class definition would look like:

classdef myClass < hgsetget
    properties
       myProperty
    end
    methods
      function mySetFunction(obj,value)
           obj.myProperty = value;
      end

         function a = subsasgn(a,s,b)
             if strcmp(s.subs,'myProperty')
                 mySetFunction(a,b);
             end
         end
      end
  end

[Daniel]

Hi Philip,

Many thanks. This comes very close to what I need. My further question would be: with the definition that suggest is there any way that I can write

g = myClass();
g.myProperty = value;

without activating mySetFunction?

The reason of my question is this: imagine mySetFunction had a further functionalirty, say:

classdef myClass < hgsetget
    properties
       myProperty
    end
    methods
      function mySetFunction(obj,value)
           disp('myProperty has been updated');
           obj.myProperty = value;
      end

           function a = subsasgn(a,s,b)
               if strcmp(s.subs,'myProperty')
                   mySetFunction(a,b);
               end
           end
        end
    end

Then my final goal is a class where the direct assignation of a value triggers the "myProperty updated" message, BUT in which I keep the ability of updating myProperty programmatically without triggering the message.

Do you think that is possible? Daniel

[Philip Caplan]

The only way I can think of achieving this is by adding an additional property to the class:

classdef myClass < hgsetget
    properties
       myProperty
       printMessage
    end
    methods
      function mySetFunction(obj,value)
          if (obj.printMessage)
              disp('myProperty has been updated');
          end
           obj.myProperty = value;
      end

      function a = subsasgn(a,s,b)
          if strcmp(s.subs,'myProperty')
              mySetFunction(a,b);
          else
              a.(s.subs) = b;
          end
      end
   end
end

The message will not print if the property "printMessage" is false (or empty which is the default when creating an instance of the class) but will display the message if this property is true.

[Daniel]

Oh, yes, that will work. Thanks!