Eschatology

Here’s another interesting thing I’ve learned about Clascal and Object Pascal: It went through exactly the same evolution from combining object allocation & initialization to separating them that Objective-C did a decade later!

In early 1983 Clascal, classes were expected to implement a New method as a function returning that type, taking zero or more parameters, and returning an instance of that type by assigning to SELF—sound familiar? This was always implemented as a “standard” method (one without dynamic dispatch) so you couldn’t call the wrong one. A cited advantage of this is that it would prevent use of the standard Pascal built-in New() within methods—which I suspect turned out not to be what people wanted, since it would prevent interoperability.

A class could also choose to implement an OVERRIDE of the also-included Free method to release any resources it had acquired, like file handles or other object instances. And each overridden Free method had to include SUPERSELF.Free; after it did so in order to ensure that its superclass would also release any resources it had acquired.

INTERFACE

  TYPE

    Object = SUBCLASS OF NIL
      FUNCTION New: Object; STANDARD;
      PROCEDURE Free; DEFAULT;
    END;

    Person = SUBCLASS OF Object
      name: String;
      FUNCTION New(name: String): Person; STANDARD;
      PROCEDURE Free; OVERRIDE;
    END;

  VAR

    gHeap: Heap;

IMPLEMENTATION

  METHODS OF Object;

    FUNCTION New{: Object}
    BEGIN
      SELF := Object(HAllocate(gHeap, Size(THISCLASS)));
    END;

    PROCEDURE Free
    BEGIN
      HFree(Handle(SELF));
    END;

  END;

  METHODS OF Person;

    FUNCTION New{(theName: String): Person;}
    BEGIN
      SELF := Person(HAllocate(gHeap, Size(THISCLASS)));
      IF SELF <> NIL THEN
        name := theName.Clone;
    END;

    PROCEDURE Free
    BEGIN
      name.Free;
      SUPERSELF.Free;
    END;
  END;

By mid-1984, Clascal changed this to the CREATE method, which was declared as ABSTRACT in the base class. Note that it still doesn’t use the standard Pascal built-in New() to create object instances. However, it takes a potentially-already-initialized object so that it’s easier for a subclass to call through to its superclass for initialization, since CREATE is still not a dynamically-dispatched method. Also, instead of referencing a global variable for a heap zone in which to perform allocation, it takes the heap zone, providing some amount of locality-of-reference that may be helpful to the VM system.

There was also a change in style to prefix class names with T.

INTERFACE

  TYPE

    TObject = SUBCLASS OF NIL
      FUNCTION CREATE(object: TObject; heap: THeap): TObject; ABSTRACT;
      PROCEDURE Free; DEFAULT;
    END;

    TPerson = SUBCLASS OF TObject
      name: TString;
      FUNCTION CREATE(theName: TString; object: TObject; heap: THeap): TPerson; STANDARD;
      PROCEDURE Free; OVERRIDE;
    END;

IMPLEMENTATION

  METHODS OF TObject;

    PROCEDURE Free
    BEGIN
      FreeObject(SELF);
    END;

  END;

  METHODS OF TPerson;

    FUNCTION CREATE{(theName: TString; object: TObject; heap: THeap): TPerson;}
    BEGIN
      IF object = NIL
        object := NewObject(heap, THISCLASS);
      SELF := TPerson(object);
      WITH SELF DO
        name := theName.Clone(heap);
    END;

    PROCEDURE Free
    BEGIN
      name.Free;
      SUPERSELF.Free;
    END;
  END;

This is starting to look even more familiar to Objective-C developers, isn’t it?

The final form of the language, Object Pascal, actually backed off on the Smalltalk terminology a little bit and renamed “classes” to “objects” and went so far as to introduce an OBJECT keyword used for defining a class. It also changed SUPERSELF. to INHERITED—yes, with whitespace instead of a dot!—as, again, developers new to OOP found “superclass” confusing.

Object Pascal also, at long last, adopted the standard Pascal built-in New() to perform object allocation (along with its counterpart Free() for deallocation) directly instead of introducing a separate function for it, since the intent can be inferred by the compiler from the type system. It also removed the need to use the METHODS OF construct to add methods, instead just prefixing the method with the class name and a period.

The final major change from Clascal to Object Pascal is that, with New() used for object allocation, the CREATE methods were changed into initialization methods instead since they just initialize the object after its allocation. They were also made procedures rather than functions returning values, and since the standard Pascal built-in New() is being used they no longer take a potentially-already-allocated object nor do they take a heap zone in which to perform the allocation. The convention is that for a class TFoo the initialization method has the form IFoo.

There was also another stylistic change, prepending field names with f to make them easy to distinguish from zero-argument function methods at a glance.

There was also a switch from not including the parameter list in the IMPLEMENTATION section to including it directly instead of in a comment.

Here’s what that looks like:

INTERFACE

  TYPE

    TObject = OBJECT
      PROCEDURE IObject; ABSTRACT;
      PROCEDURE Free; DEFAULT;
    END;

    TPerson = OBJECT(TObject)
      fName: TString;
      PROCEDURE IPerson(theName: TString); STANDARD;
      PROCEDURE Free; OVERRIDE;
    END;

IMPLEMENTATION

    PROCEDURE TObject.Free
    BEGIN
      Free(SELF);
    END;

    PROCEDURE TPerson.IPerson(theName: TString)
    BEGIN
      fName := theName.Clone();
    END;

    PROCEDURE TPerson.Free
    BEGIN
      fName.Free;
      INHERITED Free;
    END;

Based on the documentation I’ve read, it wouldn’t surprise me if the only reason initialization methods aren’t consistently named Initialize is that the language design didn’t support an OVERRIDE of a method using a different parameter list.