How can I use multiple containers in a range-based for?

Question

I've got multiple containers of different types.
I'd like to perform the same action on all of the elements across all the containers.
Normally, this involves multiple range-based for loops with duplicated code:

#include <iostream>
#include <vector>
#include <set>

int main() {

    //containers
    std::vector<int> v1{1,2,3,4,5};
    std::set<float> v2{6,7,8,9,10};

    //perform iterations
    for (auto & it: v1){
        std::cout << it << ' ';
    }
    for (auto & it: v2){
        std::cout << it << ' ';
    }
}

I'd like to be able to do this instead,
by providing multiple containers to the same range-based for loop.

This of course does not work:

for (auto & it: v1,v2){
    std::cout << it << ' ';
}

Is there a library solution I could use to achieve this?

Answer 1

You could use boost range's combine:

for(auto&& el : boost::combine(v1, v2)) {
    std::cout << boost::get<0>(el) << ", " << boost::get<1>(el) << '\n';
}

demo

Or, range-v3's zip view:

for(auto&& el : view::zip(v1, v2)) {
    std::cout << std::get<0>(el) << ", " << std::get<1>(el) << '\n';
}

demo

---

Alternatively, you could make a range from zip iterators the hard way:

auto b = boost::make_zip_iterator(boost::make_tuple(v1.begin(), v2.begin()));
auto e = boost::make_zip_iterator(boost::make_tuple(v1.end(), v2.end()));
for(auto&& tup : boost::make_iterator_range(b, e)) {
    std::cout << boost::get<0>(tup) << ", " << boost::get<1>(tup) << '\n';
}

demo