9 Declarations [dcl.dcl]

Declarations generally specify how names are to be interpreted.

Declarations have the form

Certain declarations contain one or more scopes ([basic.scope.scope]).

Unless otherwise stated, utterances in [dcl.dcl] about components in, of, or contained by a declaration or subcomponent thereof refer only to those components of the declaration that are not nested within scopes nested within the declaration.

A simple-declaration or nodeclspec-function-declaration of the form is divided into three parts.

Attributes are described in [dcl.attr].

decl-specifiers, the principal components of a decl-specifier-seq, are described in [dcl.spec].

declarators, the components of an init-declarator-list, are described in [dcl.decl].

The attribute-specifier-seq appertains to each of the entities declared by the declarators of the init-declarator-list.

If a declarator-id is a name, the init-declarator and (hence) the declaration introduce that name.

The defining-type-specifiers ([dcl.type]) in the decl-specifier-seq and the recursive declarator structure describe a type ([dcl.meaning]), which is then associated with the declarator-id.

In a simple-declaration, the optional init-declarator-list can be omitted only when declaring a class ([class.pre]) or enumeration ([dcl.enum]), that is, when the decl-specifier-seq contains either a class-specifier, an elaborated-type-specifier with a class-key ([class.name]), or an enum-specifier.

In these cases and whenever a class-specifier or enum-specifier is present in the decl-specifier-seq, the identifiers in these specifiers are also declared (as class-names, enum-names, or enumerators, depending on the syntax).

In such cases, the decl-specifier-seq shall (re)introduce one or more names into the program.

A simple-declaration with an identifier-list is called a structured binding declaration ([dcl.struct.bind]).

If the decl-specifier-seq contains any decl-specifier other than static, thread_­local, auto ([dcl.spec.auto]), or cv-qualifiers, the program is ill-formed.

The initializer shall be of the form “= assignment-expression”, of the form “{ assignment-expression }”, or of the form “( assignment-expression )”, where the assignment-expression is of array or non-union class type.

If the decl-specifier-seq contains the typedef specifier, the declaration is called a typedef declaration and each declarator-id is declared to be a typedef-name, synonymous with its associated type ([dcl.typedef]).

If the decl-specifier-seq contains no typedef specifier, the declaration is called a function declaration if the type associated with a declarator-id is a function type ([dcl.fct]) and an object declaration otherwise.

Syntactic components beyond those found in the general form of simple-declaration are added to a function declaration to make a function-definition.

An object declaration, however, is also a definition unless it contains the extern specifier and has no initializer ([basic.def]).

An object definition causes storage of appropriate size and alignment to be reserved and any appropriate initialization ([dcl.init]) to be done.

A nodeclspec-function-declaration shall declare a constructor, destructor, or conversion function.

In a static_assert-declaration, the constant-expression is contextually converted to bool and the converted expression shall be a constant expression ([expr.const]).

If the value of the expression when so converted is true, the declaration has no effect.

Otherwise, the program is ill-formed, and the resulting diagnostic message ([intro.compliance]) shall include the text of the string-literal, if one is supplied, except that characters not in the basic source character set are not required to appear in the diagnostic message.

An empty-declaration has no effect.

Except where otherwise specified, the meaning of an attribute-declaration is implementation-defined.