Argument Dependent Lookup (ADL) is also known as Koenig Lookup. When an unqualified call (without any namespaces) is made to a function with user defined types, it will lookup the function inside the type’s namespace also.
This is what enables cout’s stream operator.
std::cout << "hello";In the above code, there’s no operator<< in the global namespace. However, due to ADL, we lookup in the std namespace where we find std::operator<<() defined and that is used.