Name lookup is the procedure by which aname, when encountered in a program, is associated with thedeclaration that introduced it.

For example, to compilestd::cout<<std::endl;, the compiler performs:

• unqualified name lookup for the namestd, which finds the declaration of namespace std in the header<iostream>
• qualified name lookup for the namecout, which finds a variable declaration in the namespacestd
• qualified name lookup for the nameendl, which finds a function template declaration in the namespacestd
• bothargument-dependent lookup for the nameoperator<<, which finds multiple function template declarations in the namespacestd, and qualified name lookup for the namestd::ostream::operator<<, which finds multiple member function declarations in classstd::ostream.

For function and function template names, name lookup can associate multiple declarations with the same name, and may obtain additional declarations fromargument-dependent lookup.Template argument deduction may also apply, and the set of declarations is passed tooverload resolution, which selects the declaration that will be used.Member access rules, if applicable, are considered only after name lookup and overload resolution.

For all other names (variables, namespaces, classes, etc), name lookup can associate multiple declarations only if they declare the sameentity, otherwise it must produce a single declaration in order for the program to compile. Lookup for a name in a scope finds all declarations of that name, with one exception, known as the "struct hack" or "type/non-type hiding": Within the same scope, some occurrences of a name may refer to a declaration of a class/struct/union/enum that is not atypedef, while all other occurrences of the same name either all refer to the same variable, non-static data member, or enumerator, or they all refer to possibly overloaded function or function template names. In this case, there is no error, but the type name is hidden from lookup (the code must useelaborated type specifier to access it).

[edit]Types of lookup

If the name appears immediately to the right of the scope resolution operator:: or possibly after:: followed by the disambiguating keywordtemplate, see

• Qualified name lookup

Otherwise, see

• Unqualified name lookup

• (which, for function names, includesArgument-dependent lookup)

[edit]Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[edit]See also

• Scope
• Argument-dependent lookup (ADL)
• Template argument deduction
• Overload resolution

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