Main.cc: In function 'int main()': Main.cc:16:13: error: no match for 'operator<' (operand types are 'std::basic_ostream' and 'const char [2]') cout< constexpr bool std::operator<(const std::pair<_T1, _T2>&, const std::pair<_T1, _T2>&) operator<(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y) ^ /usr/include/c++/5/bits/stl_pair.h:220:5: note: template argument deduction/substitution failed: Main.cc:16:14: note: 'std::basic_ostream' is not derived from 'const std::pair<_T1, _T2>' cout<