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.

An example of ADL looks like this:

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 operator<< without the std:: qualifier. However, in this case, the overload of operator<< that works for string is in the std namespace, so ADL is required for it to be used.

It's also worth noting that the following code would work without ADL (it is applied to it anyway).

It works because the output operator for integers is a member function of the std::ostream class, which is the type of cout. Thus, the compiler interprets this statement as

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