Compilare: main.cpp: In function 'std::ofstream& operator<<(std::ofstream&, BigNum)': main.cpp:40:19: warning: ISO C++ says that these are ambiguous, even though the worst conversion for the first is better than the worst conversion for the second: out<<a.v[i]; ^ In file included from /usr/include/c++/6/ostream:638:0, from /usr/include/c++/6/iostream:39, from main.cpp:1: /usr/include/c++/6/bits/ostream.tcc:105:5: note: candidate 1: std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(int) [with _CharT = char; _Traits = std::char_traits<char>] basic_ostream<_CharT, _Traits>:: ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ main.cpp:38:17: note: candidate 2: std::ofstream& operator<<(std::ofstream&, BigNum) std::ofstream & operator << (std::ofstream &out,BigNum a){ ^~~~~~~~