thoughts on onCleanup and handle classes

[Jaroslav Hajek]

hi all,

as many of you know, Matlab since v. 2008 or something implements the
onCleanup object, that can be used for doing cleanup actions. Although
Octave's unwind_protect is better for most normal usages, there are
situations where the object nature of onCleanup would be an advantage,
primarily if you want to export the cleanup action outside the current
scope.

Now, in Matlab, I think onCleanup is simply a handle class with a
destructor, although I can't check. However, handle classes are not
supported in Octave, and I don't think they will be in near future.
First of all, we'd need to implement the new-style classes, and that
alone is a task that nobody just picked up.
Handle classes bring a lot more additional complexity, with their
dynamic properties and listeners and destructors. Another big problem
with handle classes is that simple reference counting is no longer
enough to avoid memory leaks, a handle-based container can contain
itself (possibly through other objects), much like in Python, so
circular references are possible (you can't make a circular reference
with cells and structs). I do not know how Matlab solves this, because
it is said there is no garbage collection. If it solves it at all.
Because of this complexity, it may make sense for Octave to mimick
certain useful handle classes of Matlab. onCleanup could be an
instance.
An existing instance is the ftp object by D.B. (here the handle is
actually an internal property, ftp.curlhandle).
Another useful thing would be a file object that can close itself
automatically. Actually, with onCleanup, one can simulate the
destructor in a rather neat way:

function fileobj (filename)
  obj.fid = fopen (filename);
  obj.close = onCleanup (@() fclose(obj.fid));
  obj = class (obj, "fileobj");
endfunction

what do you think? Is it worth implementing onCleanup into Octave in C++?
some two years back D.B. gave a sketch implementation, but I don't
think it will work anymore.
Btw., this assumes my understanding of how onCleanup works is correct.
Can anyone test the following code in Matlab 2008+ for me?

outer.m:
function outer ()
  x{1} = inner ('wild');
  disp ('to be');
  x{2} = x{1};

inner.m:
function x = inner (word)
  c = onCleanup (@() disp (word));
  disp ('born');
  x = c;


>> outer


-- 
RNDr. Jaroslav Hajek, PhD
computing expert & GNU Octave developer
Aeronautical Research and Test Institute (VZLU)
Prague, Czech Republic
url: www.highegg.matfyz.cz