ArgumentDependentNameLookup is also known as Argument Dependent Lookup (ADL) or KoenigLookup. It is a feature of CeePlusPlus, one of the CeePlusPlusIdioms.

See http://en.wikipedia.org/wiki/Argument_dependent_name_lookup for a definition and discussion.

See also 

* CppCommonKnowledge Item 25 p.89
* CppTemplatesTheCompleteGuide Chapter 9 
* CppCodingStandards item 57 p.104
* ExceptionalCpp, Items 31 and 32 p.119.

It is used in the C++ Standard Library. See TheCppStandardLibrary p.17
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CategoryLanguageFeature CategoryCpp CategoryIdiom