Argument-dependent name lookup

In the C++ programming language, 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 also known as Koenig lookup, as it is often attributed to Andrew Koenig, though he is not its inventor.

During argument-dependent lookup, other namespaces not considered during normal lookup may be searched where the set of namespaces to be searched depends on the types of the function arguments. Specifically, the set of declarations discovered during the ADL process, and considered for resolution of the function name, is the union of the declarations found by normal lookup with the declarations found by looking in the set of namespaces associated with the types of the function arguments.

Example
An example of ADL looks like this:

Even though the main function is not in namespace NS, nor is namespace NS in scope, the function NS::f(A&, int) is found because of the declared types of the actual arguments in the function call statement.

A common pattern in the C++ Standard Library is to declare overloaded operators that will be found in this manner. For example, this simple Hello World program would not compile if it weren't for ADL: Using  is equivalent to calling   without the   qualifier. However, in this case, the overload of operator<< that works for  is in the   namespace, so ADL is required for it to be used.

The following code would work without ADL (which is applied to it anyway):

It works because the output operator for integers is a member function of the  class, which is the type of. Thus, the compiler interprets this statement as which it can resolve during normal lookup. However, consider that e.g. the  overloaded   is a non-member function in the   namespace and, thus, requires ADL for a correct lookup:

The  namespace overloaded non-member   function to handle strings is another example: As Koenig points out in a personal note, without ADL the compiler would indicate an error stating it could not find  as the statement doesn't explicitly specify that it is found in the   namespace.

Interfaces
Functions found by ADL are considered part of a class's interface. In the C++ Standard Library, several algorithms use unqualified calls to  from within the   namespace. As a result, the generic  function is used if nothing else is found, but if these algorithms are used with a third-party class, , found in another namespace that also contains  , that overload of   will be used.

Criticism
While ADL makes it practical for functions defined outside of a class to behave as if they were part of the interface of that class, it makes namespaces less strict and so can require the use of fully qualified names when they would not otherwise be needed. For example, the C++ standard library makes extensive use of unqualified calls to  to swap two values. The idea is that then one can define an own version of  in one's own namespace and it will be used within the standard library algorithms. In other words, the behavior of may or may not be the same as the behavior of (where  and   are of type  ) because if   exists, the second of the above examples will call it while the first will not. Furthermore, if for some reason both  and   are defined, then the first example will call   but the second will not compile because   would be ambiguous.

In general, over-dependence on ADL can lead to semantic problems. If one library,, expects unqualified calls to   to have one meaning and another library,   expects it to have another, then namespaces lose their utility. If, however,  expects   to have one meaning and   does likewise, then there is no conflict, but calls to   would have to be fully qualified (i.e.   as opposed to  ) lest ADL get in the way.