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--- at:tutorial:objects [2007/07/04 16:27]
jorge
+++ at:tutorial:objects [2013/05/17 20:23]
tvcutsem updated
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-In this section, we explain how the object-oriented programming paradigm is implemented in AmbientTalk.
-===== Objects, fields and methods =====
-In AmbientTalk, objects are not instantiated from
-classes. Rather, they are either created ex-nihilo or by cloning
-and adapting existing objects, like prototypes in the SELF programming language. The definition of such a prototypical object contains a number of fields and methods that represent the object's state and behaviour respectively.
-The following code illustrates the ex-nihilo creation of an object:
-<code>
-> def point := object: {
-    def x := 0;
-    def y := 0;
-    def init(aX,aY) {
-      x := aX;
-      y := aY;
-    };
-    def sumOfSquares() { x*x + y*y };
-  }
-</code>
-As all definitions in AmbientTalk, objects, fields and methods are defined using the **def** keyword. Fields are defined using a ''def name := value'' syntax while methods are defined using a ''name(parameters) {body}'' syntax.
-<note important>
-AmbientTalk not only supports traditional canonical syntax (e.g. ''o.m(a,b,c)'') but also keyworded syntax (e.g. ''o.at: key put: value'') for method definitions and message sends, as in SmallTalk.
-</note>
-In the example above, the state of the ''point'' object is composed of ''x'' and ''y'' fields while its behaviour corresponds to the ''init'' and ''sumOfSquares'' methods.
-===== Sending messages =====
-In AmbientTalk, computation is expressed in terms of objects sending messages to one another. Messages are used to invoke the fields and methods of the objects.
-<code>
-> point.x
->>2
-> point.sumOfSquares()
->>13
-</code>
-This code shows two messages sent to the ''point'' object defined above in this section. The ''x'' message acts as an accessor for the ''x'' field. The ''sumOfSquares'' message selects the ''sumOfSquares'' method and evaluates its body.
-===== Cloning and instantiation =====
-As said before in this section, AmbientTalk objects are created [[objects#Objects,_fields_and_methods|ex-nihilo]] or by cloning and adapting an existing object. The code below shows the instatiation of a new ''point'' object by using the cloning semantics.
-<code>
-> def anotherPoint := point.new(2,3)
-</code>
-Every object understands the message ''new'', which creates a clone (a shallow copy) of the receiver object and initializes the clone by invoking its ''init'' method with the arguments that were passed to new (''aX'' and ''aY'' in the example of the ''point'' object). Hence, the ''init'' method plays the role of “constructor” for AmbientTalk objects. AmbientTalk’s object instantiation protocol closely corresponds to class instantiation in class-based languages, except that the new object is a clone of an existing object, rather than an empty object allocated from a class.
-AmbientTalk also provides a ''clone'' language contsruct which only creates a clone of the receiver object without calling the ''init'' method (as a matter of fact the ''new'' message desribed above does nothing more but invoking this construct and the ''init'' method subsequently).
-<code>
-> def clonedPoint := clone: point
-</code>
-===== Delegation and cloning =====
-AmbientTalk features object inheritance or delegation. By means of delegation, an object can reuse and extend the defintion of another establishing a parent-child relationship. We identify two kinds of delegation relationships: **IS-A** and **SHARE-A**. These relationships define two different semantics for clonning child objects. Whereas clonning a **IS-A** child also clones its parent, **SHARE-A** child shares the parent of the cloned object (see the figure below).
-{{:at:tutorial:isaversussharea.png|:at:tutorial:isaversussharea.png}}
-The following code shows how to extend objects with a **IS-A** relationship. It uses the ''extend: with:'' language construct.
-<code>
-> def point3D := extend: point with: {
-    def z := 0;
-    def sumOfSquares() {
-      super^sumOfSquares() + z*z
-    }
-  }
-</code>
-The following code shows how to extend objects with a **SHARE-A** relationship. It uses the ''share: with:'' language construct.
-<code>
-> def point3D := share: point with: {
-    def z := 0;
-    def sumOfSquares() {
-      super^sumOfSquares() + z*z
-    }
-  }
-</code>
-===== Delegation and dynamic inheritance =====
-The parent of an object is bound to a field named ''super''. The delegation chain defined by an object and its parent (or chain of parents) determines the scope in which the message is looked up. As any field in AmbientTalk objects, the ''super'' field can be dynamically modified.
-<code>
-> def openConnection := object: {...};
-> def closedConnection := object: {...};
-> def connection := object: {
-    def open() {
-      super := openConnection.new();
-    };
-    def close() {
-      super := closedConnection.new();
-    };
-  }
-</code>
-<note important>
-In AmbientTalk, ''self'' and ''super'' indicate the current object and its parent respectively. While the former corresponds to a language keyword the latter is just a field name of the object.
-</note>
-===== First-class delegation =====
-AmbientTalk provides an explicit delegation operator ''^'' (the "caret" or "hat" symbol). The code below illustrates the use of the ''^'' operator in the implementation of the ''init'' method of the ''point3D'' object.
-<code>
-> def point3D := extend: point with: {
-    def z := 0;
-    def init(aX, aY, aZ) {
-      super^init(aX, aY);
-      z := aZ;
-    };
-  }
-</code>
-A message sent to an object using the ''^'' symbol (e.g. to the parent object in the example above) will start the method lookup in this object (and its parents) and then execute the method body in the lexical scope of the message sender (''self'' is bound to the message sender).
-<note important>
-The ''^'' delegation operator does not have the same semantics as the ''.'' (dot) notation. A message sent to ''super'' using the ''.'' notation will not only start the method lookup in the object bound the ''super'' field but also bind the ''self'' pseudo variable to this object.
-</note>
-===== Encapsulation =====
-In AmbientTalk, all fields and methods are "public" via selection. Still, a field or method can be made "private" by means of lexical scoping. The following code shows the definition of an object inside the definition of a function. The fields and methods of this object cannot be accessed directly from outside the funuction.
-<code>
-> def makeObject(hidden) {
-    object: {
-      def foo() { /* use hidden */ }
-    }
-  }
-</code>
-Due to the encapsulation of this object the following instruction fails:
-<code>
-> makeObject(5).hidden;
->>Lookup failure : selector hidden could not be found in
-  <object:5068254>
-</code>

Ambient-Oriented Programming

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