How is Enumerator created with for in construction destroyed?

Question

I have a collection derived from TCollection, implementing GetEnumerator so I can use it in a construction like

for lElem in lCollection do

The enumerator is derived from TObject, exactly like the standard enumerators supplied by Delphi and therefor doesn't have an owner.

The Delphi help mentions that if the enumerator supports IDisposable is it will be disposed of, but that is for .NET only of course.

What I was wondering is, how and when and by who the enumerator instance is freed?

Answer 1

For each for-enum statement compiler generates code that roughly corresponds to this pseudocode:

enumerator := list.GetEnumerator;
try
  while enumerator.MoveNext do
    do something with enumerator.Current;
finally
  enumerator.Free;
end;

The code above is generated for enumerators that are implemented as class instances. If your enumerator is implemented as an interface, the last line would not call .Free but merely decrement interface reference count allowing it to be destroyed.

Answer 2

It's freed automatically once it is no longer needed. The compiler generates code to do that so that you don't need to. When the disposal happens is an implementation detail.