Re: [Boost-users] Iterator over values of std::map<K, V>

From: chun ping wang (cablepuff_at_[hidden])
Date: 2007-12-03 18:24:33

hmm what do you mean? can you give a example.

template
<
        class T, class U,
        template <typename ELEM, typename = std::allocator<ELEM> >
        class CONT
>
void getKeys(const boost::unordered_map<T, U>& data, const
CONT<T>::iterator);

On 12/3/07, Niels Aan de Brugh <nielsadb_at_[hidden]> wrote:
>
> On Fri, Nov 30, 2007 at 10:07:19AM -0800, chun ping wang wrote:
> > hey i am curious on the actual code you written. Cause i want to know
> > if doing that is faster than copying an item to a container and like
> > iterate over that.
>
> I'm sure there are cases when the extra copy performs better than the
> extra function call per element, but it'll have to be a lot of them to
> measure the difference. Aas an idiom however I think it's best to avoid
> copying if possible: for some values it may be rather expensive.
>
> > template
> >     <
> >        class T, class U,
> >        template <typename ELEM, typename = std::allocator<ELEM> >
> >        class CONT
> >     >
> > CONT<T> getKeys(const boost::unordered_map<T, U>& data)
> > {
> >     CONT<T> retCont;
> >     typedef typename boost::unordered_map<T, U>::value_type value_type;
> >     BOOST_FOREACH(value_type val, data)
> >     {
> >         retCont.push_back(val.first);
> >     }
> >     return retCont;
> > }
>
> Modulo RVO this even copies the data twice.
>
> While we're talking iterators why not accept an output iterator as an
> argument, write to it and return the resulting iterator at the end (cf.
> std::copy)?
>
> This is more generic (the concept of an iterator is simpler and in this
> case closer to what you really need than the concept of a container) and
> more flexible, especially with the Boost iterator adapters (e.g. using
> the previously mentioned transform_iterator you get some kind of
> for_each).
>
> N.
>
>
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