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
identifierin the
scopethat contains the declaration.
Opaque enum declaration resembles form (3), but the enum type is a complete type after an opaque enum declaration.
[edit] ExplanationForm (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<typename T>
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<typename T>
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 keyword must be used to refer to an enumeration type (whether scoped or unscoped)union class-key must be used to refer to a unionclass or struct class-key must be used to refer to a non-union class type (the keywords class and struct are interchangeable here).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.
RetroSearch is an open source project built by @garambo | Open a GitHub Issue
Search and Browse the WWW like it's 1997 | Search results from DuckDuckGo
HTML:
3.2
| Encoding:
UTF-8
| Version:
0.7.4