RAII vs gc (was fortran lib which provide python like data type)

Rustom Mody rustompmody at gmail.com
Fri Jan 30 19:02:55 CET 2015


On Friday, January 30, 2015 at 11:01:50 PM UTC+5:30, Rustom Mody wrote:
> On Friday, January 30, 2015 at 10:39:12 PM UTC+5:30, Michael Torrie wrote:
> > On 01/30/2015 09:27 AM, Rustom Mody wrote:
> > > ... if I restate that in other words it says that sufficiently
> > > complex data structures will be beyond the reach of the standard
> > > RAII infrastructure.
> > > 
> > > Of course this only brings up one side of memory-mgmt problems
> > > viz. unreclaimable memory.
> > > 
> > > What about dangling pointers?
> > > C++ apps are prone to segfault. Seems to suggest
> > > (to me at least) that the memory-management infrastructure
> > > is not right.
> > > 
> > > Stroustrup talks of the fact that C++ is suitable for lightweight
> > > abstractions. In view of the segfault-proneness I'd say they 
> > > are rather leaky abstractions.
> > > 
> > > But as I said at the outset I dont understand C++
> > 
> > Yes I can tell you haven't used C++.  Compared to C, I've always found
> > memory management in C++ to be quite a lot easier. The main reason is
> > that C++ guarantees objects will be destroyed when going out of scope.
> 
> I hear you and I trust you as a gentleman but I dont trust C++ :-)
> 
> The only time in some near 15 years of python use that
> I got it to segfault was when Ranting Rick gave some wx code to try
> [at that time he was in rant-against-tk mode]
> Sure enough it was related to the fact that wx is written in C++
> and some expectations were not being followed.
> 
> > So when designing a class, you put any allocation routines in the
> > constructor, and put deallocation routines in the destructor.  And it
> > just works.  This is something I miss in other languages, even Python.
> > 
> > And for many things, though it's not quite as efficient, when dealing
> > with objects you can forgo pointers altogether and just use copy
> > constructors, instead of the
> > 
> > blah *a = new blah_with_label("hello") //allocate on heap
> > //forget to "delete a" and it leaks the heap *and* anything
> > //that class blah allocated on construction.
> > 
> > just simply declare objects directly and use them:
> > 
> > blah a("hello") //assuming there's a constructor that takes a string
> >                 //deletes everything when it goes out of scope
> > 
> > So for the lightweight abstractions Stroustrup talks about, this works
> > very well. And you'll rarely have a memory leak (only in the class
> > itself) and no "dangling pointers."
> 
> And what about the grey area between lightweight and heavyweight?
> 
> You say just use copy constructors and no pointers.
> Can you (ie C++) guarantee that no pointer is ever copied out of 
> scope of these copy-constructed objects?
> 
> > 
> > For other things, though, you have to dynamically create objects.  But
> > the C++ reference-counting smart pointers offer much of the same
> > destruction semantics as using static objects.  It's really a slick
> > system.  Almost makes memory management a non-issue.  Circular
> > references will still leak (just like they do on Python).  But it
> > certainly makes life a lot more pleasant than in C from a memory
> > management perspective.

The case of RAII vs gc is hardly conclusive:

http://stackoverflow.com/questions/228620/garbage-collection-in-c-why



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