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Subject: Re: [Boost-users] [Iterator] shared_container_iterator usability
From: Pekka Seppänen (pekka.seppanen_at_[hidden])
Date: 2012-03-19 13:54:35

On 19.3.2012 18:28, Szymon Gatner wrote:
>> Erm, why wouldn't it be a good idea?
> Because one would normally just return a function-local vector by
> value which is much cheaper (with move semantics) then heap allocation
> of a container + 2 x shared_ptr construction.
>> Moreover, as vector is returned wrapped inside a shared_container_iterator
>> (well, a pair of those) one may pass it directly to Boost.Range or
>> Boost.Foreach (yep, std::pair supported) without spending too much time
>> thinking when and how to allocate and release memory as shared_ptr does that
>> for you.
> All well supported also with just returning a container by value.
>> I don't really know why you should feel bad about allocating a vector from
>> heap as std::allocator will use heap anyway.
> This way you get 2 heap allocations + shared_ptr<> construction as
> opposed to just internal allocation of vector (using allocator). This
> is especially bad if returned vector turns out to be empty. In that
> case vector might not do allocation at all so vector creation and
> return are practically free.

Well, obviously if your compiler has good and guaranteed move semantics along
with return value optimizations and you're extermely speed oriented in every
way, that's your way to go. You don't have to use everything that library has
to offer to you :) The same goes with every choice, whether to use raw
pointers (fast, but possibly allocation troubles) or some smart pointer (a
small speed penalty, but carefree usage) etc.; It all depends.

I must say, that personally I'm not really into returning anything except pod
types and trivial things by value. Naturally it happens, strings for an example.

Nevertheless, I think it's meaningless to operate on such a low level where
you calculate each and every one of your allocations and construtions in a
place that they don't usally really matter. (Say, you do a million operations
per function call - The first place to look for improvement wouldn't be the
few constructions outside the actual and heavyweight loop.) Naturally, if your
vector contains only a dozen values the return type is a good question. On the
other hand, it's again meaningless if you call your function a couple of
times. How about if you call it a million times? I think a small refactoring
could be in place etc. I think you get what I mean ;-)

>> If I'd had to optimize (as in Optimize, not some
>> needless micro optimizations) some vector / array related operation, I think
>> I'd forget any std container, let alone shared_ptr, in the first place...
> Interesting. What would you use instead of a vector + specfic
> allocator when "Optimizing"? Continuous block of memory is rather hard
> to beat in terms of data processing speed.

Well, I suppose as vectors are pretty much guaranteed to have a contiguous
memory allocation and if your platform and compiler is able to use hardware
speedups it could work; especially DSP platforms have many loop related tricks
that allow certain things to be executed in parallel + the usual (unrolling
etc.) tricks. However, this gets too much off the track here.

So, my answer final answer is to not use things just because they exist but
only if they provide something that is useful for you. What works for me might
not work for you as seen here..

-- Pekka

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