Delphi: How to add a different constructor to a descendant?

Question

Update: The example i originally had was kind of complex. Here\'s a simple 8 line example that explains everything in one code block. The following

Answer 1

Also remember that constructors don't HAVE to be called Create. Older versions of Delphi didn't have method overloading, so you had to use different names:

TComputer = class(TObject) 
public 
    constructor Create(Cup: Integer); virtual; 
end; 

TCellPhone = class(TComputer) 
private
  FTeapot: string;
public 
    constructor CreateWithTeapot(Cup: Integer; Teapot: string); virtual; 
end; 

...

constructor TCellPhone.CreateWithTeapot(Cup: Integer; Teapot: string); 
begin
  Create(Cup);
  FTeapot := Teapot;
end;

Both constructors will now be available.

Answer 2

My immediate reaction is to use the overload keyword, as in:

TCellPhone = class(TComputer)
public
    constructor Create(Cup: Integer; Teapot: string); reintroduce; overload; virtual;
end;

Edit: Thanks Ian for the edit, which makes an answer out of my answer. I would like to think that I got it for bravery, so I am going to contribute a fuller example:

program Project1;

{$APPTYPE CONSOLE}

uses
  SysUtils;

type

TComputer = class(TObject)
public
    constructor Create(Cup: Integer); virtual;
end;

TCellPhone = class(TComputer)
public
    constructor Create(Cup: Integer; Teapot: string); reintroduce; overload; virtual;
end;

{ TComputer }

constructor TComputer.Create(Cup: Integer);
begin
  writeln('constructed computer: cup = ', Cup);
end;

{ TCellPhone }

constructor TCellPhone.Create(Cup: Integer; Teapot: string);
begin
  inherited Create(Cup);
  writeln('constructed cellphone: Teapot = ', Teapot);
end;

var
  C1, C2, C3: TComputer;

begin
  C1 := TComputer.Create(1);
  Writeln;
  C2 := TCellPhone.Create(2);
  Writeln;
  C3 := TCellPhone.Create(3, 'kettle');
  Readln;
end.

with the result being:

constructed computer: cup = 1

constructed computer: cup = 2

constructed computer: cup = 3
constructed cellphone: Teapot = kettle

Answer 3

You could create two new overloaded constructors, for example:

type
  TXmlStream = class
  private
    FFileName: string;
  public
    constructor Create(const AFileName: string); virtual;
  end;

  TXhtmlStream = class(TXmlStream)
  private
    FEncoding: TEncoding;
  public
    constructor Create(const AFileName: string); overload; override;
    constructor Create(const AFileName: string; AEncoding: TEncoding); overload; virtual;
  end;

constructor TXmlStream.Create(const AFileName: string);
begin
  inherited Create;
  FFileName := AFileName;
end;

constructor TXhtmlStream.Create(const AFileName: string);
begin
  inherited Create(AFileName);
end;

constructor TXhtmlStream.Create(const AFileName: string; AEncoding: TEncoding);
begin
  inherited Create(AFileName);
  FEncoding := AEncoding;
end;

Answer 4

Another possibility is to write a new constructor with default parameter values where the part of the signature with non-default parameters matches the original constructor in the base class:

type
  TXmlStream = class
  private
    FFileName: string;
  public
    constructor Create(const AFileName: string); virtual;
  end;

  TXhtmlStream = class(TXmlStream)
  private
    FEncoding: TEncoding;
  public
    constructor Create(const AFileName: string; AEncoding: TEncoding = encDefault); reintroduce; virtual;
  end;

constructor TXmlStream.Create(const AFileName: string);
begin
  inherited Create;
  FFileName := AFileName;
end;

constructor TXhtmlStream.Create(const AFileName: string; AEncoding: TEncoding);
begin
  inherited Create(AFileName);
  FEncoding := AEncoding;
end;