Best way to return a transform iterator range

Question

Let's say I want to write a simple function keys that takes in a std::map and returns an iterator_range that provides the keys of the map. How would I do it?

template<typename A, typename B>
Range<A> keys(const std::map<A, B> & m) { ??? }

I want to apply the same pattern to various other containers, but I think this is a good prototypical example.

Edit: I'm guessing I need something in the realm of Boost's range_adapters and/or transform_iterators but I'm not familiar enough with them to apply them here.

Are you talking about [boost's `iterator_range`](http://www.boost.org/doc/libs/1_55_0/libs/range/doc/html/range/reference/utilities/iterator_range.html)? — dyp, Jan 25 '14 at 23:07
.... something like the [`boost::adaptors::keys`](http://www.boost.org/doc/libs/1_55_0/libs/range/doc/html/range/reference/adaptors/reference/map_keys.html) function? — dyp, Jan 25 '14 at 23:34
@dyp: The best things in life have already been done. ;) Thanks. I'll have to look at the implementation and see if that helps me repeat the pattern elsewhere. — Elliot Cameron, Jan 25 '14 at 23:43
@3noch - have you find a solution? if yes, answer your own question so the community will learn — NirMH, Jun 01 '14 at 08:04

score 1 · Accepted Answer · answered Jun 03 '14 at 16:15

This particular need is met with boost::adapters::keys. More generally, boost::range allows you to design your own adapters, but it's somewhat involved. So unless you're designing a new library, you might be able to get away with transformed.

If you need the result of your range back into a container, you can write a simple collect function which will "collect" the result of a boost::range pipeline.

template<typename Output, typename SinglePassRange>
Output collect(const SinglePassRange & rng)
{
    Output r;
    boost::range::copy(rng, std::inserter(r, boost::begin(r)));
    return r;
}

Now you can easily whip up some little functions and

collect<vector<int>>(numbers | filtered(odd))
collect<vector<int>>(numbers | transformed(doubled))
collect<vector<K>>(myMap | transformed(keyOf))

Best way to return a transform iterator range

1 Answers1