reverse_if and reverseOnlyLetters

reverse_if.hpp

#include <algorithm>
#include <boost/iterator/filter_iterator.hpp>

template <class Iter, class Predicate>
void reverse_if(Iter first, Iter last, Predicate pred) {
  std::reverse(boost::make_filter_iterator(pred, first, last),
               boost::make_filter_iterator(pred, last, last));
}

#include <locale>

void reverseOnlyLetters(std::string &s) {
  reverse_if(s.begin(), s.end(),
             [](char c) { return std::isalpha(c, std::locale{}); });
}