On 3/3/18 12:43 PM, Chris Angelico wrote:
On Sun, Mar 4, 2018 at 4:37 AM, Richard Damon <rich...@damon-family.org> wrote:
On 3/3/18 11:33 AM, Michael Torrie wrote:
On 03/03/2018 09:02 AM, ooom...@gmail.com wrote:
I can assure you that RAII does what it says on the tin and is relied on
in
many critical systems to release resources robustly ... given the
pre-requisite deterministic destruction.
Sure but did you read what Paul Moore wrote? He said RAII works in C++
because objects are allocated on the *stack* with strict lifetimes and
scopes. They won't ever have cycles and they are guaranteed to be
destroyed no matter what as the stack is unwound. Python has no
stack-allocated objects.
In C++, Heap-allocated objects must still be managed manually, without
the benefit of RAII, for much of the same reasons as people are giving
here for why RAII is not a good fit for Python. There are smart pointer
objects that try to give RAII semantics to heap-allocated objects, with
varying degrees of success. In other words there are some limitations.
Python does not have stack-allocated objects, so the same issues that
prevent RAII from automatically applying in C++ to heap objects exist
here.
Yes, stack allocated object in C++ have a nice lifetime to allow RAII to
work, but it doesn't just work with stack allocated objects. A lot of RAII
objects are members of a class object that may well be allocated on the
heap, and RAII makes sure that all the needed cleanup gets done when that
object gets destroyed.
How do you guarantee that the heap object is properly disposed of when
you're done with it? Your RAII object depends 100% on the destruction
of the heap object.
ChrisA
Yes, the heap object 'owns' a resource, and as long as that heap object
exists, the resource needs to exist, but as soon as it doesn't, you want
that resource freed.
That heap object might well be controlled by some other RAII object
(maybe a smart pointer with a ref count) or maybe that object is being
manually controlled by the program. But the key is that when it goes
away, all of the resources it controls via RAII are automatically
cleaned up, and you don't need all that cleanup made explicit in the
owning class.
If the resource is plentiful, like memory, we can simplify the life
management by using something like garbage collection, so if some is
used longer than needed it isn't a big issue. Other resources are much
more valuable and limited, and it is important to release them when no
longer needed. Without some method to make sure that happens
'automatically' as soon as possible, it requires that the programmer
explicitly deal with it in full detail.
Now, a lot of talk has been throw around about things like reference
cycles getting in the way of this, but practically, the resources that
this would be needed for, then not to get involved in those sorts of
issues, but tend to live in more simple structures, so the fact that
objects in a cycle might need for the garbage collector to determine
they aren't used anymore isn't an issue (and the presence of garbage
collection says that these more complicated structures, which only hold
resources that are plentiful, don't need to be precisely managed will
RAII controls).
--
Richard Damon
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