Duck typing in Delphi 2007?

Question

Question:

Is there a way to do duck typing with Delphi 2007 (i.e. without generics and advanced Rtti features)?

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Duck typing Re

Answer 1

With the help of the ObjAuto.pas and invokable variant types it should be possible (written in XE but should also run in Delphi 7 or lower):

unit DuckTyping;

interface

function Duck(Instance: TObject): Variant;

implementation

uses
  ObjAuto,
  SysUtils,
  TypInfo,
  Variants;

type
  TDuckVarData = packed record
    VType: TVarType;
    Reserved1, Reserved2, Reserved3: Word;
    VDuck: TObject;
    Reserved4: LongWord;
  end;

  TDuckVariantType = class(TPublishableVariantType)
  protected
    function GetInstance(const V: TVarData): TObject; override;
  public
    procedure Clear(var V: TVarData); override;
    procedure Copy(var Dest: TVarData; const Source: TVarData;
      const Indirect: Boolean); override;
    function DoFunction(var Dest: TVarData; const V: TVarData;
      const Name: string; const Arguments: TVarDataArray): Boolean; override;
  end;

var
  DuckVariantType: TDuckVariantType;

{ TDuckVariantType }

procedure TDuckVariantType.Clear(var V: TVarData);
begin
  V.VType := varEmpty;
  TDuckVarData(V).VDuck := nil;
end;

procedure TDuckVariantType.Copy(var Dest: TVarData; const Source: TVarData;
  const Indirect: Boolean);
begin
  if Indirect and VarDataIsByRef(Source) then
    VarDataCopyNoInd(Dest, Source)
  else
  begin
    with TDuckVarData(Dest) do
    begin
      VType := VarType;
      VDuck := TDuckVarData(Source).VDuck;
    end;
  end;
end;

function TDuckVariantType.DoFunction(var Dest: TVarData; const V: TVarData;
  const Name: string; const Arguments: TVarDataArray): Boolean;
var
  instance: TObject;
  methodInfo: PMethodInfoHeader;
  paramIndexes: array of Integer;
  params: array of Variant;
  i: Integer;
  ReturnValue: Variant;
begin
  instance := GetInstance(V);
  methodInfo := GetMethodInfo(instance, ShortString(Name));
  Result := Assigned(methodInfo);
  if Result then
  begin
    SetLength(paramIndexes, Length(Arguments));
    SetLength(params, Length(Arguments));
    for i := Low(Arguments) to High(Arguments) do
    begin
      paramIndexes[i] := i + 1;
      params[i] := Variant(Arguments[i]);
    end;

    ReturnValue := ObjectInvoke(instance, methodInfo, paramIndexes, params);
    if not VarIsEmpty(ReturnValue) then
      VarCopy(Variant(Dest), ReturnValue);
  end
  else
  begin
    VarClear(Variant(Dest));
  end;
end;

function TDuckVariantType.GetInstance(const V: TVarData): TObject;
begin
  Result := TDuckVarData(V).VDuck;
end;

function Duck(Instance: TObject): Variant;
begin
  TDuckVarData(Result).VType := DuckVariantType.VarType;
  TDuckVarData(Result).VDuck := Instance;
end;

initialization
  DuckVariantType := TDuckVariantType.Create;

finalization
  FreeAndNil(DuckVariantType);

end.

You can simply use it like this:

type
  {$METHODINFO ON}
  TDuck = class
  public // works in XE, not sure if it needs to be published in older versions
    procedure Quack;
  end;

procedure TDuck.Quack;
begin
  ShowMessage('Quack');
end;

procedure DoSomething(D: Variant);
begin
  D.Quack;
end;

var
  d: TDuck;
begin
  d := TDuck.Create;
  try
    DoSomething(Duck(d));
  finally
    d.Free;
  end;
end;