Question

I have a class Packet with an abstract method getID:

Type
  Packet = class
    ...
    function getID() : Integer;virtual;abstract;
    class procedure writePacket(par0 : Packet; par1 : TIdTCPConnection);
    ...
implementation 

class procedure Packet.writePacket(par0 : Packet; par1 : TIdTCPConnection);
// par1 is a TCP connection used to send data through the network; 
begin
  par1.writeInteger(par0.getID());
  //some code following 
end;

I have the following subclass:

type
  PacketTest = class(Packet)
    ...
    function getID() : Integer;
    ...

function PacketTest.getID():Integer;
begin
  result := {some value individual for each subclass}
end;

Now I call the superclass’s class procedure writePacket with a subclass of packet as par0, which should then call the subclass’s function getID.
Instead it calls the superclass’s function getID and (of course) throws an abstract error.

I want it to dynamically call the subclass’s function getID, which should never cause a problem because par0 is always a subclass of Packet since Packet itself is abstract.

Answer 1

The declaration of getID in the subclass must have the override keyword:

type
  PacketTest = class(Packet)
    function getID: Integer; override;
  end;

function PacketTest.getID: Integer;
begin
  Result := {some value individual for each subclass}
end;