Clone Handle Object - using MATLAB OOP

How to clone an object (deep copy) which inherits reference behaviour from the 'handle' class.

Description

The motivation for this demo is that if you have an instance (say 'a') of a custom class that subclasses 'handle', then the command 'b = a;' will produce another reference 'b' to the same object as initially referenced by 'a', effectively creating a 'shallow' copy of the object.

The link to this demo contains a test class which contains the code necessary to produce a clone or 'deep' copy of its instance by way of a method and alternatively, through its constructor.

A test script is provided to demonstrate the cloning of instances of the test class.

Written by Sam Oliver & Bobby Nedelkovski
The MathWorks Australia Pty Ltd
Copyright 2009, The MathWorks, Inc.

• MATLAB release: MATLAB 7.9 (2009b)
• HTML Files: HOW TO CREATE A COPY OF A HANDLES OBJECT
• Other Files: copyHandleObj/demoScript.m, ; copyHandleObj/TestClass.m, ; license.txt

[Volkmar Glauche]

I also faced this problem, but for more complicated objects (using e.g. protected and dependent properties). It turned out that just looping over all properties did not do the trick. I had to check all kind of meta-properties to make sure the copyobj method didn't crash.

[Volkmar Glauche]

This is the code I used for copyobj:
        function new = copyobj(obj)
            % This method acts as a copy constructor for all derived
            % classes.
            new = feval(class(obj)); % create new object of correct subclass.
            mobj = metaclass(obj);
            % Only copy properties which are
            % * not dependent or dependent and have a SetMethod
            % * not constant
            % * not abstract
            % * defined in this class or have public SetAccess - not
            % sure whether this restriction is necessary
            sel = find(cellfun(@(cProp)(~cProp.Constant && ...
                ~cProp.Abstract && ...
                (~cProp.Dependent || ...
                (cProp.Dependent && ...
                ~isempty(cProp.SetMethod))),mobj.Properties));
            for k = sel(:)'
                new.(mobj.Properties{k}.Name) = obj.(mobj.Properties{k}.Name);
            end
end

[Volkmar Glauche]

 

[Bobby Nedelkovski]

This File Exchange item is now under review. WARNING: Neither the original method (leveraging ‘TestClass.m’ with the ‘copyobj’ method as a template for your custom class) nor using Volkmar’s suggestion (inherit the Volkmar’s proposed ‘copyobj’ method by subclassing ‘TestClass.m’) produces a truly ‘deep’ copy of a handle object. Instead, both methods yield ‘deep’ copies of public properties in the custom superclass hierarchy yet they produce ‘shallow’ copies of private and protected properties. This has to do with the depth in scope of private and protected properties to an instance…

Case: Values of private properties declared in any superclass or protected properties declared in any grandparent class or higher cannot be copied. Accessor methods for private and protected properties can be customised – for instance, getMyProperty(obj). If a private property is modified with a custom set method and since this custom method is not part of the property’s meta-data (i.e. “GetMethod = []” in its corresponding “meta.property” object), one cannot copy the value of the private property using simple assignment with Dynamic Expressions. Moreover, you cannot temporarily modify the property’s “GetMethod=getMyProperty(obj)” nor “GetAccess=Public” meta-data fields to assume necessary control to copy private properties.

I am currently investigating possibilities in producing a truly ‘deep’ copy of a handle object. Please feel free to email any suggestions to [e-mail].