Question

I have a container filled with pairs. I want to iterate in it using the STL generic algorithms (in my case it would be inner_product, but consider it as a generic problem). The algorithm I am using expects iterators first and last. Can I provide special iterators first and last that will iterate not on the pairs but on the first element of each pair?

I know i can do it manually, providing a hand-made function object that will be a wrapper around the standard container iterator, deferencing it to the first member of the pair intend of the pair itself,but I think there is also a clever one-liner to do it for me. What would it be?

Answer 1

I’ve looked around and found boost::transform_iterator. I’ve come up with this code. Surprising how well it works:

#include <map> #include <algorithm> #include <iostream> #include <string> #include <iterator> #include <boost/iterator/transform_iterator.hpp> #include <boost/bind.hpp> #include <boost/function.hpp>  int main() {     typedef std::map<std::string, int>::value_type value_type;     std::map<std::string, int> a;     a['one'] = 1;     a['two'] = 2;      // returns the second element      boost::function<int(value_type&)> f = boost::bind(&value_type::second, _1);     std::copy(boost::make_transform_iterator(a.begin(), f),                boost::make_transform_iterator(a.end(), f),               std::ostream_iterator<int>(std::cout, ' '));  } 

It’s printing '1 2 ' to the standard output.