Closes #64: Create a class to represent proxy ids (#65)

1. Create `ProxyID` class.
2. Rename `ProxyID` to `Identifier`.

libmexclass/matlab/+libmexclass/+proxy/Identifier.m

@@ -0,0 +1,21 @@
+classdef Identifier < matlab.mixin.Scalar
+
+    properties(SetAccess=private, GetAccess=public)
+        % C++ Proxy ID.
+        ID = NaN
+    end
+
+    methods
+        function obj = Identifier(id)
+            if nargin == 1
+                validateattributes(id, {'numeric'}, ...
+                    {'scalar', 'nonnegative', 'integer'});
+                obj.ID = uint64(id);
+            end
+        end
+
+        function tf = ismissing(obj)
+            tf = ismissing(obj.ID);
+        end
+    end
+end

libmexclass/matlab/+libmexclass/+proxy/Proxy.m

@@ -1,18 +1,22 @@
 classdef Proxy < matlab.mixin.indexing.RedefinesDot & handle
 
-    properties
+    properties(SetAccess=private, GetAccess=public)
         % C++ Proxy ID.
-        ID uint64 = uint64.empty(0, 1);
+        Identifier(1, 1) libmexclass.proxy.Identifier;
         % C++ Proxy Name.
         Name (1,1) string;
     end
 
+    properties(Dependent)
+        ID
+    end
+
     methods
 
         function obj = Proxy(options)
             arguments
-                options.ID (1,1) uint64
-                options.Name (1,1) string
+                options.ID (1,1) libmexclass.proxy.Identifier {mustBeNonmissing}
+                options.Name (1,1) string {mustBeNonmissing}
                 options.ConstructorArguments (1,:) cell
             end
 
@@ -23,14 +27,14 @@
             obj.Name = options.Name;
 
             if isfield(options, "ID")
-                obj.ID = options.ID;
+                obj.Identifier = options.ID;
             else
                 if ~isfield(options, "ConstructorArguments")
                     error("libmexclass:proxy:NoConstructorArguments", """ConstructorArguments"" must be specified when constructing a new Proxy instance.");
                 else
                     % Create the an instance of the specified C++ Proxy class and return its
                     % Proxy ID To be stored on the MATLAB Proxy object.
-                    obj.ID = libmexclass.proxy.gateway("Create", options.Name, options.ConstructorArguments);
+                    obj.Identifier = libmexclass.proxy.gateway("Create", options.Name, options.ConstructorArguments);
                 end
             end
         end
@@ -40,10 +44,14 @@ function delete(obj)
             % Destroy the corresponding C++ Proxy instance when destroying
             % the MATLAB object. ID may be empty if an error occured during
             % construction. If so, do not call destroy.
-            if ~isempty(obj.ID)
+            if ~ismissing(obj.Identifier)
                 libmexclass.proxy.gateway("Destroy", obj.ID);
             end
         end
+
+        function id = get.ID(obj)
+            id = obj.Identifier.ID;
+        end
     end
 
     methods (Access=protected)
@@ -64,5 +72,4 @@ function delete(obj)
             n = listLength(obj.AddedFields,indexOp,indexContext);
         end
     end
-
 end