Compiler warning with -Wsign-conversion

[ettom]

Option.hpp:479:34: warning: implicit conversion changes signedness: 'int' to 'std::vector::size_type' (aka 'unsigned long') [-Wsign-conversion]
            return &(validators_[index]);
                     ~~~~~~~~~~~ ^~~~~
Option.hpp:1151:70: warning: implicit conversion changes signedness: 'int' to 'const unsigned long' [-Wsign-conversion]
                size_t trim_size = std::min<size_t>(std::max<size_t>(get_items_expected_max(), 1), original.size());
                                                    ~~~              ^~~~~~~~~~~~~~~~~~~~~~~~
Option.hpp:1153:49: warning: implicit conversion changes signedness: 'size_t' (aka 'unsigned long') to '__gnu_cxx::__normal_iterator<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > *, std::vector<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::allocator<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > > >::difference_type' (aka 'long') [-Wsign-conversion]
                    res.assign(original.end() - trim_size, original.end());
                                              ~ ^~~~~~~~~
Option.hpp:1158:66: warning: implicit conversion changes signedness: 'int' to 'const unsigned long' [-Wsign-conversion]
            size_t trim_size = std::min<size_t>(std::max<size_t>(get_items_expected_max(), 1), original.size());
                                                ~~~              ^~~~~~~~~~~~~~~~~~~~~~~~
Option.hpp:1160:65: warning: implicit conversion changes signedness: 'size_t' (aka 'unsigned long') to '__gnu_cxx::__normal_iterator<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > *, std::vector<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::allocator<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > > >::difference_type' (aka 'long') [-Wsign-conversion]
                res.assign(original.begin(), original.begin() + trim_size);
                                                              ~ ^~~~~~~~~