Indexed properties

If the property definition contains an index, then the read and write specifiers must be a function and a procedure. Moreover, these functions require an additional parameter : An integer parameter. This allows to read or write several properties with the same function. For this, the properties must have the same type. The following is an example of a property with an index:

{$mode objfpc}
Type
  TPoint = Class(TObject)
  Private
    FX,FY : Longint;
    Function GetCoord (Index : Integer): Longint;
    Procedure SetCoord (Index : Integer; Value : longint);
  Public
    Property X : Longint index 1 read GetCoord Write SetCoord;
    Property Y : Longint index 2 read GetCoord Write SetCoord;
    Property Coords[Index : Integer]:Longint Read GetCoord;
  end;

Procedure TPoint.SetCoord (Index : Integer; Value : Longint);
begin
  Case Index of
   1 : FX := Value;
   2 : FY := Value;
  end;
end;

Function TPoint.GetCoord (INdex : Integer) : Longint;
begin
  Case Index of
   1 : Result := FX;
   2 : Result := FY;
  end;
end;

Var
  P : TPoint;

begin
  P := TPoint.create;
  P.X := 2;
  P.Y := 3;
  With P do
    WriteLn (’X=’,X,’ Y=’,Y);
end.

When the compiler encounters an assignment to X, then SetCoord is called with as first parameter the index (1 in the above case) and with as a second parameter the value to be set. Conversely, when reading the value of X, the compiler calls GetCoord and passes it index 1. Indexes can only be integer values.

6.7.2 Indexed properties