In file included from prog.cpp:1:0:
/usr/include/c++/5/ostream: In function 'int main()':
/usr/include/c++/5/ostream:384:7: error: 'std::basic_ostream<_CharT, _Traits>::basic_ostream() [with _CharT = char; _Traits = std::char_traits<char>]' is protected
basic_ostream()
^
prog.cpp:7:18: error: within this context
std::ostream s;
^