MATLAB: Does the SET function behave incorrectly when used with objects of a user defined class

classdefinedinvokeMATLABobjectsetuser

I am developing a GUI in which I want to use objects of my user-defined class. I want to store an object of myclass(user defined class) in a 'UserData' structure of a GUI panel.

I create an object called "myobj" of class "myclass".

When I execute the following commands :

h = uipanel;  
set(h,'UserData',myobj);

MATLAB uses the SET function defined in the class "myclass" instead of invoking the MATLAB SET function on the uipanel handle (h).

Best Answer

There are two ways to work around this issue.

1. Wrap the user defined class object within a MATLAB structure.

h = uipanel;
myclass myobj;
a.obj=myobj;
set(h,'UserData',a);

2. Define the SET function of the user defined class object myobj such that it checks for the first argument to be myobj and if not, dispatches to the builtin set function. The modification needed to the SET function of the user defined class is shown below :

     function varargout = set(obj, varargin)
        if ~isa(obj, 'myobj')
           builtin('set', obj, varargin{:});
        end
     end

Related Solutions

MATLAB: How to use the SUBSREF function to behave as the BUILTIN only for a subset of subscripted references in MATLAB 7.8(R2009a)

This can be acheived by checking for the access information for the Properties and having an if - else statement so that SUBSREF behaves as intended.

An example of the check that you can perform is as follows:

if any(strcmp(s(1).subs,properties(TC))) || any(strcmp(s(1).subs,methods(TC)))
vals = builtin('subsref',TC,s);  
else
error(['''%s'' is not a public property or method of the ''testclass'' class.'],s(1).subs);
end

Another way to get information about the properties of the class is to use meta.class option.

mc = metaclass(TC);
p = [mc.Properties{:}];
   names = {p.Name};
p(strcmp('prop1, names)).GetAccess

MATLAB: Assigning to object variables from within-object functions

MATLAB always makes a copy when you assign an object, or pass an object to a function. Here's another example of the same thing, but with a simple array:

 a = [1 2 3];
 b = a;
 b(2) = 99;
 disp(a)

which prints

     1     2     3

- that is, a has not changed. In just the same way, myObj has been copied and not changed in your code. In general, to change the value of an element of an array or a property of an object, the name of the array or object must appear on the left side of an assignment.

Although this is different to some languages, it's an enormously valuable aspect of MATLAB, and must have saved vast amounts of programmer time over the years. It's consistent, and it entirely avoids a significant class of bugs.

This sounds inefficient, but it isn't - the copy is only made when it's actually necessary.

You can override the default and have the behaviour you are used to by declaring your class to be a handle class. For consistency with the rest of the language this is best avoided, but there are situations where a handle class is called for. For more details see this part of the documentation.

Best Answer

Related Solutions

MATLAB: How to use the SUBSREF function to behave as the BUILTIN only for a subset of subscripted references in MATLAB 7.8(R2009a)

MATLAB: Assigning to object variables from within-object functions

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