Hi all,

I cannot find a solution to a problem I've got programming a library. I want to initialze a class which is passed as a parameter to another class method. It looks like this :

tCoDec = CLASS
  constructor init; VIRTUAL;
  destructor done; VIRTUAL;
end;
tmyCoDec = CLASS (tCoDec)
  constructor init; OVERRIDE;
  destructor done; OVERRIDE;
end;
pCodecList = ^tCodecList;
tCodecList = RECORD
  codec :tCoDec;
  next :pCodecList;
end;
tMain = CLASS
  codec_list :pCodecList;
  constructor init;
  destructor done;
  registerCoDec (var cd :tCoDec);
end;
tMain.registerCoDec(var cd:tCoDec);
begin
  cd := "class of cd".init;
  AddToCoDecsList(cd);
end;

I don't know how to do this or if it's possible. It's the first time I use classes and maybe the first I use OOP at all, so maybe this is completely impossible to do.

Any help will be appreciated, thks so much.
bye, Ivan

Ivan Montes wrote

tCoDec = CLASS   constructor init; VIRTUAL;   destructor done; VIRTUAL; end;

When using classes, you should use "create" as constructor name and you *must* use "destroy" as destructor name (if you don't do the latter, the destructor won't be called automatically when the class instance is freed). Also, your destructor should be declared as "override", because the destructor over TObject (of which all classes are derived) is already virtual. Have a look at the manuals to see the differences between "virtual" and "override" (in short: the first time a virtual method appears in a class hierarchy, you should declare it as "virtual" and afterwards, when you want to override it in a child class, use "override" instead).

Add the following to your program for the solution:

tTypeOfCoDec = class of tCoDec;
tMain.registerCoDec(var cd: tCoDec; mytype: tTypeOfCoDec);
begin
   cd := mytype.create;
  AddToCoDecsList(cd);
end;

Jonas

It does, see the Classes unit.

Michael.

Taking on board what Jonas has said (you have mixed Turbo Pascal syntax with that of Object Pascal/Delphi)..

Does FPC have a TList? Surely that would work out better.

by reference.

Try:

var
  Main: tMain;

{...}

procedure Test;
begin
  Main.registerCoDec(tCoDec.Create);
  //or
 Main.registerCoDec(tMyCoDec.Create);
end;

because tMyCoDec inherits from tCoDec, you may pass it as a tCoDec. You will then need to typecast it to get at any 'added' features, but this is normal even in languages such as C/C++.

I don't know how to do this or if it's possible. It's the first time I use classes and maybe the first I use OOP at all, so maybe this is completely impossible to do.

Nope, I do this all the time. I have just written a fairly complex administration tool that uses small data classes to pass information around in a similar wat to above. In all cases the routine that accepts the data uses the base class as the param, so that I can typecast where needed, but simplify the overall routines.

Matt