Question

I m doing an event manager in Freepascal
Each event is an object type TEvent (=object), each kind of event must derive from this class.
Events are differentiated by an integer identificator, assigned dynamically.
The problem is that i want to retrieve the event id of an instance, and i can t do it well.

All instances of a class(object) have a unique id => so it should be static field.
All classes have a diferent id => so it should be virtual.
Event ids are assignated in run time, and can change => so it can t be a simple method

In sum, I can t put all this together.
I m looking for an elegant solution, i don t want to write a hardcoded table, actualizing it in every constructor... etc, i d prefer something taking advantage of the polymorphism
Can anyone help me with another technical or design solution?
I remark I don t want to use class instead of object construct.(property doesn t work on objects? :(

Answer 1

You might need class var like stuff, like in newer delphi s. But that is in the development version of FPC only (2.5.1+).

Note that the object type is TP legacy, and hasn t been developed on in this millenium, and I don t expect that to change. If you need more than it offers, I suggest to use classes.

Answer 2

You can make a simple table/list like this:

unit classids;

{$mode objfpc}{$H+}

interface

function GetClassID(c:TClass):Integer;
procedure SetClassID(c:TClass; id:Integer);

property ClassID[c:TClass]:Integer read GetClassID write SetClassID;

implementation
uses Maps;

var Map:TMap;

function GetClassID(c:TClass):Integer;
begin
 if not Map.GetData(c,Result) then
  Result:=0; //Or any default you like
end;

procedure SetClassID(c:TClass; id:Integer);
begin
 Map.Delete(c);
 Map.Add(c,id);
end;

initialization
 Map:=TMap.Create(itu4,SizeOf(Integer));
finalization
 FreeAndNil(Map);
end.

Then you can get/set the id with

ClassID[TMyObject]:=12;
ShowMessage(IntToStr(ClassID[TMyObject])); //shows 12

Good luck

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