Best practice to do nested TRY / FINALLY statement

Question

Hi What is the best way to do nested try & finally statements in delphi?

var cds1  : TClientDataSet;
    cds2  : TClientDataSet;
    cds3  : TClientDataS         


        

Answer 1

I'd use something like this:

var
  Safe: IObjectSafe;
  cds1 : TClientDataSet;
  cds2 : TClientDataSet;
  cds3 : TClientDataSet;
  cds4 : TClientDataSet;
begin
  Safe := ObjectSafe;
  cds1 := Safe.Guard(TClientDataSet.Create(nil)) as TClientDataSet;
  cds2 := Safe.Guard(TClientDataSet.Create(nil)) as TClientDataSet;
  cds3 := Safe.Guard(TClientDataSet.Create(nil)) as TClientDataSet;
  cds4 := Safe.Guard(TClientDataSet.Create(nil)) as TClientDataSet;
  ///////////////////////////////////////////////////////////////////////
  ///      DO WHAT NEEDS TO BE DONE
  ///////////////////////////////////////////////////////////////////////

  // if Safe goes out of scope it will be freed and in turn free all guarded objects
end;

For the implementation of the interface see this article, but you can easily create something similar yourself.

EDIT:

I just noticed that in the linked article Guard() is a procedure. In my own code I have overloaded Guard() functions that return TObject, above sample code assumes something similar. Of course with generics much better code is now possible...

EDIT 2:

If you wonder why try ... finally is completely removed in my code: It's impossible to remove the nested blocks without introducing the possibility of memory leaks (when destructors raise exceptions) or access violations. Therefore it's best to use a helper class, and let the reference counting of interfaces take over completely. The helper class can free all objects it guards, even if some of the destructors raise exceptions.