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c++dictionaryiteratoriterator-range

Best way to return a transform iterator range

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.

Solution

  • 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))