How to assign pointers to an object's property in MATLAB?

Question

I want to create a finite element object oriented program. I have a class Node. Since the nodes in a finite element mesh (represented by class Mesh) are distinct, I created the Node class to be a value class. When I instantiate the array of objects from class Node, I assign that object array to the nodes property of Mesh. I have an Element class too, representing a finite element. I also create an object array from this class and assign it to the element property of Mesh. It is clear up to now.

Since the finite element nodes also belong to the elements, I want to assign some of the nodes to the appropriate elements. But copying the nodes results in data redundancy, therefore I want to assign pointers to the Node objects so that the localNodes property of Element contains an array of pointers to the specific nodes. How should I modify my classes below to achieve it?

The Node class:

classdef Node

   properties
      coordinate;
   end

   methods
     % Not interesting for this example
   end

end

The Element class:

classdef Element

   properties
      localNodes; % the object instantiated from the class Element
                  % will store an array of pointers to the
                  % appropriate elements of the object array stored
                  % in Mesh.nodes. How can I assign these pointers
                  % to Element.localNodes?
   end

   methods
     % Not interesting for this example
   end

end

The Mesh class:

classdef Mesh

   properties
      nodes;    % its object will contain an object array of Node
      elements; % its object will contain an object array of Element
   end

   methods
     % Not interesting for this example
   end

end

Answer 1

Finally, following some discussion here is a starting point how I would solve this:

classdef Node < handle

   properties
      coordinate;
   end

   methods
       function obj=Node(id)
           obj.coordinate=id;
       end
   end

end

.

classdef Mesh < handle

   properties
      nodes;    % its object will contain an object array of Node
      elements; % its object will contain an object array of Element
   end

   methods
       function obj=Mesh(nodes,elements)
           pnodes=cell(1,nodes);
           for idx=1:nodes
               pnodes{idx}=Node(idx);
           end
           obj.nodes=[pnodes{:}];
           pelements=cell(1,numel(elements));
           for idx=1:numel(elements)
               pelements{idx}=Element(obj.nodes(elements{idx}));
           end
           obj.elements=[pelements{:}];
       end
       function non_deleted_nodes=get.nodes(obj)
           %getter to return only not-deleted nodes
           obj.nodes=obj.nodes(arrayfun(@isvalid,(obj.nodes)));
           non_deleted_nodes=obj.nodes;
       end
       function non_deleted_nodes=get.elements(obj)
           %getter to return only not-deleted nodes
           obj.elements=obj.elements(arrayfun(@isvalid,(obj.elements)));
           non_deleted_nodes=obj.elements;
       end

   end

end

.

classdef Element < handle

   properties
      localNodes; % the object instantiated from the class Element
                  % will store an array of pointers to the
                  % appropriate elements of the object array stored
                  % in Mesh.nodes. How can I assign these pointers
                  % to Element.localNodes?
   end

   methods
       function obj=Element(localNodes)
           obj.localNodes=localNodes;
       end
       function non_deleted_nodes=get.localNodes(obj)
           %getter to return only not-deleted nodes
           obj.localNodes=obj.localNodes(arrayfun(@isvalid,(obj.localNodes)));
           non_deleted_nodes=obj.localNodes
       end
       function delete(obj)
           for ix=1:numel(obj.localNodes)
               %The 1 is not a typo, we will delete always the first
               %element until the list is empty
               obj.localNodes(1).delete();
           end
           delete@handle(obj);
       end
   end

end

And finally a short demonstration:

m=Mesh(10,{[1,2],[2,3],[3,4]})
m.elements(1).localNodes
m.elements(1).localNodes(1).delete()
%now the node is deleted from the element and the mesh
m.elements(2).delete()
%now element 2 together with the nodes is deleted.

Answer 2

You have to use the superclass handle which let's you use references instead of copying the data.

x=MyValueClass(); %assume handle not superclass
y=x; %creates a copy
x=MyHandleClass(); %assume handle is superclass
y=x; %creates a reference to the same instance