I think DELPHI language offers a very clear and beautiful implementation of data encapsulation via the property methods and clear getter and setter functions.

I want to access a class string list strlst via a getter and setter functions. Option #1 in the code sample works fine as expected , but option #2 calling the Stringlist.Commatext function does actually not work.

Is this a DELPHI bug or did I miss something with respect to class design?

Target : fill class stringlist without creating an external Tstringlist class, just pass commatext.

program Project2;
{$APPTYPE CONSOLE}
{$R *.res}

uses
  System.SysUtils, classes;

type
  TSimpleClass = class
    FStrlst: TStringList;

  private
    procedure SetStrList(const Value: TStringList);

  public
    constructor Create;

    property strlst: TStringList read FStrlst  write SetStrList;

  end;

  { TSimpleClass }

constructor TSimpleClass.Create;
begin
  inherited;

  FStrlst := TStringList.Create;

end;

procedure TSimpleClass.SetStrList(const Value: TStringList);
var
  i: Integer;
begin
  Writeln('class  getter / setter function is called ');
  for i := 0 to Value.Count - 1 do
  begin
    FStrlst.Add(Value[i]);
  end;

  writeln ('content of internal strlist : '   FStrlst.CommaText )
end;

begin
  var
    aSimpleClass: TSimpleClass;
  var
    testLst: TStringList;

  try
    aSimpleClass := TSimpleClass.Create;
    try

       //   option #1  is working
      testLst:=TStringList.Create;
      testLst.CommaText := 'a,b,c';
      aSimpleClass.strlst := testLst;

     //    option #2  not working ...  this code should also call  the 
     //    setter function   
     aSimpleClass.strlst.commatext := 'd,e,f';


    finally
      aSimpleClass.Free;
    end;
    writeln ('done sample code !');
    Readln;

  except
    on E: Exception do
      Writeln(E.ClassName, ': ', E.Message);
  end;

end.

cmd line window sample output

output of this sample code

CodePudding user response：

Is this a DELPHI bug or did I miss something with respect to class design?

No, it is not a Delphi bug, but a misunderstanding on your part. You say "property, getter and setter function not called", and I understand you refer to getter and setter for aSimpleClass.strlst.

You have defined direct read access (of FStrLst) for TSimpleClass.strlst, thus there is no getter to be called when reading (as in // option #2), and aSimpleClass.strlst refers directly to FStrLst.

The CommaText property of TStringList has its own setter and that is called in // option #2

Edit

Or, if your intention is to not expose the TStringList at all, but only its CommaText property, that is OK as follows:

Make sure the TStringList is private, as well as a getter and setter for CommaText, and add the property CommaTxt

TSimpleClass
private
  FStrlst: TStringList;
  ...
  function GetCommaText: string;
  procedure SetCommaText(s: string);
public
  property CommaTxt: string read GetCommaText write SetCommaText;
  ...
end;

Implement the getter and setter:

function TSimpleClass.GetCommaText: string;
begin
  result := FStrlst.CommaText;
end;

procedure TSimpleClass.SetCommaText(s: string);
begin
  FStrLst.CommaText := s;
end;

Now you can access the CommaTxt property without exposing the TStringList.

CodePudding user response：

The reason why your option #2 doesn't fire the setter mthod is because in your option #2 you are not assigning any value to your strlst but insteadyou are interacting with the object that is being returned by the said property.

You see when you call aSimpleClass.strlst.commatext := 'd,e,f'; what happens is next:

1. Getter method of the strlst property is being called in order to return the reference to the TStringList object (the object thype that is defined by property)
2. Then the string value of d,e,f is assigned to the ComaText property that belongs to the String List whose reference was returned by your property getter method.

Now if at this time the TStringList that is part of your class would still not have been created an Access Violation would have been raised since you would be trying to access property of a non existent object.