# Name lookup

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

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

- unqualified name lookup for the name `std`, which finds the declaration of namespace std in the header `<iostream>`
- qualified name lookup for the name `cout`, which finds a variable declaration in the namespace `std`
- qualified name lookup for the name `endl`, which finds a function template declaration in the namespace `std`
- argument-dependent lookup for the name `operator <<`, which finds multiple function template declarations in the namespace std

For function and function template names, name lookup can associate multiple declarations with the same name, and may obtain additional declarations from argument-dependent lookup. Template argument deduction may also apply, and the set of declarations is passed to overload 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 must produce a single declaration in order for the program to compile, with one exception, known as the "struct hack" or "type/non-type hiding": some occurrences of a name may refer to a declaration of a class/struct/union/enum that is not a typedef, while all other occurrences of the same name either all refer to the same variable, non-static data member (since C++14), 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 use elaborated type specifier to access it)

### Types of lookup

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

- Qualified name lookup

Otherwise, see

- Unqualified name lookup

  - (which, for function names, includes Argument-dependent lookup)

### See also

- Scope
- Argument-dependent lookup
- Template argument deduction
- Overload resolution

C documentation for Lookup and name spaces

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