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Subject: Re: [boost] [review] The review of Boost.DoubleEnded starts today: September 21 - September 30
From: Thorsten Ottosen (tottosen_at_[hidden])
Date: 2017-10-06 09:53:01

Den 05-10-2017 kl. 22:42 skrev Ion Gaztañaga via Boost:
> On 05/10/2017 19:02, Thorsten Ottosen via Boost wrote:
>>> I understand the idea to avoid zero initialization for buffers, but
>>> what's the use case for types with non-trivial constructors? The
>>> documentation shows a socket example, and that's covered ith
>>> default_init_t.
>> Well, I don't know if my compiler is wrong, but vc 2017 says:
>>          class IntClass
>>          {
>>              int i;
>>          public:
>>              IntClass() : i(0) {}
>>              IntClass( int i ) : i( i )
>>              {}
>>          };
>> is trivially copyable (hence we can copy arrays of it as fast as
>> arrays of ints). But it will be initialized with zero if I use
>> default_init_t, right?
> Right.

But I agree we don't need two overloads, the existing one just has to
work in terms of trivially copyable. We can also put a static assertion
in there to lock it down to such types for now.
>> If reserve is not an alias for reserve_back, the I think we must
>> remove it.
> reserve() means that a container won't reallocate if size() <
> capacity(). Where memory is reserved is irrelevant for the user.
>> I was thinking along generic code along the line
>> template< class BacKInsertionContiguousContainer >
>> void foo( BacKInsertionContiguousContainer& cont )
>> {
>>     cont.reserve( source.size() );
>>     for( ... )
>>        cont.push_back( ... );
>> }
>> why should that not work with both devector and vector?
> It would work, but it would not be as efficient as vector.

It may be. But I guess my concern was to make it compile. Sure, if
a call to reserve is conditioned on d.capacity() - d.size(), it can
behave differently. Reserve maybe easier to do something about.

We may need to tweak the definition of reserve_back/reserve_front a little.

Let's say we have

   x x 1 1 x

so front_free_capacity is 2, back free capacity is 1 ans size is 2.


     d.reserve( d.size() + 3 );

devector uses the expression

   n = new_capacity - size()

so we get n = 5 - 2 = 3 and we get

   x x 1 1 x x x

. A vector 1 1 x does the same. So is your concern that users use
capacity to determine what/if to reserve?

>>>> This is tricky. I think the best is to treat both ends independently.
>>>> If you start moving things around, you can break the O(1) amortized
>>>> guarantee by interleaving push_front/push_back.
>>> What do you mean by "treat both ends independtly"?
>> I mean that buffer extensions at either end should never affect each
>> other. That is, a push_back can never remove free capacity from the
>> front, a push_front can never remove free capacity from the back.
> Ok. I still have no clear idea which allocation strategy would be the
> "natural" one. I've found this blog with a similar discussion and
> alternatives:
> also linked from the blog post:

Certainly a good analysis.

> Moving elements just one step when one side is full and the other side
> has capacity has bad quadratic behavior while current capacity is being
> exhausted and we continue inserting in the same side. Moving elements to
> the middle of the buffer seems a simple operation that reduces this to
> something closer to O(NlogN). In any case, I will always bad insertion
> sequence that hurts our strategy, so optimizing the common case seems
> the way to go.

I'm ok with inserts in the middle to move stuff around since it already
has O(n) worst case behavior. For push_back/push_front I don't know.

It would have to be really simple to be worth the effort IMO. Say, only
when other_size_free_capacity >= size().

The simple thing would be to grow independently, and then the user calls
shrink to fit when done if he cares about it.

BTW: Before the review some people complained that the container could
not work as deque when you repeatedly pop from one end and push another.

Something to ponder ...

kind regards


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