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##### map_keys

Syntax | Code
--- | ---
Pipe | `rng | boost::adaptors::map_keys`
Function | `boost::adaptors::keys(rng)`

- Precondition: The `value_type` of the range is an instantiation of `std::pair`.
- Postcondition: For all elements `x` in the returned range, `x` is the result of `y.first` where `y` is the corresponding element in the original range.
- Range Category: Single Pass Range
- Range Return Type: `boost::select_first_range<typeof(rng)>`
- Returned Range Category: The range category of `rng`.

###### map_keys example

    #include <boost/range/adaptor/map.hpp>
    #include <boost/range/algorithm/copy.hpp>
    #include <boost/assign.hpp>
    #include <algorithm>
    #include <iostream>
    #include <map>
    #include <vector>

    int main(int argc, const char* argv[])
    {
        using namespace boost::assign;
        using namespace boost::adaptors;

        std::map<int,int> input;
        for (int i = 0; i < 10; ++i)
            input.insert(std::make_pair(i, i * 10));

        boost::copy(
            input | map_keys,
            std::ostream_iterator<int>(std::cout, ","));

        return 0;
    }

This would produce the output:

    0,1,2,3,4,5,6,7,8,9