Implementation Strategies for Object Orientation

Question

I'm currently learning Smalltalk in the Squeak environment and I'm reading "Squeak - A Quick Trip To ObjectLand". I enter the object-oriented paradigm with some prior knowledge from Python and Java and this sentence from the book on page 36 has made me think:

Smalltalk is a class-based implementation of an object-oriented language.

Short sentence but very interesting. In OO all terms like class, object, instance seem to be well-defined and seem to point to the one and only true meaning and you're likely to come across generic sentences like "objects are instances of a class".
But you hear seldom about implementation strategies. What does implementation of the object-oriented concept mean in this case? Are there implementations of OO languages other than classes?

Answer 1

Javascript is a prototype based implementation of an OO language.

Instead of subclassing a class and creating an instance of that new class, you inherit behaviour by cloning a prototype.

As a historical note I should add that while Javascript is probably the most widely used prototype-using language, the first was David Ungar's and Randall Smith's Self language.

There are several implementations of prototypes floating around for Squeak. I haven't used them, so I can't comment on the libraries.

Answer 2

I never saw, but read about Emerald, that is object-oriented but neither class- nor prototype-based but seems to construct objects "one by one" with the help of a special constructor:

However, Emerald objects do not require a Class object for their creation. In most object-based systems, the programmer first specifies a class object that defines the structure and behavior of all its instances. The class object also responds to new invocations to make new instances.

In contrast, an Emerald object is created by executing an object constructor. An object constructor is an Emerald expression that defines the representation, the operations, and the process of an object.

See Andrew Black, Norman Hutchinson, Eric Jul, and Henry Levy: "Object Structure in the Emerald System".