prog.cpp: In function ‘int main()’:
prog.cpp:9:5: error: reference to ‘less’ is ambiguous
prog.cpp:4:8: error: candidates are: struct less
In file included from /usr/include/c++/4.7/functional:50:0,
from prog.cpp:1:
/usr/include/c++/4.7/bits/stl_function.h:233:12: error: template<class _Tp> struct std::less
prog.cpp:9:10: error: expected ‘;’ before ‘a’