↑
Main Page

Multiple Inheritance support

The two highlighted lines in the previous code implement inheritance from the

Polygon

class for both

the

Triangle

and the

Rectangle

classes. The reason this works is that the

prototype

object isn’t being

overwritten when using the

inheritFrom()

method; methods are just being added to it. Using this

method, it’s possible to get around the prototype chaining restriction and implement dynamic prototyp-

ing the way it is intended.

Multiple Inheritance support

One of the most useful features of the zInherit library is its capability to support multiple inheritance,

which is not available using prototype chaining. Again, the key fact that makes this possible is that

inheritFrom()

doesn’t replace the

prototype

object.

The

inheritFrom()

method must be used in combination with object masquerading in order to inherit

properties and methods. Consider the following example:

function ClassX() {

this.messageX = “This is the X message. “;

if (typeof ClassX._initialized == “undefined”) {

ClassX.prototype.sayMessageX = function () {

alert(this.messageX);

};

ClassX._initialized = true;

}

function ClassY() {

this.messageY = “This is the Y message. “;

if (typeof ClassY._initialized == “undefined”) {

ClassY.prototype.sayMessageY = function () {

alert(this.messageY);

};

ClassY._initialized = true;

}

Both

ClassX

and

ClassY

are small classes, each with one property and one method. Suppose you now

have

ClassZ

that needs to inherit from both. The class can be defined like this:

function ClassZ() {

ClassX.apply(this);

ClassY.apply(this);

this.messageZ = “This is the Z message. “;

if (typeof ClassZ._initialized == “undefined”) {

ClassZ.prototype.inheritFrom(ClassX);

ClassZ.prototype.inheritFrom(ClassY);

119

Inheritance

07_579088 ch04.qxd 3/28/05 11:36 AM Page 119