Elaborated type specifier - cppreference.com (original) (raw)

Elaborated type specifiers may be used to refer to a previously-declared class name (class, struct, or union) or to a previously-declared enum name even if the name was hidden by a non-type declaration. They may also be used to declare new class names.

[edit] Syntax

| | | | | ------------------------------------------ | --- | | | class-key class-name | (1) | | | | | | | enum enum-name | (2) | | | | | | | class-key attr (optional) identifier ; | (3) | | | | | |

class-key - one of class, struct, union
class-name - the name of a previously-declared class type, optionally qualified, or an identifier not previously declared as a type name
enum-name - the name of a previously-declared enumeration type, optionally qualified
attr - (since C++11) any number of attributes

  1. Elaborated type specifier for a class type.

  2. Elaborated type specifier for an enumeration type.

  3. A declaration that consists solely of an elaborated type specifier always declares a class type named by identifier in the scope that contains the declaration.

Opaque enum declaration resembles form (3), but the enum type is a complete type after an opaque enum declaration.

[edit] Explanation

Form (3) is a special case of elaborated type specifier, usually referred to as forward declaration of classes, for the description of form (3), see Forward declaration. The following only apply to form (1) and (2).

The class-name or enum-name in the elaborated type specifier may either be a simple identifier or be a qualified-id. The name is looked up using unqualified name lookup or qualified name lookup, depending on their appearance. But in either case, non-type names are not considered.

class T { public: class U; private: int U; };   int main() { int T; T t; // error: the local variable T is found class T t; // OK: finds ::T, the local variable T is ignored T::U* u; // error: lookup of T::U finds the private data member class T::U* u; // OK: the data member is ignored }

If the name lookup does not find a previously declared type name, the elaborated-type-specifier is introduced by **class**, **struct**, or **union** (i.e. not by **enum**), and class-name is an unqualified identifier, then the elaborated-type-specifier is a class declaration of the class-name, and the target scope is the nearest enclosing namespace or block scope.

template struct Node { struct Node* Next; // OK: lookup of Node finds the injected-class-name struct Data* Data; // OK: declares type Data at global scope // and also declares the data member Data friend class ::List; // error: cannot introduce a qualified name enum Kind* kind; // error: cannot introduce an enum };   Data* p; // OK: struct Data has been declared

If the name refers to a typedef name, a type alias, a template type parameter, or an alias template specialization, the program is ill-formed, otherwise the elaborated type specifier introduces the name into the declaration the same way a simple type specifier introduces its type-name.

template class Node { friend class T; // error: type parameter cannot appear in an elaborated type specifier; // note that similar declaration friend T; is OK. };   class A {}; enum b { f, t };   int main() { class A a; // OK: equivalent to 'A a;' enum b flag; // OK: equivalent to 'b flag;' }

The class-key or **enum** keyword present in the elaborated-type-specifier must agree in kind with the declaration to which the name in the elaborated-type-specifier refers.

enum class E { a, b }; enum E x = E::a; // OK enum class E y = E::b; // error: 'enum class' cannot introduce an elaborated type specifier   struct A {}; class A a; // OK

When used as a template argument, class T is a type template parameter named T, not an unnamed constant parameter whose type T is introduced by elaborated type specifier.

[edit] Keywords

class,struct,union,enum

[edit] References