basic_string_ctor_ex

template <class InputIterator>
basic_string::basic_string(InputIterator begin, InputIterator end, const Allocator& a = Allocator())
 : basic_string(distance(begin, end), ' ', a){
    size_t i = 0;
    for(auto it = begin; it != end; ++it, ++i) this->s[i] = *it;
}