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 washidden by a non-type declaration. They may also be used to declare new class names.

[edit]Syntax

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 asforward declaration of classes, for the description of form(3), seeForward declaration. The following only apply to form(1) and(2).

Theclass-name orenum-name in the elaborated type specifier may either be a simple identifier or be aqualified-id. The name is looked up usingunqualified name lookup orqualified 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 foundclass T t;// OK: finds ::T, the local variable T is ignored    T::U* u;// error: lookup of T::U finds the private data memberclass 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 byclass,struct, orunion (i.e. not byenum), andclass-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-namestruct Data* Data;// OK: declares type Data at global scope// and also declares the data member Datafriendclass::List;// error: cannot introduce a qualified nameenum Kind* kind;// error: cannot introduce an enum}; Data* p;// OK: struct Data has been declared

If the name refers to atypedef name, atype alias, atemplate type parameter, or analias template specialization, the program is ill-formed, otherwise the elaborated type specifier introduces the name into the declaration the same way asimple type specifier introduces its type-name.

template<typename T>class Node{friendclass 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;'}

Theclass-key orenum keyword present in the elaborated-type-specifier must agree in kind with the declaration to which the name in the elaborated-type-specifier refers.

• theenum keyword must be used to refer to anenumeration type (whether scoped or unscoped)
• theunionclass-key must be used to refer to aunion
• either theclass orstructclass-key must be used to refer to a non-union class type (the keywordsclass andstruct are interchangeable here).

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

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

[edit]Keywords

class,struct,union,enum

[edit]References

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