/in/foo.cc: In function 'int main()':
/in/foo.cc:11:8: error: reference to 'plus' is ambiguous
cout<<plus(a,b);
^~~~
/in/foo.cc:3:5: note: candidates are: int plus(int, int)
int plus(int a,int b)
^~~~
In file included from /usr/include/c++/6/string:48:0,
from /usr/include/c++/6/bits/locale_classes.h:40,
from /usr/include/c++/6/bits/ios_base.h:41,
from /usr/include/c++/6/ios:42,
from /usr/include/c++/6/ostream:38,
from /usr/include/c++/6/iostream:39,
from /in/foo.cc:1:
/usr/include/c++/6/bits/stl_function.h:167:12: note: template<class _Tp> struct std::plus
struct plus : public binary_function<_Tp, _Tp, _Tp>
^~~~