Using Interfaces

It is important to note that interfaces support a slightly different OOP model than classes. Interfaces provide a less restricted implementation of polymorphism. Object reference polymorphism is based around a specific branch of a hierarchy. Interface polymorphism works across an entire hierarchy. Certainly, interfaces favor encapsulation and provide a looser connection between classes than inheritance. Notice that the most recent OOP languages, from Java to C#, have the notion of interfaces.

Here is the syntax of the declaration of an interface (which, by convention, starts with the letter I ):

type

ICanFly =

interface

[

'{EAD9C4B4-E1C5-4CF4-9FA0-3B812C880A21}'

]

function

Fly: string;

end;

This interface has a GUID (Globally Unique Identifier)—a numeric ID following its declaration and based on Windows conventions. You can generate these identifiers by pressing Ctrl+Shift+G in the Delphi editor.

Once you've declared an interface, you can define a class to implement it:

type

TAirplane =

class

(TInterfacedObject, ICanFly)

function

Fly: string;

end;

The RTL already provides a few base classes to implement the basic behavior required by the IInterface interface. For internal objects, use the TInterfacedObject class I've used in this code.

You can implement interface methods with static methods (as in the previous code) or with virtual methods. You can override virtual methods in inherited classes by using the override directive. If you don't use virtual methods, you can still provide a new implementation in an inherited class by redeclaring the interface type in the inherited class, rebinding the interface methods to new versions of the static methods. At first sight, using virtual methods to implement interfaces seems to allow for smoother coding in inherited classes, but both approaches are equally powerful and flexible. However, the use of virtual methods affects code size and memory.

Now that you have defined an implementation of the interface, you can write some code to use an object of this class, through an interface-type variable:

var

Flyer1: ICanFly;

begin

Flyer1 := TAirplane.Create; Flyer1.Fly;

end;

As soon as you assign an object to an interface-type variable, Delphi automatically checks to see whether the object implements that interface, using the as operator. You can explicitly express this operation as follows :

Flyer1 := TAirplane.Create

as

ICanFly;