Copy of an handle object

Hello,

I have some problem copy handle objcet.

How can I edit the example code in: https://www.mathworks.com/help/matlab/matlab_oop/custom-copy-behavior.html here a copy:

classdef HandleCopy < matlab.mixin.Copyable
    properties
        Prop1 % Shallow copy
        Prop2 % Handle copy
    end
    methods (Access = protected)
        function cp = copyElement(obj)
            % Shallow copy object
            cp = [e-mail](obj);
            % Get handle from Prop2
            hobj = obj.Prop2;
            % Create default object
            new_hobj = eval(class(hobj));
            % Add public property values from orig object
            HandleCopy.propValues(new_hobj,hobj);
            % Assign the new object to property
            cp.Prop2  = new_hobj;
        end
    end
    methods (Static)
        function propValues(newObj,orgObj)
            pl = properties(orgObj);
            for k = 1:length(pl)
                if isprop(newObj,pl{k})
                    newObj.(pl{k}) = orgObj.(pl{k});
                end
            end
        end
    end
end

How can I edit the code above to get working with my own class? Here my code:

%handleClass.m
classdef handleClass < handle
    properties
        p1
    end
    methods
        function obj = handleClass(in)
            obj.p1 = in;
        end
    end
end

% handleClassHandlePropCopyable1.m
classdef handleClassHandlePropCopyable1 < matlab.mixin.Copyable
    properties
        hndProp handleClass
    end
    
    methods
        function obj = handleClassHandlePropCopyable1(in)
            obj.hndProp = handleClass(in);
        end
    end
    methods (Access = protected)
        function cp = copyElement(obj)
            % Shallow copy object
            cp = [e-mail](obj);
            
            % Copy handle
            % Get handle from hndProp
            hobj = obj.hndProp;
            % Create default object
            eval(class(hobj));
            % Add public property values from orig object
            handleClassHandlePropCopyable1.propValues(new_hobj,hobj);
            % Assign the new object to property
            cp.hndProp  = new_hobj;
        end
    end
    methods (Static)
        function propValues(newObj,orgObj)
            pl = properties(orgObj);
            for k = 1:length(pl)
                if isprop(newObj,pl{k})
                    newObj.(pl{k}) = orgObj.(pl{k});
                end
            end
        end
    end
end

Best regards,

Alex

Answer 1

[Abhaya]

Hi Alessandro,

To copy the properties that contain handles, it is essential to create a new default object of ‘handleClass’ and then assign the property values from the current object to this new object.

In the code you provided, the line ‘eval(class(hobj))’ attempts to create a default object of class ‘handleClass’. However, an error is raised because there is no default constructor defined in ‘handleClass’.  Without a default constructor, MATLAB cannot instantiate the object when no input arguments are provided.

Adding a default constructor will resolve this error by specifying how to initialize the object with default properties.

To modify the constructor of ‘handleClass’, please refer to the code given below.

function obj = handleClass(in) 
    if nargin==0 
       obj.p1=0; 
    else  
       obj.p1=in; 
    end 
end 

In the ‘handleClassHandlePropCopyable1’ class, you can create default object ‘new_hobj’ by using the constructor given above.

 new_hobj = eval(class(hobj)); 

For more information, please refer to the MATLAB documentation given below.

https://www.mathworks.com/help/matlab/matlab_oop/custom-copy-behavior.html

Hope this solves your query.