at:tutorial:objects
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at:tutorial:objects [2007/06/29 13:28] – jorge | at:tutorial:objects [2007/07/09 21:38] – rewrote 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 Point := object: { |
- | def x := 0; | + | def x := 0; |
- | def y := 0; | + | def y := 0; |
- | def init(aX,aY) { | + | def init(aX,aY) { |
- | x := aX; | + | x := aX; |
- | y := aY; | + | y := aY; |
- | }; | + | }; |
- | def sumOfSquares() { x*x + y*y }; | + | def sumOfSquares() { x*x + y*y }; |
- | } | + | } |
</ | </ | ||
- | As all definitions in AmbientTalk, objects, fields and methods are defined using the **def** keyword. Fields are defined using a '' | + | The above code defines an // |
- | <note important> | + | In the example above, the state of the point object is composed of '' |
- | AmbientTalk not only supports traditional canonical syntax (e.g. '' | + | |
- | </ | + | |
- | In the example above, the state of the '' | + | < |
+ | As already explained in the [[at: | ||
+ | |||
+ | For Smalltalk/ | ||
+ | </ | ||
===== Sending messages ===== | ===== Sending messages ===== | ||
Line 32: | Line 34: | ||
< | < | ||
- | > point.x | + | > Point.x |
- | >>2 | + | >>0 |
- | > point.sumOfSquares() | + | > Point.sumOfSquares() |
- | >>13 | + | >>0 |
</ | </ | ||
- | This code shows two messages sent to the '' | + | This code shows two messages sent to the point object defined above. The '' |
+ | |||
+ | Note that the " | ||
===== Cloning and instantiation ===== | ===== Cloning and instantiation ===== | ||
- | As said before in this section, AmbientTalk objects are created [[objects# | + | As noted above, AmbientTalk objects are created [[# |
< | < | ||
- | > def anotherPoint := point.new(2,3) | + | def anotherPoint := Point.new(2,3) |
</ | </ | ||
- | Every object understands the message '' | + | Every object understands the message '' |
- | ===== 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 child-parent 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. | + | > anotherPoint.x |
+ | >> 2 | ||
+ | > Point.x | ||
+ | >> 0 | ||
+ | > anotherPoint.x | ||
+ | >> nil | ||
+ | > Point.x | ||
+ | >> 0 | ||
+ | </ | ||
- | The following code shows how to extend objects with a **IS-A** relationship. It uses the '' | + | < |
+ | AmbientTalk' | ||
+ | </ | ||
+ | |||
+ | AmbientTalk also provides a '' | ||
< | < | ||
- | > def point3D | + | def clonedPoint := clone: Point |
- | def z := 0; | + | > clonedPoint.x |
- | def sumofsquares() { | + | >> 0 |
- | super.sumofsquares() + z*z | + | > 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 | ||
+ | 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 and dynamic inheritance ===== | ||
+ | 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(); | ||
+ | }; | ||
} | } | ||
</ | </ | ||
- | The following code shows how to extend objects with a **SHARE-A** relationship. It uses the '' | + | <note important> |
+ | In AmbientTalk, | ||
+ | </ | ||
+ | |||
+ | ===== First-class delegation ===== | ||
+ | AmbientTalk provides an explicit delegation operator '' | ||
< | < | ||
- | > def point3D := share: point with: { | + | > def point3D := extend: point with: { |
def z := 0; | def z := 0; | ||
- | def sumofsquares() { | + | def init(aX, aY, aZ) { |
- | super.sumofsquares() + z*z | + | super^init(aX, aY); |
- | } | + | |
+ | }; | ||
} | } | ||
</ | </ | ||
+ | A message sent to an object using the '' | ||
- | ===== Delegation and dynamic inheritance ===== | + | <note warning> |
- | + | The delegation operator does not have the same semantics as the dot notation. A message sent to '' | |
- | ===== First-class Delegation ===== | + | </ |
===== 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