Object oriented programming using the old (pre 2008a) syntax

We now describe how to create classes and use objects using the old OO style. Although this was replaced in the 2008a edition, it is still supported. The old framework is much more limited and cumbersome but in some ways it is more consistent with the rest of the Matlab syntax. It does bridge the inherent connection with structs much more explicitly, for example.

Keep in mind that if you write classes in the old style, the following features, available in the new framework, are not supported: protected, abstract, static/constant, sealed, or hidden methods or properties; single file class definitions; events; handle classes; packages; special set and get methods; object.method() syntax; or meta-classes. However, you can still perform operator overloading; in fact, if anything, it plays a more prominent role.

Defining a Class
Property Access
Subsref & Subsasgn
Displaying Objects
Methods
Private Methods
Inheritance Syntax
Dynamic Dispatch

Defining a Class

To begin, every method must be saved in its own file, including the constructor, and these files must be stored in a directory with the same name as the class but preceded by the @ symbol as in @date. The parent directory containing this @ directory, must be on the Matlab path.

Every class needs to have a constructor method, which constructs the objects of the class. The constructor must have the same name as the class and be saved in an m-file by the same name, (e.g. create a function called date and save it in date.m stored in the @date directory). Here is a sample definition:

function obj = date(minute, hour, day, month, year)

We define the properties of the class by creating a struct with fields by the same name and we must add one field for every property, even if we leave the data initially blank, (i.e. equal to []). We then convert the struct to an object by using the class() function.

 function obj = mydate(minute, hour, day, month, year)
 % constructor saved in file date.m

       obj = struct;
       obj.minute = [];                % leave blank for whatever reason
       obj.hour = hour;
       obj.day = day;
       obj.month = month;
       obj. year = year;
       obj = class(obj,'date');
 end

We can create objects by calling the constructor.

 obj = mydate(12,2,3,11,2008);         % create a new date object

Note that in earlier versions of Matlab, special care had to be taken to allow for objects to be saved and loaded from disk. This is no longer the case, even when using the old syntax. If you are using an older version of Matlab and run into this problem, check out the following site for a work around: http://www.cs.ubc.ca/~murphyk/Software/matlabObjects.html

When changes are made to a class definition, existing objects of that class are not automatically updated; they must be cleared and recreated. In older version of Matlab, it was necessary to use the clear classes command after making definitional changes. If you experience odd behavior, this can sometimes help. Also in older versions, date objects, for example, could not be created if @mydate were the current directory. Moving up one level may do the trick.

Property Access

Technically, the creation of the constructor method in the @ directory is sufficient to create a class and objects from it, however, our date objects are not yet very useful because Matlab protects the properties from access outside of the class. They are effectively private. Only class methods can access properties in the old style, which includes any function stored inside of the @ directory.

Subsref & Subsasgn

As we mentioned in the section on Operator Overloading ,every use of a dot such as obj.prop, is actually shorthand for a call to either the subsasgn() or the subrsref() function just like calls involving the '+' operator are shorthand for the plus() function. Subsref is called in indexing operations, (i.e. when data is requested) and subsasgn is used in assignment operations, (i.e. when data is changed). These same functions are called when parentheses are used as well, as in a = date(3,2) , or date(3,2) = 5 ,(and similarly with {} braces when dealing with structs).

One of the most powerful features of OOP in Matlab is the ability to overload such functions so that they perform customized behavior when used with objects of our class. If we create functions by these names and store them inside of the @ directory containing our class files, these functions will be executed when calls involving a . or () are made as opposed to the default implementations. We can then use these as our getter and setter methods so that users can access and set object properties with the same syntax as they would struct fields, for instance. More complicated behavior is certainly possible too.

The default behavior of these functions, for objects, is to restrict access to properties completely, but we can 'recover' the lax behavior we enjoyed with structs and effectively make the all of the properties public with the following simple implementations. We make use the the builtin() function to do so, which bypasses the redefined or 'overloaded' object versions, (versions overriden by Mathworks in this case). In general builtin() can be very useful when a function has been overloaded but you want the original behavior. Here we simply pass the inputs to the functions directly to builtin(). For more detail on these functions, (e.g. on the S arguement), see the Operator Overloading section above.

  function obj = subsasgn(obj, S, value)
     obj = builtin('subsasgn', obj, S, value);
  end

  function value = subsref(obj, S)
      value = builtin('subsref', obj, S);
  end

With these functions in place, properties of our object can then be accessed and assigned values as follows.

d1 = mydate(0,3,27,2,1998);        % construct a new object
min = d1.minute                  % get the minute property using . notation
d1.hour = 4;                     % set the hour property using the . notation

min =
     0

Displaying Objects

In the old OO style, objects are not displayed for you. You can overload the display() function so that basic info about the object is displayed in calls that do not suppress output with a semicolon. Here is a simple implementation. The struct() function when applied to an object, converts that object into a struct with fields corresponding to the properties of the object. Struct can also be used with objects created via the new OO framework.

 function display(obj)
     disp(sprintf('%s object', class(obj)))
     disp(struct(obj))
 end

The section on Operator Overloading discusses several other functions that can be overloaded.

Methods

To define methods, we simply create functions with definitions like the following and save them in the @ directory. The first argument should be an object of the containing class. Methods that change object properties must return the object, (because they are passed by value). We do not have the option in the old framework, as we do with the new, of writing classes whose objects can be passed by reference.

 obj = rollDay(obj,3);

 function obj = rollDay(obj,numdays)
      obj.day = obj.day + numdays;
 end

Similarly, calls such as obj2 = obj, result in a copy of the object obj stored in the variable obj2.

If you choose not to overload subsasgn() and subsref() but still want to provide outside access to certain properties you will need to write java style get and set functions as in the following. This approach, while less concise, does maintain the separation between implementation and interface without necessitating complicated implementations of subasgn and subsref.

 function day = getDay(obj)
    % we could change this code later without affecting users of getDay.
    day = obj.day;
 end

 function obj = setDay(obj,newDay)
     obj.day = newDay;
 end

Private Methods

Private methods are functions that can only be called by other methods of the class, not end users; they are not intended to act as part of the class interface. To make a method private in the old OO framework, create a sub-directory called 'private' in the @ directory and save it inside. You can call private methods, (from within other class methods) as you would any other method.

Inheritance Syntax

Inheritance in the old OO framework is quite limited and you cannot subclass built in classes like handle. Superclasses are specified as inputs to the class() function called in the constructor. You must pass in instances of the superclass objects.

 p1 = parent1(); p2 = parent2();   % create instances of the parent classes
 obj = class(obj,'date',p1,p2);    % pass these instances into the class function

Type doc class for additional calling sequences including an option to create an array of objects from an array of structs.

For more general information on inheritance, see the several related sections written regarding the new OO style.

Dynamic Dispatch

The old OO framework supports dynamic dispatch in much the same way as the new framework. When multiple objects of different types are passed to a method, the superior-inferior relation is used to determine which method to invoke, just as with the new OO design. However, in the old style, we must specify this relation by using the using the inferiorto() ,and superiorto() functions from within the constructor as in the following.

inferiorto('class1','class2');

Again, when the superiority of two classes is the same, the left most takes precedence.