Re: [boost] Interest in StaticVector - fixed capacity vector

Subject: Re: [boost] Interest in StaticVector - fixed capacity vector
From: Nevin Liber (nevin_at_[hidden])
Date: 2011-10-12 02:50:04

On 11 October 2011 20:01, Nathan Ridge <zeratul976_at_[hidden]> wrote:
>
>> If you are checking all the preconditions yourself, why do you care if it throws or not?
>> [ That sounds glib - but it's a serious question. ]
>>
>> Why do you care if it ASSERTs, calls abort (), throws, or gets your cat pregnant?
>
> You care because if it throws, it is incurring the runtime cost of checking
> whether the precondition has been met.

So does the assert.  The "complication" is there (performance is a
different issue) unless you make it undefined behavior AND ignore it.

> The design choice between throwing and undefined behaviour (in release
> builds, where asserts become no-ops) boils down to the choice of who
> should check the precondition: the callee, or the caller. Tradition has
> favoured the caller checking the precondition whenever possible, because
> the caller has more information and can sometimes check more efficiently
> (e.g. by eliding the check altogether in cases where it already knows the
> precondition is met).

And sometimes is less efficient.  For instance, if you are populating
from StaticVector from list iterators (or any non-random access
iterators), it's an O(n) check ahead of time or a complicated wrapper
around a one-at-a-time insert.  (Note:  this is a bug in the current
implementation of StaticVector, as its iterator constructor currently
requires random access iterators).

> Note that vector::push_back() throwing on out-of-memory
> is *not* a counterexample to this - unlike exceeding the capacity of a
> StaticVector, running out of memory is not a condition that can be checked
> beforehand by the caller.

Sure it is.  You can duplicate the functionality of exponential growth
by calling size(), capacity() and reserve() before doing push_back().

I'd really like to see the interface be as close to C++11 std::vector
as possible (other than vector<bool>).
-- 
 Nevin ":-)" Liber  <mailto:nevin_at_[hidden]>  [phone]