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 · 2010-02-22 17:56:33Z

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, 2010 at 17:56

answered Feb 22, 2010 at 10:12

kludg

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1 Comment

Barry Kelly Over a year ago

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<string> would be useless.

Community · Accepted Answer · 2017-05-23 12:06:59Z

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, 2017 at 12:06

CommunityBot

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answered Feb 22, 2010 at 9:35

gabr

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