Chapter: Objects and Classes
...Section: Creating Objects

If the preceding sections are clear, then the creation of class instances such as aPoint and abPoint should be straightforward. To summarize:

• We create an object or instance of a specific class type by calling the class name as a procedure. In fact, we can see that the value of a class such as Point-class is indeed a procedure:

  > Point-class
  #procedure 
  >
  

  For example if we have a class definition called My-class, the Scheme entity My-class is a procedure that constructs and then returns a new instance of that class. If we have class parameters in our definition we simply provide corresponding arguments to the construction function. We did this with Point-class when we provide the initial x and y values.

  Note that an instance is just another Scheme entity (in fact it is represented as a list) and thus can be passed to procedures and stored in data structures. In the Point-class example we simply created two instances using define:

  (define aPoint (Point-class 2 4))
  

  We could also store point objects in a list. For example try out the following:

  > (define two-points-list (list (Point-class 0 0) (Point-class 0 0)))
  > (call location? (car two-points-list))
  (0 0)
  > (call moveTo (cadr two-points-list) 19 45)
  (19 45)
  

  

  Q. 2

  What does

  (map (lambda (obj) (call moveRel obj 3 4)) two-points-list)
  

  do? Why might this be useful?

  ---

  

• To call methods of a specific instance we use the keyword call. The syntax is:

  (call METHOD-NAME OBJECT-NAME ARG1 ARG2 ... ARGN)
  

  where METHOD-NAME is the method to be called, OBJECT is an instance of a class and ARG1 ARG2 ... ARGN are the expressions providing values that are passed to the method.

Exercise 2

• Check that the arguments supplied with the moveTo message are numbers.
• Now, instead of just checking that the arguments are numbers, check that they are integers between 0 and 1023 (inclusive).