shapes.~pas

《Kylix程序设计》一书中附带的例程源代码

unit Shapes;

interface

uses
  SysUtils, Classes, Types, QGraphics;

type
  TGeoShape = class( TObject )
  public
    function Draw(): string; virtual;
  end;

  TGeoPoint = class( TGeoShape )
  protected
    FPoint: TPoint;
  public
// Error 1.
//    function Draw(): string; virtual; override;
// Error. Field definition not allowed after methods or properties
// Only use
//    function Draw(): string; override;
// NOT allowed to define it using virtual again.

// Error 2.
//    procedure Draw( Canvas: TCanvas );
// Error. Overloaded procedure 'Draw' must be marked with the 'overload' directive

// Error 3.
//    procedure Draw( Canvas: TCanvas ); overload;
// Error. Previous declaration of 'Draw' was not marked with the 'overload' directive

// Error 4.
//    function Draw( Canvas: TCanvas ): string; reintroduce; overload;
// This statement can work if no virtual method Draw has been defined again.
// But if:
// If the virtual method Draw has been defined again in the current class,
// Error. Previous declaration of 'Draw' was not marked with the 'overload' directive
// We Can NOT use it.

// Error 5.
// Error. Overloaded procedure 'Draw' must be marked with the 'overload' directive
// If needs an overload Draw and override, must use
    function Draw( Canvas: TCanvas ): string; reintroduce; overload;
    function Draw(): string; reintroduce; overload; override;
// This statement is error.
//    function Draw(): string; override;
  end;

  TGeoLine = class( TGeoPoint )
  protected
    FToPoint: TPoint;
  public
    function Draw(): string; override;
  end;

  TGeoRect = class( TGeoShape )
  protected
    FRect: TRect;
  public
    function Draw(): string; override;
  end;
{
var
  Shape: TGeoShape;
  Point: TGeoPoint;
  Line: TGeoLine;
  Rect: TGeoRect;
}
implementation

{ TGeoShape }

function TGeoShape.Draw: string;
begin
  Result := 'TGeoShape.Draw called.';
end;

{ TGeoPoint }

function TGeoPoint.Draw: string;
begin
  Result := 'TGeoPoint.Draw called.';
end;

function TGeoPoint.Draw(Canvas: TCanvas): string;
begin
  Result := 'TGeoPoint.Draw(Canvas) called.';
end;

{ TGeoLine }

function TGeoLine.Draw: string;
begin
  Result := 'TGeoLine.Draw called.';
end;

{ TGeoRect }

function TGeoRect.Draw: string;
begin
  Result := 'TGeoLine.Draw called.';
end;

end.