MATLAB Programming

Designing User Classes in MATLAB
No Input Arguments. If there are no input arguments, the constructor
shouldcreateadefaultobject. Sincetherearenoinputs,youhavenodata
fromwhichtocreatetheobject,soyousimplyinitializetheobject’sdata
structures with empty or default values, call the class function to instantiate
the object, and return the object as the output argument. Support for this
syntax is required for two reasons:
• When loading objects into the workspace, the load function calls the class
constructor with no arguments.
• When creating arrays of objects, MATLAB calls the class constructor to add
objects to the array.
Object Input Argument. If the first input argument in the argument list is
an object of the same class, the constructor should simply return the object.
Use the isa function to determine if an argument is a member of a class. See
“Overloading the + Operator” on page 9-32 for an example of a method that
uses this constructor syntax.
Data Input Arguments. If the input arguments exist and are not objects of
the same class, then the constructor creates the object using the input data.
Of course, as in any function, you should perform proper argument checking
in your constructor function. A typical approach is to use a varargin input
argument and a switch statement to control program flow. This provides an
easy way to accommodate the three cases: no inputs, object input, or the
data inputs used to create an object.
It is in this part of the constructor that you assign values to the object’s data
structure, call the class function to instantiate the object, and return the
object as the output argument. If necessary, place the object in an object
hierarchy using the superiorto and inferiorto functions.
Using the class Function in Constructors
Within a constructor method, you use the class function to associate an object
structure with a particular class. This is done using an internal class tag that
is only accessible using the class and isa functions. For example, this call to
the class function identifies the object p to be of type polynom.
p = class(p,'polynom');
9-11