at:tutorial:objects
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at:tutorial:objects [2007/07/02 13:44] – jorge | at:tutorial:objects [2007/07/19 07:59] – * 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 '' |
+ | |||
+ | < | ||
+ | > anotherPoint.x | ||
+ | >> 2 | ||
+ | > Point.x | ||
+ | >> 0 | ||
+ | > anotherPoint.x := 3 | ||
+ | >> nil | ||
+ | > Point.x | ||
+ | >> 0 | ||
+ | </ | ||
+ | |||
+ | < | ||
+ | AmbientTalk's object instantiation protocol closely corresponds to class instantiation in class-based languages. The major difference lies in the evaluation context of the '' | ||
+ | </ | ||
- | AmbientTalk also provides a '' | + | AmbientTalk also provides a '' |
< | < | ||
- | > def clonedPoint := clone: | + | def clonedPoint := clone: |
+ | > clonedPoint.x | ||
+ | >> 0 | ||
+ | > clonedPoint.x := 2 | ||
+ | >> nil | ||
+ | > Point.x | ||
+ | >> 0 | ||
</ | </ | ||
- | ===== Delegation and cloning | + | ===== 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: | + | |
+ | 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, | ||
- | The following code shows how to extend objects with a **IS-A** relationship. | + | An **IS-A** |
< | < | ||
- | > def point3D | + | def Point3D |
- | def z := 0; | + | def z := 0; |
- | def sumOfSquares() { | + | def sumOfSquares() { |
- | super^sumOfSquares() + z*z | + | super^sumOfSquares() + z*z; |
- | } | + | }; |
- | } | + | } |
</ | </ | ||
- | The following code shows how to extend objects with a **SHARE-A** relationship. It uses the '' | + | In this example, '' |
+ | |||
+ | A **SHARES-A** relationship between two objects signifies that an object only delegates to another object purely for reasons of code or state 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 | ||
< | < | ||
- | > def point3D | + | def Collection |
- | def z := 0; | + | def elements |
- | def sumOfSquares() { | + | ... |
- | | + | } |
- | } | + | |
- | | + | |
</ | </ | ||
- | ===== Delegation and dynamic inheritance | + | In this code example, the '' |
- | The parent of an object is bound to a field named '' | + | |
+ | 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 | ||
+ | |||
+ | In AmbientTalk, | ||
+ | |||
+ | Because '' | ||
< | < | ||
- | > def openConnection := object: {...}; | + | def openConnection := object: |
- | > def closedConnection := object: {...}; | + | def send(msg) |
- | > def connection := object: { | + | }; |
- | def open() { | + | def closedConnection := object: |
- | super := openConnection.new(); | + | def send(msg) |
- | }; | + | }; |
- | def close() { | + | def connection := object: |
- | super := closedConnection.new(); | + | def init() |
- | }; | + | |
- | } | + | }; |
+ | | ||
+ | super := openConnection; | ||
+ | }; | ||
+ | def close() { | ||
+ | super := closedConnection; | ||
+ | }; | ||
+ | } | ||
</ | </ | ||
- | < | + | In the above example, the '' |
- | In AmbientTalk, | + | |
+ | < | ||
+ | In AmbientTalk, | ||
</ | </ | ||
- | ===== First-class | + | ===== First-class |
- | AmbientTalk provides | + | |
+ | AmbientTalk provides | ||
< | < | ||
- | > def point3D | + | def Enumerable |
- | def z := 0; | + | def collect: closure |
- | | + | def c := self.new([]); |
- | | + | |
- | z := aZ; | + | |
}; | }; | ||
- | | + | c; |
+ | | ||
+ | }; | ||
+ | def Array := object: { | ||
+ | def elements := []; | ||
+ | def init(a) { elements := a; }; | ||
+ | def add(v) { elements := elements + [v]; self }; | ||
+ | def collect: closure { | ||
+ | Enumerable^collect: | ||
+ | }; | ||
+ | def each: clo { | ||
+ | 1.to: elements.length + 1 do: { |i| | ||
+ | clo(elements[i]); | ||
+ | }; | ||
+ | }; | ||
+ | }; | ||
</ | </ | ||
- | A message sent to an object using this symbol (e.g. to the parent | + | A message sent to an object using the '' |
+ | |||
+ | < | ||
+ | > Array.add(1).add(2).add(3) | ||
+ | >> < | ||
+ | > def c := Array.collect: | ||
+ | >> < | ||
+ | > c.each: { |v| system.print(v)} | ||
+ | 234 | ||
+ | >> | ||
+ | </ | ||
+ | |||
+ | Of course, | ||
+ | |||
+ | Having described the semantics of '' | ||
+ | |||
+ | < | ||
+ | def Point3D := extend: Point with: { | ||
+ | def z := 0; | ||
+ | def init(aX, aY, aZ) { | ||
+ | super^init(aX, | ||
+ | z := aZ; | ||
+ | }; | ||
+ | }; | ||
+ | </ | ||
+ | |||
+ | <note warning> | ||
+ | AmbientTalk, | ||
+ | |||
+ | Keep in mind, however, that '' | ||
+ | </ | ||
===== Encapsulation ===== | ===== Encapsulation ===== | ||
- | In AmbientTalk, all fields and methods are " | + | |
+ | AmbientTalk | ||
< | < | ||
- | > def makeObject(hidden) { | + | def makeBankAccount(balance) { |
- | object: { | + | object: { |
- | def foo() { /* use hidden */ } | + | def deposit(amnt) { |
- | } | + | balance := balance + amnt; |
+ | " | ||
+ | }; | ||
} | } | ||
+ | } | ||
</ | </ | ||
- | Due to the encapsulation of this object the following | + | Because |
< | < | ||
- | > makeObject(5).hidden; | + | > makeBankAccount(100).balance; |
- | >> | + | >> |
< | < | ||
</ | </ |
at/tutorial/objects.txt · Last modified: 2013/05/17 20:23 by tvcutsem