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at:tutorial:objects [2007/06/27 12:04] jorgeat:tutorial:objects [2007/06/29 13:06] jorge
Line 9: Line 9:
  
 <code> <code>
-> def Point := object: { +> def point := object: { 
     def x := 0;     def x := 0;
     def y := 0;     def y := 0;
-    def init(xCoord,yCoord) { +    def init(aX,aY) { 
-      x := xCoord+      x := aX
-      y := yCoord;+      y := aY;
     };     };
     def sumOfSquares() { x*x + y*y };     def sumOfSquares() { x*x + y*y };
   }   }
->><object:439658> 
 </code> </code>
  
-As all definitions in AmbientTalk, objects are defined using the **def** keyword. In the example above, the state of the ''Point'' object is composed of ''x'' and ''y'' fields and its behaviour corresponds to the ''init'' and ''sumOfSquares'' methods. +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.
-This object can be instantiated to create new points as follows:+
  
-<code+<note important
-> def initialPoint := Point.new(0,0) +AmbientTalk not only supports traditional canonical syntax (e.g. ''o.m(a,b,c)'') but also keyworded syntax (e.g. ''o.atkey put: value'') for method definitions and message sends, as in SmallTalk. 
->><object:13393187> +</note>
-</code>+
  
-Every object understands the message newwhich 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. 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.+In the example abovethe state of the ''point'' object is composed of ''x'' and ''y'' fields while its behaviour corresponds to the ''init'' and ''sumOfSquares'' methods.
  
 ===== Sending messages ===== ===== Sending messages =====
-In AmbientTalk, computation is expressed in terms of object sending messages to one another.+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 ===== ===== 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.
  
 ===== Delegation and Dynamic Inheritance ===== ===== Delegation and Dynamic Inheritance =====
 +AmbientTalk features object inheritance or delegation. By means of delegation, an object can reuse and extend the defintion of another establishing a child-parent relationship. We identify two kinds of delegation relationships: **IS-A** and **SHARE-A**, embodied by the ''extend: with:'' and ''share: with:'' language constructs respectively.
 +
 +The following code shows how to extend objects with a **IS-A** relationship.
 +
 +<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.
 +
 +<code>
 +> def point3D := share: point with: {
 +    def z := 0;
 +    def sumofsquares() {
 +      super.sumofsquares() + z*z
 +    }
 +  }
 +</code>
 +
 +
 +These relationships defines two different semantics for clonning child objects
 +
 +
 + in the way 
 +
 +
 +
 +. While both relationships 
 +
 + Extending an object with a **is-a** relationship 
 +
  
 ===== Delegation and cloning ===== ===== Delegation and cloning =====
at/tutorial/objects.txt · Last modified: 2013/05/17 20:23 by tvcutsem