[dcl.pre] (original) (raw)

9 Declarations [dcl]

9.1 Preamble [dcl.pre]

Declarations generally specify how names are to be interpreted.

Declarations have the form

Unless otherwise stated, utterances in[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.

Asimple-declaration ornodeclspec-function-declaration of the form

is divided into three parts.

[Note 2:

In the declaration for an entity, attributes appertaining to that entity can appear at the start of the declaration and after thedeclarator-id for that declaration.

— _end note_]

[Example 1: [[noreturn]] void f [[noreturn]] (); — _end example_]

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

[Example 2: enum { }; typedef class { }; — _end example_]

The declaration shall contain at most one sb-identifierwhose identifier is preceded by an ellipsis.

If the declaration contains any such sb-identifier, it shall declare a templated entity ([temp.pre]).

[Example 3: template<class T> concept C = true; C auto [x, y] = std::pair{1, 2}; — _end example_]

Otherwise, if the type associated with a declarator-idis a function type ([dcl.fct]), the declaration is a function declaration.

Otherwise, if the type associated with a declarator-idis an object or reference type, the declaration is an object declaration.

Otherwise, the program is ill-formed.

[Example 4: int f(), x; extern void g(), y; — _end example_]

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

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

[Note 5:

If the declaration acquires a function type through template instantiation, the program is ill-formed; see [temp.spec.general].

The function type of a function definition cannot be specified with a typedef-name ([dcl.fct]).

— _end note_]

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

Otherwise, a static_assert-message shall be an expression Msuch that

• the expression M.size() is implicitly convertible to the type std​::​size_t, and
• the expression M.data() is implicitly convertible to the type “pointer to const char”.

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

If the value of the expression E when so converted is true or the expression is evaluated in the context of a template definition, the declaration has no effect and the static_assert-message is an unevaluated operand ([expr.context]).

Otherwise, the static_assert-declaration fails and

• the program is ill-formed, and
• if the static_assert-message is a constant-expression M,
  • M.size() shall be a converted constant expression of type std​::​size_t and let N denote the value of that expression,
  • M.data(), implicitly converted to the type “pointer to const char”, shall be a core constant expression and let D denote the converted expression,
  • for each i where ,D[i] shall be an integral constant expression, and
  • the text of the static_assert-message is formed by the sequence of N code units, starting at D, of the ordinary literal encoding ([lex.charset]).

Recommended practice: When a static_assert-declaration fails, the resulting diagnostic message should include the text of the static_assert-message, if one is supplied.

[Example 5: static_assert(sizeof(int) == sizeof(void*), "wrong pointer size");static_assert(sizeof(int[2])); template <class T> void f(T t) { if constexpr (sizeof(T) == sizeof(int)) { use(t);} else { static_assert(false, "must be int-sized");} } void g(char c) { f(0); f(c); } — _end example_]

An empty-declaration has no effect.

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