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Subject: Re: [boost] [variant2] Need rationale for never-empty guarantee
From: Andrzej Krzemienski (akrzemi1_at_[hidden])
Date: 2019-03-02 10:44:08

sob., 2 mar 2019 o 10:36 Andrey Semashev via Boost <boost_at_[hidden]>

> On 3/2/19 11:56 AM, Andrzej Krzemienski via Boost wrote:
> > sob., 2 mar 2019 o 07:35 Emil Dotchevski via Boost <
> boost_at_[hidden]>
> > napisał(a):
> >
> >> On Fri, Mar 1, 2019 at 9:37 PM Andrzej Krzemienski via Boost <
> >> boost_at_[hidden]> wrote:
> >>> My hypothesis is that reading valid-but-unspecified can only happen in
> a
> >>> buggy program in an unintended path.
> >>
> >> Running out of memory, or out of some other resource, does not indicate
> a
> >> bug. In response, under the basic exception guarantee, you may get a
> state
> >> which I'm saying shouldn't be merely "destructable" but also valid. For
> >> example, if this was a vector<T>, it shouldn't explode if you call
> .size(),
> >> or if you iterate over whatever elements it ended up with.
> >>
> >
> > This is where my imagination fails me. I cannot imagine why upon
> bad_alloc
> > I would be stopping the stack unwinding and determining size of my
> vectors.
> > This is why I ask about others' experience with real-world correct code.
> That is not an unimaginable scenario. If you have two branches of code,
> one requiring more memory but better performance, and the other that is
> slower (or maybe lacking some other qualities but still acceptable) and
> less resource consuming, operating on the same vector, you will want the
> vector to stay valid if memory allocation fails. Although not
> specifically with vectors, I had cases like this in real world.

Thanks for sharing your experience.
I am not sure we are on the same page here. Are you describing an operation
where an operation on vector fails to allocate more storage and therefore
throws and leaves the *value* of the vector unchanged (i.e., an operation
with strong exception safety guarantee)? Or are you describing an operation
that reuses memory owned by a vector and discards the vector's value? Boith
these cases can be described as "never observing the value after a failed
operation with a basic exception safety guarantee". Or are you describing a
different situation?


> However, in my experience, if I want to handle OOM condition gracefully,
> I tend to not trust any third party components except the lowest level
> ones, like C runtime, and write the relevant code myself. Especially,
> this concerns components that allocate memory, like containers.
> Unfortunately, it is often the case that either I don't trust
> implementations to take OOM into account and handle it well or I want
> some specific guarantees about how much memory is allocated and what the
> state of the program is when OOM happens.
> >>> And that making design compromises to
> >>> address this path is not necessarily the best approach to take.
> >>
> >> Consider that if you choose to allow, after an error, to have objects
> left
> >> in such a state that they may explode if you attempt to do anything but
> >> destroy them, there may not be any way to detect that state.
> >
> > Yes. and I do not see how this is a problem in practice. In my experience
> > objects that failed on operation with basic guarantee can only be safely
> > removed from the scope. (I do not even reset them.)
> Removing the objects may be wasteful or require expensive operations. In
> the vector example, that vector may be initially large or expensive or
> even impossible to reconstruct. If you strive for the "destroy upon
> failure" logic, you would have to duplicate the vector before attempting
> the operation that may fail with an exception. Which is a point of
> failure on its own, BTW. Generally, you want to minimize the number of
> points of failure while also minimizing amount of work needed to be done
> to complete the program. There is also a third subjective limit of code
> quality or simplicity, design quality, etc., but that is not relevant to
> my point.
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