10 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
	linkage-specification
	namespace-definition
	empty-declaration
	attribute-declaration
block-declaration:
	simple-declaration
	asm-definition
	namespace-alias-definition
	using-declaration
	using-directive
	static_assert-declaration
	alias-declaration
	opaque-enum-declaration
nodeclspec-function-declaration:
	attribute-specifier-seq declarator ;
alias-declaration:
	using identifier attribute-specifier-seq = defining-type-id ;
simple-declaration:
	decl-specifier-seq init-declarator-list ;
	attribute-specifier-seq decl-specifier-seq init-declarator-list ;
	attribute-specifier-seq decl-specifier-seq ref-qualifier [ identifier-list ] initializer ;
static_assert-declaration:
	static_assert ( constant-expression ) ;
	static_assert ( constant-expression , string-literal ) ;
empty-declaration:
	;
attribute-declaration:
	attribute-specifier-seq ;
[Note
:
asm-definitions are described in [dcl.asm], and linkage-specifications are described in [dcl.link].
end note
]
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.
[Note
:
In the declaration for an entity, attributes appertaining to that entity may appear at the start of the declaration and after the declarator-id for that declaration.
end note
]
[Example
:
[[noreturn]] void f [[noreturn]] ();    // OK
end example
]
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.
[Example
:
enum { };           // ill-formed
typedef class { };  // ill-formed
end example
]
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.
[Example
:
static_assert(char(-1) < 0, "this library requires plain 'char' to be signed");
end example
]
An empty-declaration has no effect.
The decl-specifier-seq shall contain only the type-specifier auto and cv-qualifiers.
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]).
A definition causes the appropriate amount of storage to be reserved and any appropriate initialization ([dcl.init]) to be done.
A nodeclspec-function-declaration shall declare a constructor, destructor, or conversion function.93
The “implicit int” rule of C is no longer supported.

10.1 Specifiers [dcl.spec]

The specifiers that can be used in a declaration are
decl-specifier:
	storage-class-specifier
	defining-type-specifier
	function-specifier
	friend
	typedef
	constexpr
	inline
decl-specifier-seq:
	decl-specifier attribute-specifier-seq
	decl-specifier decl-specifier-seq
The optional attribute-specifier-seq in a decl-specifier-seq appertains to the type determined by the preceding decl-specifiers ([dcl.meaning]).
The attribute-specifier-seq affects the type only for the declaration it appears in, not other declarations involving the same type.
Each decl-specifier shall appear at most once in a complete decl-specifier-seq, except that long may appear twice.
If a type-name is encountered while parsing a decl-specifier-seq, it is interpreted as part of the decl-specifier-seq if and only if there is no previous defining-type-specifier other than a cv-qualifier in the decl-specifier-seq.
The sequence shall be self-consistent as described below.
[Example
:
typedef char* Pc;
static Pc;                      // error: name missing
Here, the declaration static Pc is ill-formed because no name was specified for the static variable of type Pc.
To get a variable called Pc, a type-specifier (other than const or volatile) has to be present to indicate that the typedef-name Pc is the name being (re)declared, rather than being part of the decl-specifier sequence.
For another example,
void f(const Pc);               // void f(char* const) (not const char*)
void g(const int Pc);           // void g(const int)
end example
]
[Note
:
Since signed, unsigned, long, and short by default imply int, a type-name appearing after one of those specifiers is treated as the name being (re)declared.
[Example
:
void h(unsigned Pc);            // void h(unsigned int)
void k(unsigned int Pc);        // void k(unsigned int)
end example
]
end note
]

10.1.1 Storage class specifiers [dcl.stc]

The storage class specifiers are
storage-class-specifier:
	static
	thread_local
	extern
	mutable
At most one storage-class-specifier shall appear in a given decl-specifier-seq, except that thread_­local may appear with static or extern.
If thread_­local appears in any declaration of a variable it shall be present in all declarations of that entity.
If a storage-class-specifier appears in a decl-specifier-seq, there can be no typedef specifier in the same decl-specifier-seq and the init-declarator-list or member-declarator-list of the declaration shall not be empty (except for an anonymous union declared in a named namespace or in the global namespace, which shall be declared static ([class.union.anon])).
The storage-class-specifier applies to the name declared by each init-declarator in the list and not to any names declared by other specifiers.
A storage-class-specifier other than thread_­local shall not be specified in an explicit specialization or an explicit instantiation directive.
[Note
:
A variable declared without a storage-class-specifier at block scope or declared as a function parameter has automatic storage duration by default.
end note
]
The thread_­local specifier indicates that the named entity has thread storage duration.
It shall be applied only to the names of variables of namespace or block scope and to the names of static data members.
When thread_­local is applied to a variable of block scope the storage-class-specifier static is implied if no other storage-class-specifier appears in the decl-specifier-seq.
The static specifier can be applied only to names of variables and functions and to anonymous unions.
There can be no static function declarations within a block, nor any static function parameters.
A static specifier used in the declaration of a variable declares the variable to have static storage duration, unless accompanied by the thread_­local specifier, which declares the variable to have thread storage duration.
A static specifier can be used in declarations of class members; [class.static] describes its effect.
For the linkage of a name declared with a static specifier, see [basic.link].
The extern specifier can be applied only to the names of variables and functions.
The extern specifier cannot be used in the declaration of class members or function parameters.
For the linkage of a name declared with an extern specifier, see [basic.link].
[Note
:
The extern keyword can also be used in explicit-instantiations and linkage-specifications, but it is not a storage-class-specifier in such contexts.
end note
]
The linkages implied by successive declarations for a given entity shall agree.
That is, within a given scope, each declaration declaring the same variable name or the same overloading of a function name shall imply the same linkage.
Each function in a given set of overloaded functions can have a different linkage, however.
[Example
:
static char* f();               // f() has internal linkage
char* f()                       // f() still has internal linkage
  { /* ... */ }

char* g();                      // g() has external linkage
static char* g()                // error: inconsistent linkage
  { /* ... */ }

void h();
inline void h();                // external linkage

inline void l();
void l();                       // external linkage

inline void m();
extern void m();                // external linkage

static void n();
inline void n();                // internal linkage

static int a;                   // a has internal linkage
int a;                          // error: two definitions

static int b;                   // b has internal linkage
extern int b;                   // b still has internal linkage

int c;                          // c has external linkage
static int c;                   // error: inconsistent linkage

extern int d;                   // d has external linkage
static int d;                   // error: inconsistent linkage
end example
]
The name of a declared but undefined class can be used in an extern declaration.
Such a declaration can only be used in ways that do not require a complete class type.
[Example
:
struct S;
extern S a;
extern S f();
extern void g(S);

void h() {
  g(a);                         // error: S is incomplete
  f();                          // error: S is incomplete
}
end example
]
The mutable specifier shall appear only in the declaration of a non-static data member whose type is neither const-qualified nor a reference type.
[Example
:
class X {
  mutable const int* p;         // OK
  mutable int* const q;         // ill-formed
};
end example
]
The mutable specifier on a class data member nullifies a const specifier applied to the containing class object and permits modification of the mutable class member even though the rest of the object is const ([dcl.type.cv]).

10.1.2 Function specifiers [dcl.fct.spec]

Function-specifiers can be used only in function declarations.
function-specifier:
	virtual
	explicit
The virtual specifier shall be used only in the initial declaration of a non-static class member function; see [class.virtual].
The explicit specifier shall be used only in the declaration of a constructor or conversion function within its class definition; see [class.conv.ctor] and [class.conv.fct].

10.1.3 The typedef specifier [dcl.typedef]

Declarations containing the decl-specifier typedef declare identifiers that can be used later for naming fundamental or compound types.
The typedef specifier shall not be combined in a decl-specifier-seq with any other kind of specifier except a defining-type-specifier, and it shall not be used in the decl-specifier-seq of a parameter-declaration nor in the decl-specifier-seq of a function-definition ([dcl.fct.def]).
If a typedef specifier appears in a declaration without a declarator, the program is ill-formed.
typedef-name:
	identifier
A name declared with the typedef specifier becomes a typedef-name.
Within the scope of its declaration, a typedef-name is syntactically equivalent to a keyword and names the type associated with the identifier in the way described in [dcl.decl].
A typedef-name is thus a synonym for another type.
A typedef-name does not introduce a new type the way a class declaration ([class.name]) or enum declaration does.
[Example
:
After
typedef int MILES, *KLICKSP;
the constructions
MILES distance;
extern KLICKSP metricp;
are all correct declarations; the type of distance is int and that of metricp is “pointer to int.
end example
]
A typedef-name can also be introduced by an alias-declaration.
The identifier following the using keyword becomes a typedef-name and the optional attribute-specifier-seq following the identifier appertains to that typedef-name.
Such a typedef-name has the same semantics as if it were introduced by the typedef specifier.
In particular, it does not define a new type.
[Example
:
using handler_t = void (*)(int);
extern handler_t ignore;
extern void (*ignore)(int);         // redeclare ignore
using cell = pair<void*, cell*>;    // ill-formed
end example
]
The defining-type-specifier-seq of the defining-type-id shall not define a class or enumeration if the alias-declaration is the declaration of a template-declaration.
In a given non-class scope, a typedef specifier can be used to redefine the name of any type declared in that scope to refer to the type to which it already refers.
[Example
:
typedef struct s { /* ... */ } s;
typedef int I;
typedef int I;
typedef I I;
end example
]
In a given class scope, a typedef specifier can be used to redefine any class-name declared in that scope that is not also a typedef-name to refer to the type to which it already refers.
[Example
:
struct S {
  typedef struct A { } A;       // OK
  typedef struct B B;           // OK
  typedef A A;                  // error
};
end example
]
If a typedef specifier is used to redefine in a given scope an entity that can be referenced using an elaborated-type-specifier, the entity can continue to be referenced by an elaborated-type-specifier or as an enumeration or class name in an enumeration or class definition respectively.
[Example
:
struct S;
typedef struct S S;
int main() {
  struct S* p;                  // OK
}
struct S { };                   // OK
end example
]
In a given scope, a typedef specifier shall not be used to redefine the name of any type declared in that scope to refer to a different type.
[Example
:
class complex { /* ... */ };
typedef int complex;            // error: redefinition
end example
]
Similarly, in a given scope, a class or enumeration shall not be declared with the same name as a typedef-name that is declared in that scope and refers to a type other than the class or enumeration itself.
[Example
:
typedef int complex;
class complex { /* ... */ };    // error: redefinition
end example
]
[Note
:
A typedef-name that names a class type, or a cv-qualified version thereof, is also a class-name ([class.name]).
If a typedef-name is used to identify the subject of an elaborated-type-specifier, a class definition, a constructor declaration, or a destructor declaration, the program is ill-formed.
end note
]
[Example
:
struct S {
  S();
  ~S();
};

typedef struct S T;

S a = T();                      // OK
struct T * p;                   // error
end example
]
If the typedef declaration defines an unnamed class (or enum), the first typedef-name declared by the declaration to be that class type (or enum type) is used to denote the class type (or enum type) for linkage purposes only ([basic.link]).
[Example
:
typedef struct { } *ps, S;      // S is the class name for linkage purposes
end example
]

10.1.4 The friend specifier [dcl.friend]

The friend specifier is used to specify access to class members; see [class.friend].

10.1.5 The constexpr specifier [dcl.constexpr]

The constexpr specifier shall be applied only to the definition of a variable or variable template or the declaration of a function or function template.
A function or static data member declared with the constexpr specifier is implicitly an inline function or variable ([dcl.inline]).
If any declaration of a function or function template has a constexpr specifier, then all its declarations shall contain the constexpr specifier.
[Note
:
An explicit specialization can differ from the template declaration with respect to the constexpr specifier.
end note
]
[Note
:
Function parameters cannot be declared constexpr.
end note
]
[Example
:
constexpr void square(int &x);  // OK: declaration
constexpr int bufsz = 1024;     // OK: definition
constexpr struct pixel {        // error: pixel is a type
  int x;
  int y;
  constexpr pixel(int);         // OK: declaration
};
constexpr pixel::pixel(int a)
  : x(a), y(x)                  // OK: definition
  { square(x); }
constexpr pixel small(2);       // error: square not defined, so small(2)
                                // not constant ([expr.const]) so constexpr not satisfied

constexpr void square(int &x) { // OK: definition
  x *= x;
}
constexpr pixel large(4);       // OK: square defined
int next(constexpr int x) {     // error: not for parameters
     return x + 1;
}
extern constexpr int memsz;     // error: not a definition
end example
]
A constexpr specifier used in the declaration of a function that is not a constructor declares that function to be a constexpr function.
Similarly, a constexpr specifier used in a constructor declaration declares that constructor to be a constexpr constructor.
The definition of a constexpr function shall satisfy the following requirements:
[Example
:
constexpr int square(int x)
  { return x * x; }             // OK
constexpr long long_max()
  { return 2147483647; }        // OK
constexpr int abs(int x) {
  if (x < 0)
    x = -x;
  return x;                     // OK
}
constexpr int first(int n) {
  static int value = n;         // error: variable has static storage duration
  return value;
}
constexpr int uninit() {
  int a;                        // error: variable is uninitialized
  return a;
}
constexpr int prev(int x)
  { return --x; }               // OK
constexpr int g(int x, int n) { // OK
  int r = 1;
  while (--n > 0) r *= x;
  return r;
}
end example
]
The definition of a constexpr constructor shall satisfy the following requirements:
In addition, either its function-body shall be = delete, or it shall satisfy the following requirements:
  • either its function-body shall be = default, or the compound-statement of its function-body shall satisfy the requirements for a function-body of a constexpr function;
  • every non-variant non-static data member and base class subobject shall be initialized ([class.base.init]);
  • if the class is a union having variant members ([class.union]), exactly one of them shall be initialized;
  • if the class is a union-like class, but is not a union, for each of its anonymous union members having variant members, exactly one of them shall be initialized;
  • for a non-delegating constructor, every constructor selected to initialize non-static data members and base class subobjects shall be a constexpr constructor;
  • for a delegating constructor, the target constructor shall be a constexpr constructor.
[Example
:
struct Length {
  constexpr explicit Length(int i = 0) : val(i) { }
private:
  int val;
};
end example
]
For a constexpr function or constexpr constructor that is neither defaulted nor a template, if no argument values exist such that an invocation of the function or constructor could be an evaluated subexpression of a core constant expression, or, for a constructor, a constant initializer for some object ([basic.start.static]), the program is ill-formed, no diagnostic required.
[Example
:
constexpr int f(bool b)
  { return b ? throw 0 : 0; }           // OK
constexpr int f() { return f(true); }   // ill-formed, no diagnostic required

struct B {
  constexpr B(int x) : i(0) { }         // x is unused
  int i;
};

int global;

struct D : B {
  constexpr D() : B(global) { }         // ill-formed, no diagnostic required
                                        // lvalue-to-rvalue conversion on non-constant global
};
end example
]
If the instantiated template specialization of a constexpr function template or member function of a class template would fail to satisfy the requirements for a constexpr function or constexpr constructor, that specialization is still a constexpr function or constexpr constructor, even though a call to such a function cannot appear in a constant expression.
If no specialization of the template would satisfy the requirements for a constexpr function or constexpr constructor when considered as a non-template function or constructor, the template is ill-formed, no diagnostic required.
A call to a constexpr function produces the same result as a call to an equivalent non-constexpr function in all respects except that
The constexpr specifier has no effect on the type of a constexpr function or a constexpr constructor.
[Example
:
constexpr int bar(int x, int y)         // OK
    { return x + y + x*y; }
// ...
int bar(int x, int y)                   // error: redefinition of bar
    { return x * 2 + 3 * y; }
end example
]
A constexpr specifier used in an object declaration declares the object as const.
Such an object shall have literal type and shall be initialized.
In any constexpr variable declaration, the full-expression of the initialization shall be a constant expression.
[Example
:
struct pixel {
  int x, y;
};
constexpr pixel ur = { 1294, 1024 };    // OK
constexpr pixel origin;                 // error: initializer missing
end example
]

10.1.6 The inline specifier [dcl.inline]

The inline specifier can be applied only to the declaration or definition of a variable or function.
A function declaration ([dcl.fct], [class.mfct], [class.friend]) with an inline specifier declares an inline function.
The inline specifier indicates to the implementation that inline substitution of the function body at the point of call is to be preferred to the usual function call mechanism.
An implementation is not required to perform this inline substitution at the point of call; however, even if this inline substitution is omitted, the other rules for inline functions specified in this section shall still be respected.
A variable declaration with an inline specifier declares an inline variable.
A function defined within a class definition is an inline function.
The inline specifier shall not appear on a block scope declaration.94
If the inline specifier is used in a friend function declaration, that declaration shall be a definition or the function shall have previously been declared inline.
An inline function or variable shall be defined in every translation unit in which it is odr-used and shall have exactly the same definition in every case ([basic.def.odr]).
[Note
:
A call to the inline function or a use of the inline variable may be encountered before its definition appears in the translation unit.
end note
]
If the definition of a function or variable appears in a translation unit before its first declaration as inline, the program is ill-formed.
If a function or variable with external linkage is declared inline in one translation unit, it shall be declared inline in all translation units in which it appears; no diagnostic is required.
An inline function or variable with external linkage shall have the same address in all translation units.
[Note
:
A static local variable in an inline function with external linkage always refers to the same object.
A type defined within the body of an inline function with external linkage is the same type in every translation unit.
end note
]
The inline keyword has no effect on the linkage of a function.

10.1.7 Type specifiers [dcl.type]

As a general rule, at most one defining-type-specifier is allowed in the complete decl-specifier-seq of a declaration or in a defining-type-specifier-seq, and at most one type-specifier is allowed in a type-specifier-seq.
The only exceptions to this rule are the following:
  • const can be combined with any type specifier except itself.
  • volatile can be combined with any type specifier except itself.
  • signed or unsigned can be combined with char, long, short, or int.
  • short or long can be combined with int.
  • long can be combined with double.
  • long can be combined with long.
Except in a declaration of a constructor, destructor, or conversion function, at least one defining-type-specifier that is not a cv-qualifier shall appear in a complete type-specifier-seq or a complete decl-specifier-seq.95
[Note
:
enum-specifiers, class-specifiers, and typename-specifiers are discussed in [dcl.enum], [class], and [temp.res], respectively.
The remaining type-specifiers are discussed in the rest of this section.
end note
]
There is no special provision for a decl-specifier-seq that lacks a type-specifier or that has a type-specifier that only specifies cv-qualifiers.
The “implicit int” rule of C is no longer supported.

10.1.7.1 The cv-qualifiers [dcl.type.cv]

There are two cv-qualifiers, const and volatile.
Each cv-qualifier shall appear at most once in a cv-qualifier-seq.
If a cv-qualifier appears in a decl-specifier-seq, the init-declarator-list or member-declarator-list of the declaration shall not be empty.
[Note
:
[basic.type.qualifier] and [dcl.fct] describe how cv-qualifiers affect object and function types.
end note
]
Redundant cv-qualifications are ignored.
[Note
:
For example, these could be introduced by typedefs.
end note
]
[Note
:
Declaring a variable const can affect its linkage ([dcl.stc]) and its usability in constant expressions.
As described in [dcl.init], the definition of an object or subobject of const-qualified type must specify an initializer or be subject to default-initialization.
end note
]
A pointer or reference to a cv-qualified type need not actually point or refer to a cv-qualified object, but it is treated as if it does; a const-qualified access path cannot be used to modify an object even if the object referenced is a non-const object and can be modified through some other access path.
[Note
:
Cv-qualifiers are supported by the type system so that they cannot be subverted without casting.
end note
]
Except that any class member declared mutable can be modified, any attempt to modify a const object during its lifetime results in undefined behavior.
[Example
:
const int ci = 3;                       // cv-qualified (initialized as required)
ci = 4;                                 // ill-formed: attempt to modify const

int i = 2;                              // not cv-qualified
const int* cip;                         // pointer to const int
cip = &i;                               // OK: cv-qualified access path to unqualified
*cip = 4;                               // ill-formed: attempt to modify through ptr to const

int* ip;
ip = const_cast<int*>(cip);             // cast needed to convert const int* to int*
*ip = 4;                                // defined: *ip points to i, a non-const object

const int* ciq = new const int (3);     // initialized as required
int* iq = const_cast<int*>(ciq);        // cast required
*iq = 4;                                // undefined: modifies a const object
For another example,
struct X {
  mutable int i;
  int j;
};
struct Y {
  X x;
  Y();
};

const Y y;
y.x.i++;                                // well-formed: mutable member can be modified
y.x.j++;                                // ill-formed: const-qualified member modified
Y* p = const_cast<Y*>(&y);              // cast away const-ness of y
p->x.i = 99;                            // well-formed: mutable member can be modified
p->x.j = 99;                            // undefined: modifies a const member
end example
]
The semantics of an access through a volatile glvalue are implementation-defined.
If an attempt is made to access an object defined with a volatile-qualified type through the use of a non-volatile glvalue, the behavior is undefined.
[Note
:
volatile is a hint to the implementation to avoid aggressive optimization involving the object because the value of the object might be changed by means undetectable by an implementation.
Furthermore, for some implementations, volatile might indicate that special hardware instructions are required to access the object.
See [intro.execution] for detailed semantics.
In general, the semantics of volatile are intended to be the same in C++ as they are in C.
end note
]

10.1.7.2 Simple type specifiers [dcl.type.simple]

The simple type specifiers are
simple-type-specifier:
	nested-name-specifier type-name
	nested-name-specifier template simple-template-id
	nested-name-specifier template-name
	char
	char16_t
	char32_t
	wchar_t
	bool
	short
	int
	long
	signed
	unsigned
	float
	double
	void
	auto
	decltype-specifier
type-name:
	class-name
	enum-name
	typedef-name
	simple-template-id
decltype-specifier:
	decltype ( expression )
	decltype ( auto )
The simple-type-specifier auto is a placeholder for a type to be deduced ([dcl.spec.auto]).
A type-specifier of the form typename nested-name-specifier template-name is a placeholder for a deduced class type ([dcl.type.class.deduct]).
The template-name shall name a class template that is not an injected-class-name.
The other simple-type-specifiers specify either a previously-declared type, a type determined from an expression, or one of the fundamental types.
Table 11 summarizes the valid combinations of simple-type-specifiers and the types they specify.
Table 11simple-type-specifiers and the types they specify
Specifier(s)
Type
type-name
the type named
simple-template-id
the type as defined in [temp.names]
template-name
placeholder for a type to be deduced
char
char
unsigned char
unsigned char
signed char
signed char
char16_­t
char16_­t
char32_­t
char32_­t
bool
bool
unsigned
unsigned int
unsigned int
unsigned int
signed
int
signed int
int
int
int
unsigned short int
unsigned short int
unsigned short
unsigned short int
unsigned long int
unsigned long int
unsigned long
unsigned long int
unsigned long long int
unsigned long long int
unsigned long long
unsigned long long int
signed long int
long int
signed long
long int
signed long long int
long long int
signed long long
long long int
long long int
long long int
long long
long long int
long int
long int
long
long int
signed short int
short int
signed short
short int
short int
short int
short
short int
wchar_­t
wchar_­t
float
float
double
double
long double
long double
void
void
auto
placeholder for a type to be deduced
decltype(auto)
placeholder for a type to be deduced
decltype(expression)
the type as defined below
When multiple simple-type-specifiers are allowed, they can be freely intermixed with other decl-specifiers in any order.
[Note
:
It is implementation-defined whether objects of char type are represented as signed or unsigned quantities.
The signed specifier forces char objects to be signed; it is redundant in other contexts.
end note
]
For an expression e, the type denoted by decltype(e) is defined as follows:
  • if e is an unparenthesized id-expression naming a structured binding ([dcl.struct.bind]), decltype(e) is the referenced type as given in the specification of the structured binding declaration;
  • otherwise, if e is an unparenthesized id-expression or an unparenthesized class member access, decltype(e) is the type of the entity named by e.
    If there is no such entity, or if e names a set of overloaded functions, the program is ill-formed;
  • otherwise, if e is an xvalue, decltype(e) is T&&, where T is the type of e;
  • otherwise, if e is an lvalue, decltype(e) is T&, where T is the type of e;
  • otherwise, decltype(e) is the type of e.
The operand of the decltype specifier is an unevaluated operand.
[Example
:
const int&& foo();
int i;
struct A { double x; };
const A* a = new A();
decltype(foo()) x1 = 17;        // type is const int&&
decltype(i) x2;                 // type is int
decltype(a->x) x3;              // type is double
decltype((a->x)) x4 = x3;       // type is const double&
end example
]
[Note
:
The rules for determining types involving decltype(auto) are specified in [dcl.spec.auto].
end note
]
If the operand of a decltype-specifier is a prvalue, the temporary materialization conversion is not applied and no result object is provided for the prvalue.
The type of the prvalue may be incomplete.
[Note
:
As a result, storage is not allocated for the prvalue and it is not destroyed.
Thus, a class type is not instantiated as a result of being the type of a function call in this context.
In this context, the common purpose of writing the expression is merely to refer to its type.
In that sense, a decltype-specifier is analogous to a use of a typedef-name, so the usual reasons for requiring a complete type do not apply.
In particular, it is not necessary to allocate storage for a temporary object or to enforce the semantic constraints associated with invoking the type's destructor.
end note
]
[Note
:
Unlike the preceding rule, parentheses have no special meaning in this context.
end note
]
[Example
:
template<class T> struct A { ~A() = delete; };
template<class T> auto h()
  -> A<T>;
template<class T> auto i(T)     // identity
  -> T;
template<class T> auto f(T)     // #1
  -> decltype(i(h<T>()));       // forces completion of A<T> and implicitly uses A<T>​::​~A()
                                // for the temporary introduced by the use of h().
                                // (A temporary is not introduced as a result of the use of i().)
template<class T> auto f(T)     // #2
  -> void;
auto g() -> void {
  f(42);                        // OK: calls #2. (#1 is not a viable candidate: type deduction
                                // fails ([temp.deduct]) because A<int>​::​~A() is implicitly used in its
                                // decltype-specifier)
}
template<class T> auto q(T)
  -> decltype((h<T>()));        // does not force completion of A<T>; A<T>​::​~A() is not implicitly
                                // used within the context of this decltype-specifier
void r() {
  q(42);                        // Error: deduction against q succeeds, so overload resolution selects
                                // the specialization “q(T) -> decltype((h<T>())) [with T=int]”.
                                // The return type is A<int>, so a temporary is introduced and its
                                // destructor is used, so the program is ill-formed.
}
end example
]

10.1.7.3 Elaborated type specifiers [dcl.type.elab]

elaborated-type-specifier:
	class-key attribute-specifier-seq nested-name-specifier identifier
	class-key simple-template-id
	class-key nested-name-specifier template simple-template-id
	enum nested-name-specifier identifier
An attribute-specifier-seq shall not appear in an elaborated-type-specifier unless the latter is the sole constituent of a declaration.
If an elaborated-type-specifier is the sole constituent of a declaration, the declaration is ill-formed unless it is an explicit specialization, an explicit instantiation or it has one of the following forms:
class-key attribute-specifier-seq identifier ;
friend class-key :: identifier ;
friend class-key :: simple-template-id ;
friend class-key nested-name-specifier identifier ;
friend class-key nested-name-specifier template simple-template-id ;
In the first case, the attribute-specifier-seq, if any, appertains to the class being declared; the attributes in the attribute-specifier-seq are thereafter considered attributes of the class whenever it is named.
[basic.lookup.elab] describes how name lookup proceeds for the identifier in an elaborated-type-specifier.
If the identifier resolves to a class-name or enum-name, the elaborated-type-specifier introduces it into the declaration the same way a simple-type-specifier introduces its type-name.
If the identifier resolves to a typedef-name or the simple-template-id resolves to an alias template specialization, the elaborated-type-specifier is ill-formed.
[Note
:
This implies that, within a class template with a template type-parameter T, the declaration
friend class T;
is ill-formed.
However, the similar declaration friend T; is allowed ([class.friend]).
end note
]
The class-key or enum keyword present in the elaborated-type-specifier shall agree in kind with the declaration to which the name in the elaborated-type-specifier refers.
This rule also applies to the form of elaborated-type-specifier that declares a class-name or friend class since it can be construed as referring to the definition of the class.
Thus, in any elaborated-type-specifier, the enum keyword shall be used to refer to an enumeration, the union class-key shall be used to refer to a union, and either the class or struct class-key shall be used to refer to a class declared using the class or struct class-key.
[Example
:
enum class E { a, b };
enum E x = E::a;                // OK
end example
]

10.1.7.4 The auto specifier [dcl.spec.auto]

The auto and decltype(auto) type-specifiers are used to designate a placeholder type that will be replaced later by deduction from an initializer.
The auto type-specifier is also used to introduce a function type having a trailing-return-type or to signify that a lambda is a generic lambda ([expr.prim.lambda]).
The auto type-specifier is also used to introduce a structured binding declaration.
The placeholder type can appear with a function declarator in the decl-specifier-seq, type-specifier-seq, conversion-function-id, or trailing-return-type, in any context where such a declarator is valid.
If the function declarator includes a trailing-return-type ([dcl.fct]), that trailing-return-type specifies the declared return type of the function.
Otherwise, the function declarator shall declare a function.
If the declared return type of the function contains a placeholder type, the return type of the function is deduced from non-discarded return statements, if any, in the body of the function ([stmt.if]).
The type of a variable declared using auto or decltype(auto) is deduced from its initializer.
This use is allowed in an initializing declaration ([dcl.init]) of a variable.
auto or decltype(auto) shall appear as one of the decl-specifiers in the decl-specifier-seq and the decl-specifier-seq shall be followed by one or more declarators, each of which shall be followed by a non-empty initializer.
In an initializer of the form
( expression-list )
the expression-list shall be a single assignment-expression.
[Example
:
auto x = 5;                     // OK: x has type int
const auto *v = &x, u = 6;      // OK: v has type const int*, u has type const int
static auto y = 0.0;            // OK: y has type double
auto int r;                     // error: auto is not a storage-class-specifier
auto f() -> int;                // OK: f returns int
auto g() { return 0.0; }        // OK: g returns double
auto h();                       // OK: h's return type will be deduced when it is defined
end example
]
A placeholder type can also be used in the type-specifier-seq in the new-type-id or type-id of a new-expression and as a decl-specifier of the parameter-declaration's decl-specifier-seq in a template-parameter.
A program that uses auto or decltype(auto) in a context not explicitly allowed in this section is ill-formed.
If the init-declarator-list contains more than one init-declarator, they shall all form declarations of variables.
The type of each declared variable is determined by placeholder type deduction, and if the type that replaces the placeholder type is not the same in each deduction, the program is ill-formed.
[Example
:
auto x = 5, *y = &x;            // OK: auto is int
auto a = 5, b = { 1, 2 };       // error: different types for auto
end example
]
If a function with a declared return type that contains a placeholder type has multiple non-discarded return statements, the return type is deduced for each such return statement.
If the type deduced is not the same in each deduction, the program is ill-formed.
If a function with a declared return type that uses a placeholder type has no non-discarded return statements, the return type is deduced as though from a return statement with no operand at the closing brace of the function body.
[Example
:
auto  f() { }                   // OK, return type is void
auto* g() { }                   // error, cannot deduce auto* from void()
end example
]
If the type of an entity with an undeduced placeholder type is needed to determine the type of an expression, the program is ill-formed.
Once a non-discarded return statement has been seen in a function, however, the return type deduced from that statement can be used in the rest of the function, including in other return statements.
[Example
:
auto n = n;                     // error, n's type is unknown
auto f();
void g() { &f; }                // error, f's return type is unknown
auto sum(int i) {
  if (i == 1)
    return i;                   // sum's return type is int
  else
    return sum(i-1)+i;          // OK, sum's return type has been deduced
}
end example
]
Return type deduction for a function template with a placeholder in its declared type occurs when the definition is instantiated even if the function body contains a return statement with a non-type-dependent operand.
[Note
:
Therefore, any use of a specialization of the function template will cause an implicit instantiation.
Any errors that arise from this instantiation are not in the immediate context of the function type and can result in the program being ill-formed ([temp.deduct]).
end note
]
[Example
:
template <class T> auto f(T t) { return t; }    // return type deduced at instantiation time
typedef decltype(f(1)) fint_t;                  // instantiates f<int> to deduce return type
template<class T> auto f(T* t) { return *t; }
void g() { int (*p)(int*) = &f; }               // instantiates both fs to determine return types,
                                                // chooses second
end example
]
Redeclarations or specializations of a function or function template with a declared return type that uses a placeholder type shall also use that placeholder, not a deduced type.
[Example
:
auto f();
auto f() { return 42; }                         // return type is int
auto f();                                       // OK
int f();                                        // error, cannot be overloaded with auto f()
decltype(auto) f();                             // error, auto and decltype(auto) don't match

template <typename T> auto g(T t) { return t; } // #1
template auto g(int);                           // OK, return type is int
template char g(char);                          // error, no matching template
template<> auto g(double);                      // OK, forward declaration with unknown return type

template <class T> T g(T t) { return t; }       // OK, not functionally equivalent to #1
template char g(char);                          // OK, now there is a matching template
template auto g(float);                         // still matches #1

void h() { return g(42); }                      // error, ambiguous

template <typename T> struct A {
  friend T frf(T);
};
auto frf(int i) { return i; }                   // not a friend of A<int>
end example
]
A function declared with a return type that uses a placeholder type shall not be virtual.
An explicit instantiation declaration does not cause the instantiation of an entity declared using a placeholder type, but it also does not prevent that entity from being instantiated as needed to determine its type.
[Example
:
template <typename T> auto f(T t) { return t; }
extern template auto f(int);    // does not instantiate f<int>
int (*p)(int) = f;              // instantiates f<int> to determine its return type, but an explicit
                                // instantiation definition is still required somewhere in the program
end example
]

10.1.7.4.1 Placeholder type deduction [dcl.type.auto.deduct]

Placeholder type deduction is the process by which a type containing a placeholder type is replaced by a deduced type.
A type T containing a placeholder type, and a corresponding initializer e, are determined as follows:
  • for a non-discarded return statement that occurs in a function declared with a return type that contains a placeholder type, T is the declared return type and e is the operand of the return statement.
    If the return statement has no operand, then e is void();
  • for a variable declared with a type that contains a placeholder type, T is the declared type of the variable and e is the initializer.
    If the initialization is direct-list-initialization, the initializer shall be a braced-init-list containing only a single assignment-expression and e is the assignment-expression;
  • for a non-type template parameter declared with a type that contains a placeholder type, T is the declared type of the non-type template parameter and e is the corresponding template argument.
In the case of a return statement with no operand or with an operand of type void, T shall be either decltype(auto) or cv auto.
If the deduction is for a return statement and e is a braced-init-list ([dcl.init.list]), the program is ill-formed.
If the placeholder is the auto type-specifier, the deduced type T' replacing T is determined using the rules for template argument deduction.
Obtain P from T by replacing the occurrences of auto with either a new invented type template parameter U or, if the initialization is copy-list-initialization, with std​::​initializer_­list<U>.
Deduce a value for U using the rules of template argument deduction from a function call, where P is a function template parameter type and the corresponding argument is e.
If the deduction fails, the declaration is ill-formed.
Otherwise, T' is obtained by substituting the deduced U into P.
[Example
:
auto x1 = { 1, 2 };             // decltype(x1) is std​::​initializer_­list<int>
auto x2 = { 1, 2.0 };           // error: cannot deduce element type
auto x3{ 1, 2 };                // error: not a single element
auto x4 = { 3 };                // decltype(x4) is std​::​initializer_­list<int>
auto x5{ 3 };                   // decltype(x5) is int
end example
]
[Example
:
const auto &i = expr;
The type of i is the deduced type of the parameter u in the call f(expr) of the following invented function template:
template <class U> void f(const U& u);
end example
]
If the placeholder is the decltype(auto) type-specifier, T shall be the placeholder alone.
The type deduced for T is determined as described in [dcl.type.simple], as though e had been the operand of the decltype.
[Example
:
int i;
int&& f();
auto           x2a(i);          // decltype(x2a) is int
decltype(auto) x2d(i);          // decltype(x2d) is int
auto           x3a = i;         // decltype(x3a) is int
decltype(auto) x3d = i;         // decltype(x3d) is int
auto           x4a = (i);       // decltype(x4a) is int
decltype(auto) x4d = (i);       // decltype(x4d) is int&
auto           x5a = f();       // decltype(x5a) is int
decltype(auto) x5d = f();       // decltype(x5d) is int&&
auto           x6a = { 1, 2 };  // decltype(x6a) is std​::​initializer_­list<int>
decltype(auto) x6d = { 1, 2 };  // error, { 1, 2 } is not an expression
auto          *x7a = &i;        // decltype(x7a) is int*
decltype(auto)*x7d = &i;        // error, declared type is not plain decltype(auto)
end example
]

10.1.7.5 Deduced class template specialization types [dcl.type.class.deduct]

If a placeholder for a deduced class type appears as a decl-specifier in the decl-specifier-seq of an initializing declaration ([dcl.init]) of a variable, the placeholder is replaced by the return type of the function selected by overload resolution for class template deduction ([over.match.class.deduct]).
If the decl-specifier-seq is followed by an init-declarator-list or member-declarator-list containing more than one declarator, the type that replaces the placeholder shall be the same in each deduction.
A placeholder for a deduced class type can also be used in the type-specifier-seq in the new-type-id or type-id of a new-expression, or as the simple-type-specifier in an explicit type conversion (functional notation).
A placeholder for a deduced class type shall not appear in any other context.
[Example
:
template<class T> struct container {
    container(T t) {}
    template<class Iter> container(Iter beg, Iter end);
};
template<class Iter>
container(Iter b, Iter e) -> container<typename std::iterator_traits<Iter>::value_type>;
std::vector<double> v = { /* ... */ };

container c(7);                         // OK, deduces int for T
auto d = container(v.begin(), v.end()); // OK, deduces double for T
container e{5, 6};                      // error, int is not an iterator
end example
]

10.2 Enumeration declarations [dcl.enum]

An enumeration is a distinct type ([basic.compound]) with named constants.
Its name becomes an enum-name within its scope.
enum-name:
	identifier
enum-specifier:
	enum-head { enumerator-list }
	enum-head { enumerator-list , }
enum-head:
	enum-key attribute-specifier-seq enum-head-name enum-base
enum-head-name:
	nested-name-specifier identifier
opaque-enum-declaration:
	enum-key attribute-specifier-seq nested-name-specifier identifier enum-base ;
enum-key:
	enum
	enum class
	enum struct
enum-base:
	: type-specifier-seq
enumerator-list:
	enumerator-definition
	enumerator-list , enumerator-definition
enumerator-definition:
	enumerator
	enumerator = constant-expression
enumerator:
	identifier attribute-specifier-seq
The optional attribute-specifier-seq in the enum-head and the opaque-enum-declaration appertains to the enumeration; the attributes in that attribute-specifier-seq are thereafter considered attributes of the enumeration whenever it is named.
A : following “enum nested-name-specifier identifier” within the decl-specifier-seq of a member-declaration is parsed as part of an enum-base.
[Note
:
This resolves a potential ambiguity between the declaration of an enumeration with an enum-base and the declaration of an unnamed bit-field of enumeration type.
[Example
:
   struct S {
     enum E : int {};
     enum E : int {};           // error: redeclaration of enumeration
   };
end example
]
end note
]
If an opaque-enum-declaration contains a nested-name-specifier, the declaration shall be an explicit specialization.
The enumeration type declared with an enum-key of only enum is an unscoped enumeration, and its enumerators are unscoped enumerators.
The enum-keys enum class and enum struct are semantically equivalent; an enumeration type declared with one of these is a scoped enumeration, and its enumerators are scoped enumerators.
The optional identifier shall not be omitted in the declaration of a scoped enumeration.
The type-specifier-seq of an enum-base shall name an integral type; any cv-qualification is ignored.
An opaque-enum-declaration declaring an unscoped enumeration shall not omit the enum-base.
The identifiers in an enumerator-list are declared as constants, and can appear wherever constants are required.
An enumerator-definition with = gives the associated enumerator the value indicated by the constant-expression.
If the first enumerator has no initializer, the value of the corresponding constant is zero.
An enumerator-definition without an initializer gives the enumerator the value obtained by increasing the value of the previous enumerator by one.
[Example
:
enum { a, b, c=0 };
enum { d, e, f=e+2 };
defines a, c, and d to be zero, b and e to be 1, and f to be 3.
end example
]
The optional attribute-specifier-seq in an enumerator appertains to that enumerator.
An opaque-enum-declaration is either a redeclaration of an enumeration in the current scope or a declaration of a new enumeration.
[Note
:
An enumeration declared by an opaque-enum-declaration has fixed underlying type and is a complete type.
The list of enumerators can be provided in a later redeclaration with an enum-specifier.
end note
]
A scoped enumeration shall not be later redeclared as unscoped or with a different underlying type.
An unscoped enumeration shall not be later redeclared as scoped and each redeclaration shall include an enum-base specifying the same underlying type as in the original declaration.
If the enum-key is followed by a nested-name-specifier, the enum-specifier shall refer to an enumeration that was previously declared directly in the class or namespace to which the nested-name-specifier refers (i.e., neither inherited nor introduced by a using-declaration), and the enum-specifier shall appear in a namespace enclosing the previous declaration.
Each enumeration defines a type that is different from all other types.
Each enumeration also has an underlying type.
The underlying type can be explicitly specified using an enum-base.
For a scoped enumeration type, the underlying type is int if it is not explicitly specified.
In both of these cases, the underlying type is said to be fixed.
Following the closing brace of an enum-specifier, each enumerator has the type of its enumeration.
If the underlying type is fixed, the type of each enumerator prior to the closing brace is the underlying type and the constant-expression in the enumerator-definition shall be a converted constant expression of the underlying type.
If the underlying type is not fixed, the type of each enumerator prior to the closing brace is determined as follows:
  • If an initializer is specified for an enumerator, the constant-expression shall be an integral constant expression.
    If the expression has unscoped enumeration type, the enumerator has the underlying type of that enumeration type, otherwise it has the same type as the expression.
  • If no initializer is specified for the first enumerator, its type is an unspecified signed integral type.
  • Otherwise the type of the enumerator is the same as that of the preceding enumerator unless the incremented value is not representable in that type, in which case the type is an unspecified integral type sufficient to contain the incremented value.
    If no such type exists, the program is ill-formed.
An enumeration whose underlying type is fixed is an incomplete type from its point of declaration to immediately after its enum-base (if any), at which point it becomes a complete type.
An enumeration whose underlying type is not fixed is an incomplete type from its point of declaration to immediately after the closing } of its enum-specifier, at which point it becomes a complete type.
For an enumeration whose underlying type is not fixed, the underlying type is an integral type that can represent all the enumerator values defined in the enumeration.
If no integral type can represent all the enumerator values, the enumeration is ill-formed.
It is implementation-defined which integral type is used as the underlying type except that the underlying type shall not be larger than int unless the value of an enumerator cannot fit in an int or unsigned int.
If the enumerator-list is empty, the underlying type is as if the enumeration had a single enumerator with value 0.
For an enumeration whose underlying type is fixed, the values of the enumeration are the values of the underlying type.
Otherwise, for an enumeration where is the smallest enumerator and is the largest, the values of the enumeration are the values in the range to , defined as follows: Let K be 1 for a two's complement representation and 0 for a ones' complement or sign-magnitude representation.
is the smallest value greater than or equal to and equal to , where M is a non-negative integer.
is zero if is non-negative and otherwise.
The size of the smallest bit-field large enough to hold all the values of the enumeration type is if is zero and otherwise.
It is possible to define an enumeration that has values not defined by any of its enumerators.
If the enumerator-list is empty, the values of the enumeration are as if the enumeration had a single enumerator with value 0.96
Two enumeration types are layout-compatible enumerations if they have the same underlying type.
The value of an enumerator or an object of an unscoped enumeration type is converted to an integer by integral promotion.
[Example
:
  enum color { red, yellow, green=20, blue };
  color col = red;
  color* cp = &col;
  if (*cp == blue)              // ...
makes color a type describing various colors, and then declares col as an object of that type, and cp as a pointer to an object of that type.
The possible values of an object of type color are red, yellow, green, blue; these values can be converted to the integral values 0, 1, 20, and 21.
Since enumerations are distinct types, objects of type color can be assigned only values of type color.
color c = 1;                    // error: type mismatch, no conversion from int to color
int i = yellow;                 // OK: yellow converted to integral value 1, integral promotion
Note that this implicit enum to int conversion is not provided for a scoped enumeration:
enum class Col { red, yellow, green };
int x = Col::red;               // error: no Col to int conversion
Col y = Col::red;
if (y) { }                      // error: no Col to bool conversion
end example
]
Each enum-name and each unscoped enumerator is declared in the scope that immediately contains the enum-specifier.
Each scoped enumerator is declared in the scope of the enumeration.
These names obey the scope rules defined for all names in [basic.scope] and [basic.lookup].
[Example
:
enum direction { left='l', right='r' };

void g()  {
  direction d;                  // OK
  d = left;                     // OK
  d = direction::right;         // OK
}

enum class altitude { high='h', low='l' };

void h()  {
  altitude a;                   // OK
  a = high;                     // error: high not in scope
  a = altitude::low;            // OK
}
end example
]
An enumerator declared in class scope can be referred to using the class member access operators (​::​, . (dot) and -> (arrow)), see [expr.ref].
[Example
:
struct X {
  enum direction { left='l', right='r' };
  int f(int i) { return i==left ? 0 : i==right ? 1 : 2; }
};

void g(X* p) {
  direction d;                  // error: direction not in scope
  int i;
  i = p->f(left);               // error: left not in scope
  i = p->f(X::right);           // OK
  i = p->f(p->left);            // OK
  // ...
}
end example
]
If an enum-head contains a nested-name-specifier, the enum-specifier shall refer to an enumeration that was previously declared directly in the class or namespace to which the nested-name-specifier refers, or in an element of the inline namespace set ([namespace.def]) of that namespace (i.e., not merely inherited or introduced by a using-declaration), and the enum-specifier shall appear in a namespace enclosing the previous declaration.
In such cases, the nested-name-specifier of the enum-head of the definition shall not begin with a decltype-specifier.
This set of values is used to define promotion and conversion semantics for the enumeration type.
It does not preclude an expression of enumeration type from having a value that falls outside this range.

10.3 Namespaces [basic.namespace]

A namespace is an optionally-named declarative region.
The name of a namespace can be used to access entities declared in that namespace; that is, the members of the namespace.
Unlike other declarative regions, the definition of a namespace can be split over several parts of one or more translation units.
The outermost declarative region of a translation unit is a namespace; see [basic.scope.namespace].

10.3.1 Namespace definition [namespace.def]

namespace-name:
	identifier
	namespace-alias
namespace-definition:
	named-namespace-definition
	unnamed-namespace-definition
	nested-namespace-definition
named-namespace-definition:
	inline namespace attribute-specifier-seq identifier { namespace-body }
unnamed-namespace-definition:
	inline namespace attribute-specifier-seq { namespace-body }
nested-namespace-definition:
	namespace enclosing-namespace-specifier :: identifier { namespace-body }
enclosing-namespace-specifier:
	identifier
	enclosing-namespace-specifier :: identifier
namespace-body:
	declaration-seq
Every namespace-definition shall appear in the global scope or in a namespace scope ([basic.scope.namespace]).
In a named-namespace-definition, the identifier is the name of the namespace.
If the identifier, when looked up ([basic.lookup.unqual]), refers to a namespace-name (but not a namespace-alias) that was introduced in the namespace in which the named-namespace-definition appears or that was introduced in a member of the inline namespace set of that namespace, the namespace-definition extends the previously-declared namespace.
Otherwise, the identifier is introduced as a namespace-name into the declarative region in which the named-namespace-definition appears.
Because a namespace-definition contains declarations in its namespace-body and a namespace-definition is itself a declaration, it follows that namespace-definitions can be nested.
[Example
:
namespace Outer {
  int i;
  namespace Inner {
    void f() { i++; }           // Outer​::​i
    int i;
    void g() { i++; }           // Inner​::​i
  }
}
end example
]
The enclosing namespaces of a declaration are those namespaces in which the declaration lexically appears, except for a redeclaration of a namespace member outside its original namespace (e.g., a definition as specified in [namespace.memdef]).
Such a redeclaration has the same enclosing namespaces as the original declaration.
[Example
:
namespace Q {
  namespace V {
    void f();                   // enclosing namespaces are the global namespace, Q, and Q​::​V
    class C { void m(); };
  }
  void V::f() {                 // enclosing namespaces are the global namespace, Q, and Q​::​V
    extern void h();            // ... so this declares Q​::​V​::​h
  }
  void V::C::m() {              // enclosing namespaces are the global namespace, Q, and Q​::​V
  }
}
end example
]
If the optional initial inline keyword appears in a namespace-definition for a particular namespace, that namespace is declared to be an inline namespace.
The inline keyword may be used on a namespace-definition that extends a namespace only if it was previously used on the namespace-definition that initially declared the namespace-name for that namespace.
The optional attribute-specifier-seq in a named-namespace-definition appertains to the namespace being defined or extended.
Members of an inline namespace can be used in most respects as though they were members of the enclosing namespace.
Specifically, the inline namespace and its enclosing namespace are both added to the set of associated namespaces used in argument-dependent lookup whenever one of them is, and a using-directive that names the inline namespace is implicitly inserted into the enclosing namespace as for an unnamed namespace.
Furthermore, each member of the inline namespace can subsequently be partially specialized, explicitly instantiated, or explicitly specialized as though it were a member of the enclosing namespace.
Finally, looking up a name in the enclosing namespace via explicit qualification ([namespace.qual]) will include members of the inline namespace brought in by the using-directive even if there are declarations of that name in the enclosing namespace.
These properties are transitive: if a namespace N contains an inline namespace M, which in turn contains an inline namespace O, then the members of O can be used as though they were members of M or N.
The inline namespace set of N is the transitive closure of all inline namespaces in N.
The enclosing namespace set of O is the set of namespaces consisting of the innermost non-inline namespace enclosing an inline namespace O, together with any intervening inline namespaces.
A nested-namespace-definition with an enclosing-namespace-specifier E, identifier I and namespace-body B is equivalent to
namespace E { namespace I { B } }
[Example
:
namespace A::B::C {
  int i;
}
The above has the same effect as:
namespace A {
  namespace B {
    namespace C {
      int i;
    }
  }
}
end example
]

10.3.1.1 Unnamed namespaces [namespace.unnamed]

An unnamed-namespace-definition behaves as if it were replaced by
inline namespace unique { /* empty body */ }
using namespace unique ;
namespace unique { namespace-body }
where inline appears if and only if it appears in the unnamed-namespace-definition and all occurrences of unique in a translation unit are replaced by the same identifier, and this identifier differs from all other identifiers in the translation unit.
The optional attribute-specifier-seq in the unnamed-namespace-definition appertains to unique.
[Example
:
namespace { int i; }            // unique​::​i
void f() { i++; }               // unique​::​i++

namespace A {
  namespace {
    int i;                      // A​::​unique​::​i
    int j;                      // A​::​unique​::​j
  }
  void g() { i++; }             // A​::​unique​::​i++
}

using namespace A;
void h() {
  i++;                          // error: unique​::​i or A​::​unique​::​i
  A::i++;                       // A​::​unique​::​i
  j++;                          // A​::​unique​::​j
}
end example
]

10.3.1.2 Namespace member definitions [namespace.memdef]

A declaration in a namespace N (excluding declarations in nested scopes) whose declarator-id is an unqualified-id ([dcl.meaning]), whose class-head-name or enum-head-name is an identifier, or whose elaborated-type-specifier is of the form class-key attribute-specifier-seq identifier ([dcl.type.elab]), or that is an opaque-enum-declaration, declares (or redeclares) its unqualified-id or identifier as a member of N.
[Note
:
An explicit instantiation or explicit specialization of a template does not introduce a name and thus may be declared using an unqualified-id in a member of the enclosing namespace set, if the primary template is declared in an inline namespace.
end note
]
[Example
:
namespace X {
  void f() { /* ... */ }        // OK: introduces X​::​f()

  namespace M {
    void g();                   // OK: introduces X​::​M​::​g()
  }
  using M::g;
  void g();                     // error: conflicts with X​::​M​::​g()
}
end example
]
Members of a named namespace can also be defined outside that namespace by explicit qualification ([namespace.qual]) of the name being defined, provided that the entity being defined was already declared in the namespace and the definition appears after the point of declaration in a namespace that encloses the declaration's namespace.
[Example
:
namespace Q {
  namespace V {
    void f();
  }
  void V::f() { /* ... */ }     // OK
  void V::g() { /* ... */ }     // error: g() is not yet a member of V
  namespace V {
    void g();
  }
}

namespace R {
  void Q::V::g() { /* ... */ }  // error: R doesn't enclose Q
}
end example
]
If a friend declaration in a non-local class first declares a class, function, class template or function template97 the friend is a member of the innermost enclosing namespace.
The friend declaration does not by itself make the name visible to unqualified lookup or qualified lookup.
[Note
:
The name of the friend will be visible in its namespace if a matching declaration is provided at namespace scope (either before or after the class definition granting friendship).
end note
]
If a friend function or function template is called, its name may be found by the name lookup that considers functions from namespaces and classes associated with the types of the function arguments ([basic.lookup.argdep]).
If the name in a friend declaration is neither qualified nor a template-id and the declaration is a function or an elaborated-type-specifier, the lookup to determine whether the entity has been previously declared shall not consider any scopes outside the innermost enclosing namespace.
[Note
:
The other forms of friend declarations cannot declare a new member of the innermost enclosing namespace and thus follow the usual lookup rules.
end note
]
[Example
:
// Assume f and g have not yet been declared.
void h(int);
template <class T> void f2(T);
namespace A {
  class X {
    friend void f(X);           // A​::​f(X) is a friend
    class Y {
      friend void g();          // A​::​g is a friend
      friend void h(int);       // A​::​h is a friend
                                // ​::​h not considered
      friend void f2<>(int);    // ​::​f2<>(int) is a friend
    };
  };

  // A​::​f, A​::​g and A​::​h are not visible here
  X x;
  void g() { f(x); }            // definition of A​::​g
  void f(X) { /* ... */ }       // definition of A​::​f
  void h(int) { /* ... */ }     // definition of A​::​h
  // A​::​f, A​::​g and A​::​h are visible here and known to be friends
}

using A::x;

void h() {
  A::f(x);
  A::X::f(x);                   // error: f is not a member of A​::​X
  A::X::Y::g();                 // error: g is not a member of A​::​X​::​Y
}
end example
]
this implies that the name of the class or function is unqualified.

10.3.2 Namespace alias [namespace.alias]

The identifier in a namespace-alias-definition is a synonym for the name of the namespace denoted by the qualified-namespace-specifier and becomes a namespace-alias.
[Note
:
When looking up a namespace-name in a namespace-alias-definition, only namespace names are considered, see [basic.lookup.udir].
end note
]
In a declarative region, a namespace-alias-definition can be used to redefine a namespace-alias declared in that declarative region to refer only to the namespace to which it already refers.
[Example
:
The following declarations are well-formed:
namespace Company_with_very_long_name { /* ... */ }
namespace CWVLN = Company_with_very_long_name;
namespace CWVLN = Company_with_very_long_name;  // OK: duplicate
namespace CWVLN = CWVLN;
end example
]

10.3.3 The using declaration [namespace.udecl]

using-declaration:
	using using-declarator-list ;
using-declarator-list:
	using-declarator ...
	using-declarator-list , using-declarator ...
using-declarator:
	typename nested-name-specifier unqualified-id
Each using-declarator in a using-declaration98 introduces a set of declarations into the declarative region in which the using-declaration appears.
The set of declarations introduced by the using-declarator is found by performing qualified name lookup ([basic.lookup.qual], [class.member.lookup]) for the name in the using-declarator, excluding functions that are hidden as described below.
If the using-declarator does not name a constructor, the unqualified-id is declared in the declarative region in which the using-declaration appears as a synonym for each declaration introduced by the using-declarator.
[Note
:
Only the specified name is so declared; specifying an enumeration name in a using-declaration does not declare its enumerators in the using-declaration's declarative region.
end note
]
If the using-declarator names a constructor, it declares that the class inherits the set of constructor declarations introduced by the using-declarator from the nominated base class.
Every using-declaration is a declaration and a member-declaration and can therefore be used in a class definition.
[Example
:
struct B {
  void f(char);
  void g(char);
  enum E { e };
  union { int x; };
};

struct D : B {
  using B::f;
  void f(int) { f('c'); }       // calls B​::​f(char)
  void g(int) { g('c'); }       // recursively calls D​::​g(int)
};
end example
]
In a using-declaration used as a member-declaration, each using-declarator's nested-name-specifier shall name a base class of the class being defined.
If a using-declarator names a constructor, its nested-name-specifier shall name a direct base class of the class being defined.
[Example
:
template <typename... bases>
struct X : bases... {
  using bases::g...;
};

X<B, D> x;                      // OK: B​::​g and D​::​g introduced
end example
]
[Example
:
class C {
  int g();
};

class D2 : public B {
  using B::f;                   // OK: B is a base of D2
  using B::e;                   // OK: e is an enumerator of base B
  using B::x;                   // OK: x is a union member of base B
  using C::g;                   // error: C isn't a base of D2
};
end example
]
[Note
:
Since destructors do not have names, a using-declaration cannot refer to a destructor for a base class.
Since specializations of member templates for conversion functions are not found by name lookup, they are not considered when a using-declaration specifies a conversion function ([temp.mem]).
end note
]
If a constructor or assignment operator brought from a base class into a derived class has the signature of a copy/move constructor or assignment operator for the derived class ([class.copy]), the using-declaration does not by itself suppress the implicit declaration of the derived class member; the member from the base class is hidden or overridden by the implicitly-declared copy/move constructor or assignment operator of the derived class, as described below.
A using-declaration shall not name a template-id.
[Example
:
struct A {
  template <class T> void f(T);
  template <class T> struct X { };
};
struct B : A {
  using A::f<double>;           // ill-formed
  using A::X<int>;              // ill-formed
};
end example
]
A using-declaration shall not name a namespace.
A using-declaration shall not name a scoped enumerator.
A using-declaration that names a class member shall be a member-declaration.
[Example
:
struct X {
  int i;
  static int s;
};

void f() {
  using X::i;                   // error: X​::​i is a class member and this is not a member declaration.
  using X::s;                   // error: X​::​s is a class member and this is not a member declaration.
}
end example
]
Members declared by a using-declaration can be referred to by explicit qualification just like other member names ([namespace.qual]).
[Example
:
void f();

namespace A {
  void g();
}

namespace X {
  using ::f;                    // global f
  using A::g;                   // A's g
}

void h()
{
  X::f();                       // calls ​::​f
  X::g();                       // calls A​::​g
}
end example
]
A using-declaration is a declaration and can therefore be used repeatedly where (and only where) multiple declarations are allowed.
[Example
:
namespace A {
  int i;
}

namespace A1 {
  using A::i, A::i;             // OK: double declaration
}

struct B {
  int i;
};

struct X : B {
  using B::i, B::i;             // error: double member declaration
};
end example
]
[Note
:
For a using-declaration whose nested-name-specifier names a namespace, members added to the namespace after the using-declaration are not in the set of introduced declarations, so they are not considered when a use of the name is made.
Thus, additional overloads added after the using-declaration are ignored, but default function arguments ([dcl.fct.default]), default template arguments ([temp.param]), and template specializations ([temp.class.spec], [temp.expl.spec]) are considered.
end note
]
[Example
:
namespace A {
  void f(int);
}

using A::f;         // f is a synonym for A​::​f; that is, for A​::​f(int).
namespace A {
  void f(char);
}

void foo() {
  f('a');           // calls f(int), even though f(char) exists.
}

void bar() {
  using A::f;       // f is a synonym for A​::​f; that is, for A​::​f(int) and A​::​f(char).
  f('a');           // calls f(char)
}
end example
]
[Note
:
Partial specializations of class templates are found by looking up the primary class template and then considering all partial specializations of that template.
If a using-declaration names a class template, partial specializations introduced after the using-declaration are effectively visible because the primary template is visible ([temp.class.spec]).
end note
]
Since a using-declaration is a declaration, the restrictions on declarations of the same name in the same declarative region also apply to using-declarations.
[Example
:
namespace A {
  int x;
}

namespace B {
  int i;
  struct g { };
  struct x { };
  void f(int);
  void f(double);
  void g(char);     // OK: hides struct g
}

void func() {
  int i;
  using B::i;       // error: i declared twice
  void f(char);
  using B::f;       // OK: each f is a function
  f(3.5);           // calls B​::​f(double)
  using B::g;
  g('a');           // calls B​::​g(char)
  struct g g1;      // g1 has class type B​::​g
  using B::x;
  using A::x;       // OK: hides struct B​::​x
  x = 99;           // assigns to A​::​x
  struct x x1;      // x1 has class type B​::​x
}
end example
]
If a function declaration in namespace scope or block scope has the same name and the same parameter-type-list as a function introduced by a using-declaration, and the declarations do not declare the same function, the program is ill-formed.
If a function template declaration in namespace scope has the same name, parameter-type-list, return type, and template parameter list as a function template introduced by a using-declaration, the program is ill-formed.
[Note
:
Two using-declarations may introduce functions with the same name and the same parameter-type-list.
If, for a call to an unqualified function name, function overload resolution selects the functions introduced by such using-declarations, the function call is ill-formed.
[Example
:
namespace B {
  void f(int);
  void f(double);
}
namespace C {
  void f(int);
  void f(double);
  void f(char);
}

void h() {
  using B::f;       // B​::​f(int) and B​::​f(double)
  using C::f;       // C​::​f(int), C​::​f(double), and C​::​f(char)
  f('h');           // calls C​::​f(char)
  f(1);             // error: ambiguous: B​::​f(int) or C​::​f(int)?
  void f(int);      // error: f(int) conflicts with C​::​f(int) and B​::​f(int)
}
end example
]
end note
]
When a using-declarator brings declarations from a base class into a derived class, member functions and member function templates in the derived class override and/or hide member functions and member function templates with the same name, parameter-type-list, cv-qualification, and ref-qualifier (if any) in a base class (rather than conflicting).
Such hidden or overridden declarations are excluded from the set of declarations introduced by the using-declarator.
[Example
:
struct B {
  virtual void f(int);
  virtual void f(char);
  void g(int);
  void h(int);
};

struct D : B {
  using B::f;
  void f(int);      // OK: D​::​f(int) overrides B​::​f(int);

  using B::g;
  void g(char);     // OK

  using B::h;
  void h(int);      // OK: D​::​h(int) hides B​::​h(int)
};

void k(D* p)
{
  p->f(1);          // calls D​::​f(int)
  p->f('a');        // calls B​::​f(char)
  p->g(1);          // calls B​::​g(int)
  p->g('a');        // calls D​::​g(char)
}

struct B1 {
  B1(int);
};

struct B2 {
  B2(int);
};

struct D1 : B1, B2 {
  using B1::B1;
  using B2::B2;
};
D1 d1(0);           // ill-formed: ambiguous

struct D2 : B1, B2 {
  using B1::B1;
  using B2::B2;
  D2(int);          // OK: D2​::​D2(int) hides B1​::​B1(int) and B2​::​B2(int)
};
D2 d2(0);           // calls D2​::​D2(int)
end example
]
For the purpose of forming a set of candidates during overload resolution, the functions that are introduced by a using-declaration into a derived class are treated as though they were members of the derived class.
In particular, the implicit this parameter shall be treated as if it were a pointer to the derived class rather than to the base class.
This has no effect on the type of the function, and in all other respects the function remains a member of the base class.
Likewise, constructors that are introduced by a using-declaration are treated as though they were constructors of the derived class when looking up the constructors of the derived class ([class.qual]) or forming a set of overload candidates ([over.match.ctor], [over.match.copy], [over.match.list]).
If such a constructor is selected to perform the initialization of an object of class type, all subobjects other than the base class from which the constructor originated are implicitly initialized ([class.inhctor.init]).
[Note
:
A member of a derived class is sometimes preferred to a member of a base class if they would otherwise be ambiguous ([over.match.best]).
end note
]
In a using-declarator that does not name a constructor, all members of the set of introduced declarations shall be accessible.
In a using-declarator that names a constructor, no access check is performed.
In particular, if a derived class uses a using-declarator to access a member of a base class, the member name shall be accessible.
If the name is that of an overloaded member function, then all functions named shall be accessible.
The base class members mentioned by a using-declarator shall be visible in the scope of at least one of the direct base classes of the class where the using-declarator is specified.
[Note
:
Because a using-declarator designates a base class member (and not a member subobject or a member function of a base class subobject), a using-declarator cannot be used to resolve inherited member ambiguities.
[Example
:
struct A { int x(); };
struct B : A { };
struct C : A {
  using A::x;
  int x(int);
};

struct D : B, C {
  using C::x;
  int x(double);
};
int f(D* d) {
  return d->x();    // error: overload resolution selects A​::​x, but A is an ambiguous base class
}
end example
]
end note
]
A synonym created by a using-declaration has the usual accessibility for a member-declaration.
A using-declarator that names a constructor does not create a synonym; instead, the additional constructors are accessible if they would be accessible when used to construct an object of the corresponding base class, and the accessibility of the using-declaration is ignored.
[Example
:
class A {
private:
    void f(char);
public:
    void f(int);
protected:
    void g();
};

class B : public A {
  using A::f;       // error: A​::​f(char) is inaccessible
public:
  using A::g;       // B​::​g is a public synonym for A​::​g
};
end example
]
If a using-declarator uses the keyword typename and specifies a dependent name ([temp.dep]), the name introduced by the using-declaration is treated as a typedef-name.
A using-declaration with more than one using-declarator is equivalent to a corresponding sequence of using-declarations with one using-declarator each.

10.3.4 Using directive [namespace.udir]

using-directive:
	attribute-specifier-seq using  namespace nested-name-specifier namespace-name ;
A using-directive shall not appear in class scope, but may appear in namespace scope or in block scope.
[Note
:
When looking up a namespace-name in a using-directive, only namespace names are considered, see [basic.lookup.udir].
end note
]
The optional attribute-specifier-seq appertains to the using-directive.
A using-directive specifies that the names in the nominated namespace can be used in the scope in which the using-directive appears after the using-directive.
During unqualified name lookup, the names appear as if they were declared in the nearest enclosing namespace which contains both the using-directive and the nominated namespace.
[Note
:
In this context, “contains” means “contains directly or indirectly”.
end note
]
A using-directive does not add any members to the declarative region in which it appears.
[Example
:
namespace A {
  int i;
  namespace B {
    namespace C {
      int i;
    }
    using namespace A::B::C;
    void f1() {
      i = 5;        // OK, C​::​i visible in B and hides A​::​i
    }
  }
  namespace D {
    using namespace B;
    using namespace C;
    void f2() {
      i = 5;        // ambiguous, B​::​C​::​i or A​::​i?
    }
  }
  void f3() {
    i = 5;          // uses A​::​i
  }
}
void f4() {
  i = 5;            // ill-formed; neither i is visible
}
end example
]
For unqualified lookup, the using-directive is transitive: if a scope contains a using-directive that nominates a second namespace that itself contains using-directives, the effect is as if the using-directives from the second namespace also appeared in the first.
[Note
:
For qualified lookup, see [namespace.qual].
end note
]
[Example
:
namespace M {
  int i;
}

namespace N {
  int i;
  using namespace M;
}

void f() {
  using namespace N;
  i = 7;            // error: both M​::​i and N​::​i are visible
}
For another example,
namespace A {
  int i;
}
namespace B {
  int i;
  int j;
  namespace C {
    namespace D {
      using namespace A;
      int j;
      int k;
      int a = i;    // B​::​i hides A​::​i
    }
    using namespace D;
    int k = 89;     // no problem yet
    int l = k;      // ambiguous: C​::​k or D​::​k
    int m = i;      // B​::​i hides A​::​i
    int n = j;      // D​::​j hides B​::​j
  }
}
end example
]
If a namespace is extended after a using-directive for that namespace is given, the additional members of the extended namespace and the members of namespaces nominated by using-directives in the extending namespace-definition can be used after the extending namespace-definition.
If name lookup finds a declaration for a name in two different namespaces, and the declarations do not declare the same entity and do not declare functions, the use of the name is ill-formed.
[Note
:
In particular, the name of a variable, function or enumerator does not hide the name of a class or enumeration declared in a different namespace.
For example,
namespace A {
  class X { };
  extern "C"   int g();
  extern "C++" int h();
}
namespace B {
  void X(int);
  extern "C"   int g();
  extern "C++" int h(int);
}
using namespace A;
using namespace B;

void f() {
  X(1);             // error: name X found in two namespaces
  g();              // OK: name g refers to the same entity
  h();              // OK: overload resolution selects A​::​h
}
end note
]
During overload resolution, all functions from the transitive search are considered for argument matching.
The set of declarations found by the transitive search is unordered.
[Note
:
In particular, the order in which namespaces were considered and the relationships among the namespaces implied by the using-directives do not cause preference to be given to any of the declarations found by the search.
end note
]
An ambiguity exists if the best match finds two functions with the same signature, even if one is in a namespace reachable through using-directives in the namespace of the other.99
[Example
:
namespace D {
  int d1;
  void f(char);
}
using namespace D;

int d1;             // OK: no conflict with D​::​d1

namespace E {
  int e;
  void f(int);
}

namespace D {       // namespace extension
  int d2;
  using namespace E;
  void f(int);
}

void f() {
  d1++;             // error: ambiguous ​::​d1 or D​::​d1?
  ::d1++;           // OK
  D::d1++;          // OK
  d2++;             // OK: D​::​d2
  e++;              // OK: E​::​e
  f(1);             // error: ambiguous: D​::​f(int) or E​::​f(int)?
  f('a');           // OK: D​::​f(char)
}
end example
]
During name lookup in a class hierarchy, some ambiguities may be resolved by considering whether one member hides the other along some paths ([class.member.lookup]).
There is no such disambiguation when considering the set of names found as a result of following using-directives.

10.4 The asm declaration [dcl.asm]

An asm declaration has the form
asm-definition:
	attribute-specifier-seq asm ( string-literal ) ;
The asm declaration is conditionally-supported; its meaning is implementation-defined.
The optional attribute-specifier-seq in an asm-definition appertains to the asm declaration.
[Note
:
Typically it is used to pass information through the implementation to an assembler.
end note
]

10.6 Attributes [dcl.attr]

10.6.1 Attribute syntax and semantics [dcl.attr.grammar]

If an attribute-specifier contains an attribute-using-prefix, the attribute-list following that attribute-using-prefix shall not contain an attribute-scoped-token and every attribute-token in that attribute-list is treated as if its identifier were prefixed with N​::​, where N is the attribute-namespace specified in the attribute-using-prefix.
[Note
:
This rule imposes no constraints on how an attribute-using-prefix affects the tokens in an attribute-argument-clause.
end note
]
[Example
:
[[using CC: opt(1), debug]]         // same as [[CC​::​opt(1), CC​::​debug]]
  void f() {}
[[using CC: opt(1)]] [[CC::debug]]  // same as [[CC​::​opt(1)]] [[CC​::​debug]]
  void g() {}
[[using CC: CC::opt(1)]]            // error: cannot combine using and scoped attribute token
  void h() {}
end example
]
[Note
:
For each individual attribute, the form of the balanced-token-seq will be specified.
end note
]
In an attribute-list, an ellipsis may appear only if that attribute's specification permits it.
An attribute followed by an ellipsis is a pack expansion.
An attribute-specifier that contains no attributes has no effect.
The order in which the attribute-tokens appear in an attribute-list is not significant.
If a keyword or an alternative token that satisfies the syntactic requirements of an identifier is contained in an attribute-token, it is considered an identifier.
No name lookup is performed on any of the identifiers contained in an attribute-token.
The attribute-token determines additional requirements on the attribute-argument-clause (if any).
Each attribute-specifier-seq is said to appertain to some entity or statement, identified by the syntactic context where it appears ([stmt.stmt], [dcl.dcl], [dcl.decl]).
If an attribute-specifier-seq that appertains to some entity or statement contains an attribute or alignment-specifier that is not allowed to apply to that entity or statement, the program is ill-formed.
If an attribute-specifier-seq appertains to a friend declaration, that declaration shall be a definition.
For an attribute-token (including an attribute-scoped-token) not specified in this International Standard, the behavior is implementation-defined.
Any attribute-token that is not recognized by the implementation is ignored.
[Note
:
Each implementation should choose a distinctive name for the attribute-namespace in an attribute-scoped-token.
end note
]
Two consecutive left square bracket tokens shall appear only when introducing an attribute-specifier or within the balanced-token-seq of an attribute-argument-clause.
[Note
:
If two consecutive left square brackets appear where an attribute-specifier is not allowed, the program is ill-formed even if the brackets match an alternative grammar production.
end note
]
[Example
:
int p[10];
void f() {
  int x = 42, y[5];
  int(p[[x] { return x; }()]);  // error: invalid attribute on a nested declarator-id and
                                // not a function-style cast of an element of p.
  y[[] { return 2; }()] = 2;    // error even though attributes are not allowed in this context.
  int i [[vendor::attr([[]])]]; // well-formed implementation-defined attribute.
}
end example
]

10.6.2 Alignment specifier [dcl.align]

An alignment-specifier may be applied to a variable or to a class data member, but it shall not be applied to a bit-field, a function parameter, or an exception-declaration ([except.handle]).
An alignment-specifier may also be applied to the declaration or definition of a class (in an elaborated-type-specifier or class-head, respectively) and to the declaration or definition of an enumeration (in an opaque-enum-declaration or enum-head, respectively).
An alignment-specifier with an ellipsis is a pack expansion.
When the alignment-specifier is of the form alignas( constant-expression ):
  • the constant-expression shall be an integral constant expression
  • if the constant expression does not evaluate to an alignment value ([basic.align]), or evaluates to an extended alignment and the implementation does not support that alignment in the context of the declaration, the program is ill-formed.
An alignment-specifier of the form alignas( type-id ) has the same effect as alignas(​alignofexpr.alignof( type-id )).
The alignment requirement of an entity is the strictest nonzero alignment specified by its alignment-specifiers, if any; otherwise, the alignment-specifiers have no effect.
The combined effect of all alignment-specifiers in a declaration shall not specify an alignment that is less strict than the alignment that would be required for the entity being declared if all alignment-specifiers appertaining to that entity were omitted.
[Example
:
struct alignas(8) S {};
struct alignas(1) U {
  S s;
};  // error: U specifies an alignment that is less strict than if the alignas(1) were omitted.
end example
]
If the defining declaration of an entity has an alignment-specifier, any non-defining declaration of that entity shall either specify equivalent alignment or have no alignment-specifier.
Conversely, if any declaration of an entity has an alignment-specifier, every defining declaration of that entity shall specify an equivalent alignment.
No diagnostic is required if declarations of an entity have different alignment-specifiers in different translation units.
[Example
:
// Translation unit #1:
struct S { int x; } s, *p = &s;

// Translation unit #2:
struct alignas(16) S;           // error: definition of S lacks alignment, no diagnostic required
extern S* p;
end example
]
[Example
:
An aligned buffer with an alignment requirement of A and holding N elements of type T can be declared as:
alignas(T) alignas(A) T buffer[N];
Specifying alignas(T) ensures that the final requested alignment will not be weaker than alignof(T), and therefore the program will not be ill-formed.
end example
]
[Example
:
alignas(double) void f();                           // error: alignment applied to function
alignas(double) unsigned char c[sizeof(double)];    // array of characters, suitably aligned for a double
extern unsigned char c[sizeof(double)];             // no alignas necessary
alignas(float)
  extern unsigned char c[sizeof(double)];           // error: different alignment in declaration
end example
]

10.6.3 Carries dependency attribute [dcl.attr.depend]

The attribute-token carries_­dependency specifies dependency propagation into and out of functions.
It shall appear at most once in each attribute-list and no attribute-argument-clause shall be present.
The attribute may be applied to the declarator-id of a parameter-declaration in a function declaration or lambda, in which case it specifies that the initialization of the parameter carries a dependency to each lvalue-to-rvalue conversion of that object.
The attribute may also be applied to the declarator-id of a function declaration, in which case it specifies that the return value, if any, carries a dependency to the evaluation of the function call expression.
The first declaration of a function shall specify the carries_­dependency attribute for its declarator-id if any declaration of the function specifies the carries_­dependency attribute.
Furthermore, the first declaration of a function shall specify the carries_­dependency attribute for a parameter if any declaration of that function specifies the carries_­dependency attribute for that parameter.
If a function or one of its parameters is declared with the carries_­dependency attribute in its first declaration in one translation unit and the same function or one of its parameters is declared without the carries_­dependency attribute in its first declaration in another translation unit, the program is ill-formed, no diagnostic required.
[Note
:
The carries_­dependency attribute does not change the meaning of the program, but may result in generation of more efficient code.
end note
]
[Example
:
/* Translation unit A. */

struct foo { int* a; int* b; };
std::atomic<struct foo *> foo_head[10];
int foo_array[10][10];

[[carries_dependency]] struct foo* f(int i) {
  return foo_head[i].load(memory_order_consume);
}

int g(int* x, int* y [[carries_dependency]]) {
  return kill_dependency(foo_array[*x][*y]);
}

/* Translation unit B. */

[[carries_dependency]] struct foo* f(int i);
int g(int* x, int* y [[carries_dependency]]);

int c = 3;

void h(int i) {
  struct foo* p;

  p = f(i);
  do_something_with(g(&c, p->a));
  do_something_with(g(p->a, &c));
}
The carries_­dependency attribute on function f means that the return value carries a dependency out of f, so that the implementation need not constrain ordering upon return from f.
Implementations of f and its caller may choose to preserve dependencies instead of emitting hardware memory ordering instructions (a.
k.
a.
fences).
Function g's second parameter has a carries_­dependency attribute, but its first parameter does not.
Therefore, function h's first call to g carries a dependency into g, but its second call does not.
The implementation might need to insert a fence prior to the second call to g.
end example
]

10.6.4 Deprecated attribute [dcl.attr.deprecated]

The attribute-token deprecated can be used to mark names and entities whose use is still allowed, but is discouraged for some reason.
[Note
:
In particular, deprecated is appropriate for names and entities that are deemed obsolescent or unsafe.
end note
]
It shall appear at most once in each attribute-list.
An attribute-argument-clause may be present and, if present, it shall have the form:
( string-literal )
[Note
:
The string-literal in the attribute-argument-clause could be used to explain the rationale for deprecation and/or to suggest a replacing entity.
end note
]
The attribute may be applied to the declaration of a class, a typedef-name, a variable, a non-static data member, a function, a namespace, an enumeration, an enumerator, or a template specialization.
A name or entity declared without the deprecated attribute can later be redeclared with the attribute and vice-versa.
[Note
:
Thus, an entity initially declared without the attribute can be marked as deprecated by a subsequent redeclaration.
However, after an entity is marked as deprecated, later redeclarations do not un-deprecate the entity.
end note
]
Redeclarations using different forms of the attribute (with or without the attribute-argument-clause or with different attribute-argument-clauses) are allowed.
[Note
:
Implementations may use the deprecated attribute to produce a diagnostic message in case the program refers to a name or entity other than to declare it, after a declaration that specifies the attribute.
The diagnostic message may include the text provided within the attribute-argument-clause of any deprecated attribute applied to the name or entity.
end note
]

10.6.5 Fallthrough attribute [dcl.attr.fallthrough]

The attribute-token fallthrough may be applied to a null statement; such a statement is a fallthrough statement.
The attribute-token fallthrough shall appear at most once in each attribute-list and no attribute-argument-clause shall be present.
A fallthrough statement may only appear within an enclosing switch statement.
The next statement that would be executed after a fallthrough statement shall be a labeled statement whose label is a case label or default label for the same switch statement.
The program is ill-formed if there is no such statement.
[Note
:
The use of a fallthrough statement is intended to suppress a warning that an implementation might otherwise issue for a case or default label that is reachable from another case or default label along some path of execution.
Implementations should issue a warning if a fallthrough statement is not dynamically reachable.
end note
]
[Example
:
void f(int n) {
  void g(), h(), i();
  switch (n) {
  case 1:
  case 2:
    g();
    [[fallthrough]];
  case 3:                       // warning on fallthrough discouraged
    h();
  case 4:                       // implementation may warn on fallthrough
    i();
    [[fallthrough]];            // ill-formed
  }
}
end example
]

10.6.6 Maybe unused attribute [dcl.attr.unused]

The attribute-token maybe_­unused indicates that a name or entity is possibly intentionally unused.
It shall appear at most once in each attribute-list and no attribute-argument-clause shall be present.
The attribute may be applied to the declaration of a class, a typedef-name, a variable, a non-static data member, a function, an enumeration, or an enumerator.
[Note
:
For an entity marked maybe_­unused, implementations should not emit a warning that the entity is unused, or that the entity is used despite the presence of the attribute.
end note
]
A name or entity declared without the maybe_­unused attribute can later be redeclared with the attribute and vice versa.
An entity is considered marked after the first declaration that marks it.
[Example
:
[[maybe_unused]] void f([[maybe_unused]] bool thing1,
                        [[maybe_unused]] bool thing2) {
  [[maybe_unused]] bool b = thing1 && thing2;
  assert(b);
}
Implementations should not warn that b is unused, whether or not NDEBUG is defined.
end example
]

10.6.7 Nodiscard attribute [dcl.attr.nodiscard]

The attribute-token nodiscard may be applied to the declarator-id in a function declaration or to the declaration of a class or enumeration.
It shall appear at most once in each attribute-list and no attribute-argument-clause shall be present.
[Note
:
A nodiscard call is a function call expression that calls a function previously declared nodiscard, or whose return type is a possibly cv-qualified class or enumeration type marked nodiscard.
Appearance of a nodiscard call as a potentially-evaluated discarded-value expression is discouraged unless explicitly cast to void.
Implementations should issue a warning in such cases.
This is typically because discarding the return value of a nodiscard call has surprising consequences.
end note
]
[Example
:
struct [[nodiscard]] error_info { /* ... */ };
error_info enable_missile_safety_mode();
void launch_missiles();
void test_missiles() {
  enable_missile_safety_mode(); // warning encouraged
  launch_missiles();
}
error_info &foo();
void f() { foo(); }             // warning not encouraged: not a nodiscard call, because neither
                                // the (reference) return type nor the function is declared nodiscard
end example
]

10.6.8 Noreturn attribute [dcl.attr.noreturn]

The attribute-token noreturn specifies that a function does not return.
It shall appear at most once in each attribute-list and no attribute-argument-clause shall be present.
The attribute may be applied to the declarator-id in a function declaration.
The first declaration of a function shall specify the noreturn attribute if any declaration of that function specifies the noreturn attribute.
If a function is declared with the noreturn attribute in one translation unit and the same function is declared without the noreturn attribute in another translation unit, the program is ill-formed, no diagnostic required.
If a function f is called where f was previously declared with the noreturn attribute and f eventually returns, the behavior is undefined.
[Note
:
The function may terminate by throwing an exception.
end note
]
[Note
:
Implementations should issue a warning if a function marked [[noreturn]] might return.
end note
]
[Example
:
[[ noreturn ]] void f() {
  throw "error";                // OK
}

[[ noreturn ]] void q(int i) {  // behavior is undefined if called with an argument <= 0
  if (i > 0)
    throw "positive";
}
end example
]