Anonymous methods cast as pointers

Question

can anyone explain why the code below fails?

type TIDEThemeObserverFunc = reference to procedure(foo: integer);
var fObserverFuncs: TList<TIDEThemeObserverFunc>

function RegisterEventObserver(aObserverFunc: TIDEThemeObserverFunc): Pointer;
begin
  fObserverFuncs.Add(aObserverFunc);
  Result := @aObserverFunc;

  // line below somehow fails
  assert(fObserverFuncs.IndexOf(TIDEThemeObserverFunc(Result)) <> -1);
end;

I assumed anonymous methods can simply be casted and used around via pointers but that seems like a wrong assumption. Also, any resources explaining how the anonymous methods are implemented under the hood would be great. TIA.

kludg · Accepted Answer · 2010-02-22T17:56:33.527

5

You should use PPointer(@aObserverFunc)^ instead of @aObserverFunc to avoid the failed assert.

@gabr: thanks for ref to my blog, but I should recommend first to read the Stackoverflow user Barry Kelly blog as a more competent source of information.

edited Feb 22 '10 at 17:56

answered Feb 22 '10 at 10:12

kludg

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Reference counting for the list should work, as it is of type `TList<(some method reference type)>`. If reference counting broke with generics, even trivial cases like `TList` would be useless. – Barry Kelly Feb 22 '10 at 17:12

score 4 · Answer 2 · edited May 23 '17 at 12:06

4

Anonymous methods are actually interfaces (more correct - objects implementing an interface).

Read more here: Anonymous methods in Delphi: the internals (written by Stackoverflow user Serg).

edited May 23 '17 at 12:06

Community

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answered Feb 22 '10 at 09:35

gabr

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Anonymous methods cast as pointers

2 Answers2