Initialize a container with iterator range of container with different type

Question

Let's say we have std::set<int> and we want to create a std::vector<int> with all values from that set:

std::set<int> set;
std::vector<int> vec( set.begin(), set.end() );

This is simple and elegant. But let's say I have a std::map<std::string,int> and I want to copy all values to std::vector<int>. Unfortunately there is no constructor, that accepts range of iterators and converter function. Why there is no such constructor provided? Is there another simple and elegant way to initialize one container with different type values?

Answer 1

Use transform iterators:

#include <boost/iterator/transform_iterator.hpp>
#include <vector>
#include <map>

int main() {
    std::map<int, double> m;
    auto f = [](auto&& pair) { return pair.second; };
    std::vector<double>(boost::make_transform_iterator(m.begin(), f),
                        boost::make_transform_iterator(m.end(), f));
}

Alternatively, use boost::adaptors:

#include <boost/range/adaptor/map.hpp>
#include <vector>
#include <map>

int main() {
    std::map<int, double> m;
    auto range = boost::adaptors::values(m);
    std::vector<double>(range.begin(), range.end());
}

Or the same as above:

    auto v = boost::copy_range<std::vector<double> >(boost::adaptors::values(m));

Note that using vector's range constructor is more efficient than a solution involving back_inserter.

Answer 2

With boost::transform_iterator:

#include <functional>
#include <boost/iterator/transform_iterator.hpp>

std::map<std::string, int> m{ {"a", 1}, {"b", 2} };
auto second = std::mem_fn(&std::map<std::string, int>::value_type::second);

std::vector<int> vec(boost::make_transform_iterator(std::begin(m), second)
                   , boost::make_transform_iterator(std::end(m), second));

DEMO