Delphi: Goto not considered harmful in this case?

Question

Alright, so you have a TObjectList instance. You want to loop through the items in it and delete some of the objects from the list. You can't do this:

for I := 0 to ObjectList.Count - 1 do
  if TMyClass(ObjectList[I]).ShouldRemove then
    ObjectList.Delete(I);

...because once you delete the first object the index counter I will be all wrong and the loop won't work any more.

So here is my solution:

Again:
  for I := 0 to ObjectList.Count - 1 do
    if TMyClass(ObjectList[I]).ShouldRemove then
    begin
      ObjectList.Delete(I);
      goto Again;
    end;

This is the best solution I've found to this so far. If anyone has a neater solution I'd love to see it.

Answer 1

Try this instead:

 for I := ObjectList.Count - 1 downto 0 do
   if TMyClass(ObjectList[I]).ShouldRemove then
     ObjectList.Delete(I);

That looks like a particularly bad use of goto, jumping out of the for loop like that. I assume it works (since you're using it), but it would give me the willies.

Answer 2

The only valid use of Goto I've seen is the one supplied in Delphi's help.

for I := 0 to something do
begin
  for J := 0 to something do
  begin
    For K := 0 to something do
    begin
      if SomeCondition then
        Goto NestedBreak
    end;
  end;
end;
NestedBreak:

Though Goto could be avoided in that exemple by moving the loop in a local function and using EXIT, for exemple. If a subfunction is not acceptable, you can still do that:

for I := 0 to something do
begin
  for J := 0 to something do
  begin
    For K := 0 to something do
    begin
      if SomeCondition then
      begin
        GottaBreak := True
        Break;
      end; 
    end;
    if GottaBreak then Break;
  end;
  if GottaBreak then Break;
end;

This is just sligthly less optimal.

I have yet to see a single other situation where a Goto would be the best solution.(Or any good at all).

Goto in itself is NOT bad. It's a flow control command just like EXIT, BREAK or CONTINUE. Except that those other are restricted to specific situations and are managed by the compiler correctly. (With that being said, some programmer I spoke with consider those as being as harmful as Goto, a view I don't share) Goto being unrestricted, the things you can do with it can have very negative impacts. Anyway, I think I went a bit beyond the scope of the question already. ^_^

Answer 3

You can also use

  I := 0;
  while I < ObjectList.Count do begin
    if TMyClass(ObjectList[I]).ShouldRemove then ObjectList.Delete(I)
    else Inc(I);
  end;