[Octave-bug-tracker] [bug #59192] unwind_protect objects should be replaced with lighter weight alternatives

[John W. Eaton]

Follow-up Comment #10, bug #59192 (project octave):

Rik: Yeah, me too.  But there are things like this (just an example, not real
code):


unwind_protect frame;

frame.protect_var (v1);
v1 = some_value;

frame.add_fcn (some_fcn, ...);

frame.protect_var (v2);
v2 = some_other_value;

if (some_condition)
  {
     frame.add_fcn (another_fcn, ...);

     do_some_stuff ();
  }


I could guess but wouldn't know for sure without doing some careful checking
whether it would be OK to rearrange the order in which things are
saved/restored (actual code might be much more complex than the bit above). 
In some cases it may be OK to create a new unwind_action inside an inner scope
(see the use of another_fcn above) and others adding it to the unwind_protect
object is simply conditional and the action needs to execute at the end of the
outer scope.  So we might need to be able to write something like this:


unwind_action act;

if (condition)
  act = unwind_action (...);

// OR (perhaps to avoid confusion?)

if (condition)
  act.replace_action (...);


Copying unwind_action objects is currently not allowed, probably because doing
so could cause some confusion.

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