[fpc-pascal]Polymorphism of class field

[Patrick O'Leary]

I am having trouble with polymorphism in a unit I am working on.

I have several classes defined as follows:

type

         TStackElem = class
                 public
                 constructor Init; virtual;
                 destructor Done; virtual;
                 Prolog : TRPLObjectType;
                 Next : TStackElem;
                 function Decomp : string; virtual; abstract;
         end;

         TStackReal = class(TStackElem)
                 public
                 constructor Init; override;
                 Data : real;
                 function Decomp : string; override;
         end;

         TStackCharString = class(TStackElem)
                 public
                 constructor Init; override;
                 Data : string;
                 function Decomp : string; override;
         end;

Note the Data field: it is not present in the parent class, and is defined
as two different types in the child class.

I also have defined the function:

function Parse(Command : string) : TStackElem;
begin
         // Decide type of data based on Command
         // Define Parse as being TStackReal or TStackCharString (running 
Parse := TStackReal.Init, etc.)
         // Put properly parsed data in Data field
end;

I haven't written the internal part of this function this time, but in
previous attempts, the part of this function that 'Puts properly parsed data
in Data field' of the TStackElem causes a compilation error, due to the Data
field not being defined for TStackElem. In the absence of a Variant type
(with which I would define Data : variant in TStackElem), how should I 
implement
the above function?

--
Patrick O'Leary
patrick.oleary at resnet.pleasepleasedontsendmespam.gatech.edu