How To Use Interfaces

Copy the text below and it will demonstrate how to use interfaces to write less code, avoid code repetition,..., this is a fully working program.

program interfacesygenerics;

{$mode objfpc}{$H+}

const
  STestInterface = '{3FB19775-F5FA-464C-B10C-D8137D742088}';

type
  ITestInterface = interface
    [STestInterface]
    procedure DoSomething;
    procedure DoItAll;
  end;

  { Base class Start }
  TBaseObject = class(TInterfacedObject)
    procedure DoAll;
  end;

  procedure TBaseObject.DoAll;
  begin
    Writeln('TBaseObject DoAll !');
  end;
  { Base class End }

type

  { THook Start}
  //Interface implementation on a derived class
  THook = class(TBaseObject, ITestInterface)
    procedure DoSomething;
    //Contrary to what is stated on the FPC reference manual
    //it's now possible to use aliases to indicate
    //interface implementations
    procedure ITestInterface.DoItAll = DoAll;
  end;

  procedure THook.DoSomething;
  begin
    Writeln('THook Doing something !');
  end;
  { THook End}

type

  { TRealClass Start }
  TRealClass = class(TBaseObject, ITestInterface)
  private
    FHook: ITestInterface;
  public
    constructor Create;
    destructor Destroy; override;
    //Here we tell the compiler that our class
    //interface implementation is provided by
    //a member field wich we initialize in this case
    //on the constructor
    property Hook: ITestInterface read FHook implements ITestInterface;
  end;

  constructor TRealClass.Create;
  begin
    //Creating our interface implementor
    FHook := THook.Create;
  end;

  destructor TRealClass.Destroy;
  begin
    //Releasing our interface implementor
    THook(FHook).Free;
    inherited Destroy;
  end;
  { TRealClass End }

type
    //An example of how to use generics to write
    //general use like procedures
    //as a bonus they are namespaced by the classname
    //disadvantage: can't be overloaded
    generic TFakeClass<_GT> = class
      class function gmax(a,b: _GT):_GT;
    end;

    TFakeClassInt = specialize TFakeClass<integer>;
    TFakeClassDouble = specialize TFakeClass<double>;

    class function TFakeClass.gmax(a,b: _GT):_GT;
    begin
         if a > b then result := a
         else result := b;
    end;


var
  R: TRealClass;
  I: ITestInterface;

begin
  Writeln('Using regular interfaces');

  R := TRealClass.Create;

  //callign the interfaced function directly
  R.DoAll;

  //Calling through the interface
  if R.GetInterface(STestInterface, I) then
    begin
      Writeln('Got interface OK. Calling it');
      I.DoSomething;
      I.DoItAll;
    end
    else
      Writeln('Interface not implemented !');


    //using the generated generics function
    writeln('Integer GMax:', TFakeClassInt.gmax(23,56));
    writeln('Double GMax:', TFakeClassDouble.gmax(23.89,56.5));
    readln;
end.