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--- at:tutorial:objects [2007/07/09 21:13]
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+++ at:tutorial:objects [2013/05/17 20:23]
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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, in the spirit of prototype-based programming such as in the SELF programming language. The definition of 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>
-The above code defines an //ex-nihilo// created point object and binds it to the variable ''Point''. The object itself does not carry a name (i.e. it is "anonymous"). Like all definitions in AmbientTalk, fields and methods are defined using the ''def'' keyword. Fields are defined using a ''def name := value'' syntax while methods are defined using a ''def name(parameters) {body}'' syntax.
-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.
-<note>
-As already explained in the [[at:tutorial:basic|basic programming]] part of the tutorial, 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''). Keyworded syntax can be used both for method definitions and for message sends.
-For Smalltalk/Self programmers: note that a keyworded message send does require a message sending operator (like ''.'') in between the receiver and the message, which is different from Smalltalk and Self. As will be described in later chapters, AmbientTalk features more than one message sending operator, so the programmer must explicitly specify which one to use.
-</note>
-===== 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
->>0
-> Point.sumOfSquares()
->>0
-</code>
-This code shows two messages sent to the point object defined above. The ''x'' message acts as an accessor for the ''x'' field. The ''sumOfSquares'' message looks up the ''sumOfSquares'' method in the object and applies it.
-Note that the "prototypical" point object defined above can act as a stand-alone object. This is different from a class in a class-based language, which often requires the use of ''static'' fields or methods to be used stand-alone.
-===== Cloning and instantiation =====
-As noted above, AmbientTalk objects are created [[#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 //instantiating// the ''Point'' prototype:
-<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 code). Hence, the ''init'' method plays the role of "constructor" for AmbientTalk objects. Hence, in the code above, ''anotherPoint'' shares its methods with its ''Point'' prototype, but it has its own set of fields:
-<code>
-> anotherPoint.x
->> 2
-> Point.x
->> 0
-> anotherPoint.x := 3
->> nil
-> Point.x
->> 0
-</code>
-<note>
-AmbientTalk's object instantiation protocol closely corresponds to class instantiation in class-based languages. The major difference lies in the evaluation context of the ''init'' method: in a class-based language, the constructor is ran in the context of an empty object, freshly allocated from the class blueprint. In AmbientTalk, the ''init'' method is ran in the context of a shallow copy of an original object. Hence, in the ''init'' method, fields do not necessarily contain ''nil'' values: they have the value of the clonee. This can sometimes be useful to express the state of a clone as a delta w.r.t. the state of its clonee.
-</note>
-AmbientTalk also provides a ''clone:'' construct 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
-> clonedPoint.x
->> 0
-> clonedPoint.x := 2
->> nil
-> Point.x
->> 0
-</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 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>

Ambient-Oriented Programming

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