4 Encapsulation

 

There are two major differences of interpretation of encapsulation between SK and S1.

S1 interprets per-class; that is, code may invoke a private method on another object of the same class. SK interprets per-object; it is not legal to dot into private methods. In SK private methods may only be called implicitly on . In this sense, SK has a stronger notion of privacy than S1.

S1 requires that the creation and initialization of an object execute code in the object's class, and that access to private attributes by code outside of the class be strictly impossible, even by pickling - converting entire data structures to and from a common format for persistence or debugging. In this sense, S1 has a stronger notion of privacy than SK.

The implications of these differences are far-reaching:

• Creation of objects.

  Issue:

  How are objects created?

  Choices:

  In S1, an expression may be used to create an object of the class in which it appears, which may then be filled in. The standard S1 idiom is:

   
                
  
     		            ;
  
     		            ;
  
    		             ;
  
    		             
  
              ;
  
  

  Because this is so common, S1 provides syntactic sugar for calling . This sugar also provides type inference. Because is only effective within the class, it isn't possible to create uninitialized objects outside of the class.

  In the S1 idiom, the routine creates a new object and then fills in the attributes. SK can't do this if the attributes are private. Instead, in SK, creates a new, uninitialized object, but this may occur outside of the class. For example creates a new object of class T. There is additionally an operator which creates an object of the same type as an argument. Furthermore the structure expression allows filling in of public attributes in the textual order they were defined in the class. If class T has two attributes and defined in that textual order, creates an object of class T where a is assigned to and b is assigned to . Alternatively, this can be made explicit by .

• Copying objects.

  Issue:

  How are objects copied?

  Choices:

  In S1, there is no special language support for copying; it is done in the same way as object creation. This prevents copying objects without the class getting control, for example to assert unique object ids. In SK, a special predefined method may be used to make a new object with the same attributes as .

Observation for S2: a compromise that would allow the per-object privacy of SK with the simplicity and encapsulation of S1 would be to somehow distinguish special methods such as which would automatically have a new object as .