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Subject: Re: [boost] [auto_buffer] Interest check: container with preallocated buffer
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2009-05-19 17:00:32

Thorsten Ottosen wrote
On Tuesday, May 19, 2009 4:41 PM
> Stewart, Robert skrev:
> > Thorsten Ottosen wrote
> > On Tuesday, May 19, 2009 2:17 PM
> >> Stewart, Robert skrev:
> > Is the signature actually sufficient? What if the
> > requested size exceeds the adjustment new_capacity() would make?
> >IOW, what if the requested size is greater than four times
> > the old capacity? Would this be better?
> >
> > template <class Size>
> > static Size
> > grow(Size _old, Size _size);
> >
> > This means supplying the old capacity and desired size such
> > that the result should be at least _size.
> In that case the exact requested size is allocated. The interface you
> show provides some extra flexibility, but I don't know if it
> is useful.
> Perhaps.

I haven't looked at your implementation of reserve(), but I was expecting that it would simply ask the policy for the new size and allocate it. Apparently, you're making decisions in reserve() over and above those made by new_capacity(). I think that will be surprising to those writing the policy.

> >>> The "GrowPolicy" template parameter should be named
> "GrowthPolicy."
> >> Really?
> >
> > Yes, "growth" is a noun.
> How about ResizingPolicy? The policy is also used in shrink_to_fit().


> >>> The auto_buffer nested value "N" is confusing as it will
> >>> change based upon the SizingPolicy,
> >>> but won't match that policy's "value" if using
> >>> boost::store_n_bytes, and it isn't the number of elements in,
> >>> or capacity of, the auto_buffer, particularly when memory is
> >>> allocated from the allocator.
> >>> Renaming it to "stack_capacity" should work, though.
> >> I'll think about it. I need a short name for the docs, and N
> >> seems both
> >> short and good to me. N is also documented.
> >
> > "N" suggests number of elements to me.
> Right. And that is what it is.

Isn't it just the number of elements that will fit within the stack allocation? When the buffer switches to allocator-supplied memory, N is no longer likely to be the number of elements -- which is why you provide size().

> > Why would anyone bother to create one that won't fit on the
> > stack? Why not just use vector?
> Because auto_buffer provides additional functionality not found in
> std::vector.
> > Are you thinking of generic code that sometimes will succeed
> > in using the stack and other times not?
> I don't know what you mean by this, but I'm happy if there is yet
> another reason to use the class.

I meant a function template that creates an auto_buffer<T,store_n_bytes<N>> where sizeof(T) may be greater than N, depending upon T.

> >>> How about that store_n_bytes<N>::value >= sizeof(T)? Those
> >>> will help to diagnose misuse.
> >> This might be ok to have if 0 < N < sizeof(T).
> >
> > I suppose this can occur for the same reason as above but
> > it should be documented. Otherwise, there's no sense in allowing it.
> OTOH, I would hate to put a static assertion in there if it sometimes
> complicates the use when the user actually don't mind this.

It may be worth mentioning the relationship between N and sizeof(T) when using store_n_bytes as it can lead to not using the stack space.

> >>> For small types, optimized_const_reference isn't a reference
> >>> at all, so the name is misleading.
> >>> Better to name it for the usage rather than the type.
> >> But then for large types, "by_value_type" is not a value
> >> type, but a
> >> reference. What about parameter_type or param_type?
> >
> > Yeah, it doesn't work well for the other case. "Parameter"
> > suggests only the input side of things. boost::call_traits
> > does something similar, but that only has to do with calling
> > functions. It would be odd to return a call_traits type.
> > How about "fast_value_type?"
> It's a candidate, but again, sometimes it is a reference :-)

It is being used as a value. You're returning a T, but you're using a reference to const to avoid copying. "fast_value_type" refers to the usage rather than the type.

> >>> Rather than reserve_precisely(), which should be named
> >>> "reserve_exactly" if retained,
> >>> why not just document reserve() as allocating what
> >>> GrowthPolicy::reserve() returns and leave the behavior to the
> >>> policy?
> >>> That leaves room for reserve() allocating exactly what's
> >>> requested, or rounding up according to some computation, as
> >>> the policy dictates.
> >> reserve() is used several places when the buffer needs to
> >> grow. If this
> >> was to allocate precisely what was requested, then we would e.g.
> >> allocate just one extra element.
> >>
> >> But I guess I could factor the code so it was as you
> >> suggested. I will
> >> make this a review question. This will require a small change in
> >> GrowPolicy so you can see what is requested.
> >
> > I think the "see what is requested" is the change I suggested above.
> Not quite. reserve_precisely() allows you to do extractly that, not
> matter what GrowPolicy you have installed. The user knows that the
> new buffer should be exactly what he asks for, no matter what the
> policy thinks.

OK, but if you remove reserve_precisely(), as I suggested, then you're left with reserve() and it will behave as dictated by the policy. Whenever I use reserve(), I only want that many elements, not at least that many, but I understand that when auto_buffer grows implicitly, it wants to allocate something extra to get amortized constant time for the increase. Thus, in my mind, reserve() does exactly what the caller asks, but auto_buffer itself uses a private member function that calls new_capacity() and moves the data to a new allocation.

> >>> The unchecked_*() functions deserve remarks that explain
> >>> the danger in their use and that the precondition is not checked.
> >
> > You didn't comment on the addition of remarks for these
> functions, so I'm re-raising this point.
> The precondition is checked with BOOST_ASSERT(). As for
> remarks, yeah,
> but how many other functions in Boost have a remark saying that you
> should not violate the preconditions?

OK, but you can still document the danger.

> >>> shrink_to_stack() might be a useful addition.
> >> shrink_to_fit() can already place the content on the stack.
> >
> > OK. I assume that shrink_to_fit() takes an argument, so
> that shrink_to_stack() would be shorthand for
> a.shrink_to_fit(a.stack_capacity).
> No, shrink_to_fit() never erases elements from the container
> (logically). resize() does, but this is on my todo list.
> resize() would
> probably not move elements to the stack.

Then shrink_to_stack() would be a nice addition for which shrink_to_fit() is not a replacement.

Rob Stewart robert.stewart_at_[hidden]
Software Engineer, Core Software using std::disclaimer;
Susquehanna International Group, LLP

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