Neatest way to loop over a range of integers

The neatest way is still this:

for (int i=0; i<n; ++i)

I guess you can do this, but I wouldn't call it so neat:

#include <iostream>

int main()
{
  for ( auto i : { 1,2,3,4,5 } )
  {
    std::cout<<i<<std::endl;
  }
}

With C++20 we will have ranges. If you don't have access to C++20, you can try them by downloading the lastest stable release from it's author, Eric Niebler, from his github, or go to Wandbox. What you are interested in is ranges::views::iota, which makes this code legal:

#include <range/v3/all.hpp>
#include <iostream>

int main() {
    using namespace ranges;
    
    for (int i : views::iota(1, 10)) {
        std::cout << i << ' ';
    }
}

What's great about this approach is that views are lazy. That means even though views::iota represents a range from 1 to 10 exclusive, no more than one int from that range exists at one point. The elements are generated on demand.

If you do have access to C++20, this version works out of the box:

#include <ranges>
#include <iostream>

int main() {  
    for (int i : std::views::iota(1, 10)) {
        std::cout << i << ' ';
    }
}

While its not provided by C++11, you can write your own view or use the one from boost:

#include <boost/range/irange.hpp>
#include <iostream>

int main(int argc, char **argv)
{
    for (auto i : boost::irange(1, 10))
        std::cout << i << "\n";
}

Moreover, Boost.Range contains a few more interesting ranges which you could find pretty useful combined with the new for loop. For example, you can get a reversed view.