9 Declarations [dcl.dcl]

Declarations generally specify how names are to be interpreted.

Declarations have the form

declaration-seq:
	declaration
	declaration-seq declaration

declaration:
	block-declaration
	nodeclspec-function-declaration
	function-definition
	template-declaration
	deduction-guide
	explicit-instantiation
	explicit-specialization
	export-declaration
	linkage-specification
	namespace-definition
	empty-declaration
	attribute-declaration
	module-import-declaration

block-declaration:
	simple-declaration
	asm-declaration
	namespace-alias-definition
	using-declaration
	using-enum-declaration
	using-directive
	static_assert-declaration
	alias-declaration
	opaque-enum-declaration

nodeclspec-function-declaration:
	attribute-specifier-seq${}_{opt}$ declarator ;

alias-declaration:
	using identifier attribute-specifier-seq${}_{opt}$ = defining-type-id ;

simple-declaration:
	decl-specifier-seq init-declarator-list${}_{opt}$ ;
	attribute-specifier-seq decl-specifier-seq init-declarator-list ;
	attribute-specifier-seq${}_{opt}$ decl-specifier-seq ref-qualifier${}_{opt}$ [ identifier-list ] initializer ;

static_assert-declaration:
	static_­assert ( constant-expression ) ;
	static_­assert ( constant-expression , string-literal ) ;

empty-declaration:
	;

attribute-declaration:
	attribute-specifier-seq ;

A simple-declaration or nodeclspec-function-declaration of the form

attribute-specifier-seq${}_{opt}$ decl-specifier-seq${}_{opt}$ init-declarator-list${}_{opt}$ ;

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.

[[noreturn]] void f [[noreturn]] ();    // OK

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

A declaration occurs in a scope; the scope rules are summarized in [basic.lookup].

A declaration that declares a function or defines a class, namespace, template, or function also has one or more scopes nested within it.

These nested scopes, in turn, can have declarations nested within them.

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.

In a simple-declaration, the optional init-declarator-list can be omitted only when declaring a class or enumeration, 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 among the names being declared by the declaration (as class-names, enum-names, or enumerators, depending on the syntax).

In such cases, the decl-specifier-seq shall introduce one or more names into the program, or shall redeclare a name introduced by a previous declaration.

enum { };           // error
typedef class { };  // error

In a static_assert-declaration, the constant-expression shall be a contextually converted constant expression of type bool.

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.

static_assert(sizeof(int) == sizeof(void*), "wrong pointer size");

An empty-declaration has no effect.

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.

Each init-declarator in the init-declarator-list contains exactly one declarator-id, which is the name declared by that init-declarator and hence one of the names declared by the declaration.

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

If the decl-specifier-seq contains the typedef specifier, the declaration is called a typedef declaration and the name of each init-declarator 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 the name is a function type ([dcl.fct]) and an object declaration otherwise.

Syntactic components beyond those found in the general form of 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.