at:tutorial:objects
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at:tutorial:objects [2007/06/27 11:34] – jorge | at:tutorial:objects [2007/07/10 12:39] – * tvcutsem | ||
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In AmbientTalk, | In AmbientTalk, | ||
classes. Rather, they are either created ex-nihilo or by cloning | classes. Rather, they are either created ex-nihilo or by cloning | ||
- | and adapting existing objects, | + | and adapting existing objects, |
The following code illustrates the ex-nihilo creation of an object: | The following code illustrates the ex-nihilo creation of an object: | ||
< | < | ||
- | > def point := object: { def x := 0; def y := 0; def sumofsquares() { x*x + y*y }; } | + | def Point := object: { |
- | >>< | + | |
+ | | ||
+ | | ||
+ | x := aX; | ||
+ | y := aY; | ||
+ | }; | ||
+ | def sumOfSquares() { x*x + y*y }; | ||
+ | } | ||
</ | </ | ||
+ | The above code defines an // | ||
+ | |||
+ | In the example above, the state of the point object is composed of '' | ||
+ | |||
+ | < | ||
+ | As already explained in the [[at: | ||
+ | |||
+ | For Smalltalk/ | ||
+ | </ | ||
===== Sending messages ===== | ===== Sending messages ===== | ||
- | In AmbientTalk, | + | In AmbientTalk, |
+ | |||
+ | < | ||
+ | > Point.x | ||
+ | >>0 | ||
+ | > Point.sumOfSquares() | ||
+ | >>0 | ||
+ | </ | ||
+ | |||
+ | This code shows two messages sent to the point object defined above. The '' | ||
+ | |||
+ | Note that the " | ||
===== Cloning and instantiation ===== | ===== Cloning and instantiation ===== | ||
+ | As noted above, AmbientTalk objects are created [[# | ||
+ | < | ||
+ | def anotherPoint := Point.new(2, | ||
+ | </ | ||
+ | Every object understands the message '' | ||
- | ===== Delegation and Dynamic Inheritance ===== | + | < |
+ | > anotherPoint.x | ||
+ | >> 2 | ||
+ | > Point.x | ||
+ | >> 0 | ||
+ | > anotherPoint.x := 3 | ||
+ | >> nil | ||
+ | > Point.x | ||
+ | >> 0 | ||
+ | </ | ||
- | ===== Delegation and cloning ===== | + | < |
+ | AmbientTalk' | ||
+ | </ | ||
- | ===== First-class Delegation ===== | + | AmbientTalk also provides a '' |
+ | |||
+ | < | ||
+ | def clonedPoint := clone: Point | ||
+ | > clonedPoint.x | ||
+ | >> 0 | ||
+ | > clonedPoint.x := 2 | ||
+ | >> nil | ||
+ | > Point.x | ||
+ | >> 0 | ||
+ | </ | ||
+ | |||
+ | ===== Delegation and Cloning ===== | ||
+ | |||
+ | In order to support code reuse and modular extensions between objects, AmbientTalk features // | ||
+ | |||
+ | Delegation implies that, if a message is sent to an object, but that object has no definition for the message' | ||
+ | |||
+ | AmbientTalk distinguishes between **two kinds** of delegation relationships, | ||
+ | |||
+ | An **IS-A** delegation relationship between two objects signifies that the child object " | ||
+ | |||
+ | < | ||
+ | def Point3D := extend: Point with: { | ||
+ | def z := 0; | ||
+ | def sumOfSquares() { | ||
+ | super^sumOfSquares() + z*z | ||
+ | }; | ||
+ | } | ||
+ | </ | ||
+ | |||
+ | In this example, '' | ||
+ | |||
+ | A **SHARES-A** relationship between two objects signifies that an object only delegates to another object purely for reasons of code sharing. The delegation link has no other semantics, and conceptually both parent and child can exist without one another. | ||
+ | |||
+ | The following code shows how to extend objects with a **SHARES-A** delegation relationship. It uses the '' | ||
+ | |||
+ | < | ||
+ | def Collection := share: Enumerable with: { | ||
+ | def elements := []; | ||
+ | ... | ||
+ | } | ||
+ | </ | ||
+ | |||
+ | In this code example, the '' | ||
+ | |||
+ | The **IS-A** and **SHARES-A** delegation relationships differ in their semantics for cloning child objects. Whereas cloning an **IS-A** child also clones its parent, a **SHARES-A** child shares its parent object with the clonee (see the figure below). | ||
+ | |||
+ | {{: | ||
+ | |||
+ | This cloning semantics reinforces the semantics of **IS-A** as promoting a unique link between a parent and a child object. **IS-A** delegation most closely corresponds to class-based inheritance. | ||
+ | |||
+ | ===== Delegation | ||
+ | The parent of an object is bound to a field named '' | ||
+ | |||
+ | < | ||
+ | > def openConnection := object: {...}; | ||
+ | > def closedConnection := object: {...}; | ||
+ | > def connection := object: { | ||
+ | def open() { | ||
+ | super := openConnection.new(); | ||
+ | }; | ||
+ | def close() { | ||
+ | super := closedConnection.new(); | ||
+ | }; | ||
+ | } | ||
+ | </ | ||
+ | |||
+ | <note important> | ||
+ | In AmbientTalk, | ||
+ | </ | ||
+ | |||
+ | ===== First-class delegation ===== | ||
+ | AmbientTalk provides an explicit delegation operator '' | ||
+ | |||
+ | < | ||
+ | > def point3D := extend: point with: { | ||
+ | def z := 0; | ||
+ | def init(aX, aY, aZ) { | ||
+ | super^init(aX, | ||
+ | z := aZ; | ||
+ | }; | ||
+ | } | ||
+ | </ | ||
+ | |||
+ | A message sent to an object using the '' | ||
+ | |||
+ | <note warning> | ||
+ | The delegation operator does not have the same semantics as the dot notation. A message sent to '' | ||
+ | </ | ||
===== Encapsulation ===== | ===== Encapsulation ===== | ||
+ | In AmbientTalk, | ||
+ | < | ||
+ | > def makeObject(hidden) { | ||
+ | object: { | ||
+ | def foo() { /* use hidden */ } | ||
+ | } | ||
+ | } | ||
+ | </ | ||
+ | |||
+ | Due to the encapsulation of this object the following instruction fails: | ||
+ | |||
+ | < | ||
+ | > makeObject(5).hidden; | ||
+ | >> | ||
+ | < | ||
+ | </ |
at/tutorial/objects.txt · Last modified: 2013/05/17 20:23 by tvcutsem