Argument dependent name lookup

In the C++ programming language, Koenig lookup, also known as argument dependent lookup (ADL), or argument dependent name lookup, applies to the lookup of an unqualified function name depending on the types of the arguments given to the function call. This behavior is named after Andrew Koenig.In Koenig lookup, other namespaces not considered during normal lookup may be searched. The set of namespaces to be searched depends on the types of the function arguments.For a class A, there is a set of associated classes which make up its direct and indirect base classes. The set of associated namespaces for A is the namespace that contains A together with the namespaces containing A's associated classes. It is this set of associated namespaces that is searched for a function with an argument of type A.
See more at Wikipedia.org...