Differences

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--- at:tutorial:objects [2007/06/29 09:08] – jorge
+++ at:tutorial:objects [2007/07/05 14:01] – jorge
@@ Line 18: / Line 18: @@
     def sumOfSquares() { x*x + y*y };
   }
->><object:439658>
 </code>
@@ Line 46: / Line 45: @@
 <code>
 > def anotherPoint := point.new(2,3)
->><object:13393187>
 </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.
-===== Delegation and Dynamic Inheritance =====
+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).
-AmbientTalk features object inheritance or delegation. Through delegation an object can rely upon another to provide a specified set of functionalities. We identify two different relationships between objects that
+<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).
-===== First-class Delegation =====
+{{: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 warning>
+The delegation operator does not have the same semantics as the dot notation. A message sent to ''super'' using the dot 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>