Compilare: main.cpp: In instantiation of 'bool Huge<Base>::operator<(Huge<Base>) const [with int Base = 10]': /usr/include/c++/6/bits/stl_function.h:386:20: required from 'constexpr bool std::less<_Tp>::operator()(const _Tp&, const _Tp&) const [with _Tp = Huge<10>]' /usr/include/c++/6/bits/stl_map.h:481:32: required from 'std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type& std::map<_Key, _Tp, _Compare, _Alloc>::operator[](const key_type&) [with _Key = Huge<10>; _Tp = int; _Compare = std::less<Huge<10> >; _Alloc = std::allocator<std::pair<const Huge<10>, int> >; std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type = int; std::map<_Key, _Tp, _Compare, _Alloc>::key_type = Huge<10>]' main.cpp:182:18: required from here main.cpp:59:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] if (digits.size() == num.size()) { main.cpp:64:34: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] } return digits.size() < num.size();