10 Declarations [dcl.dcl]

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 document, 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 Likelihood attributes [dcl.attr.likelihood]

The attribute-tokens likely and unlikely may be applied to labels or statements.
The attribute-tokens likely and unlikely shall appear at most once in each attribute-list and no attribute-argument-clause shall be present.
The attribute-token likely shall not appear in an attribute-specifier-seq that contains the attribute-token unlikely.
[Note
:
The use of the likely attribute is intended to allow implementations to optimize for the case where paths of execution including it are arbitrarily more likely than any alternative path of execution that does not include such an attribute on a statement or label.
The use of the unlikely attribute is intended to allow implementations to optimize for the case where paths of execution including it are arbitrarily more unlikely than any alternative path of execution that does not include such an attribute on a statement or label.
A path of execution includes a label if and only if it contains a jump to that label.
Excessive usage of either of these attributes is liable to result in performance degradation.
end note
]
[Example
:
void g(int);
int f(int n) {
  if (n > 5) [[unlikely]] {     // n > 5 is considered to be arbitrarily unlikely
    g(0);
    return n * 2 + 1;
  }

  switch (n) {
  case 1:
    g(1);
    [[fallthrough]];

  [[likely]] case 2:            // n == 3 is considered to be arbitrarily more
    g(2);                       // likely than any other value of n
    break;
  }
  return 3;
}
end example
]

10.6.7 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
]

10.6.8 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
]

10.6.9 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
]

10.6.10 No unique address attribute [dcl.attr.nouniqueaddr]

The attribute-token no_­unique_­address specifies that a non-static data member is a potentially-overlapping subobject ([intro.object]).
It shall appear at most once in each attribute-list and no attribute-argument-clause shall be present.
The attribute may appertain to a non-static data member other than a bit-field.
[Note
:
The non-static data member can share the address of another non-static data member or that of a base class, and any padding that would normally be inserted at the end of the object can be reused as storage for other members.
end note
]
[Example
:
template<typename Key, typename Value,
         typename Hash, typename Pred, typename Allocator>
class hash_map {
  [[no_unique_address]] Hash hasher;
  [[no_unique_address]] Pred pred;
  [[no_unique_address]] Allocator alloc;
  Bucket *buckets;
  // ...
public:
  // ...
};
Here, hasher, pred, and alloc could have the same address as buckets if their respective types are all empty.
end example
]