[ Note: For each individual attribute, the form of the balanced-token-seq will be specified. — end note ]
For an attribute-token not specified in this International Standard, the behavior is implementation-defined.
Two consecutive left square bracket tokens shall appear only when introducing an attribute-specifier. [ 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; }()]); y[[] { return 2; }()] = 2; }
— end example ]
7.6.2 Alignment specifier [dcl.align]When the alignment-specifier is of the form alignas( constant-expression ):
the constant-expression shall be an integral constant expression
if the constant expression evaluates to a fundamental alignment, the alignment requirement of the declared entity shall be the specified fundamental alignment
if the constant expression evaluates to an extended alignment and the implementation supports that alignment in the context of the declaration, the alignment of the declared entity shall be that alignment
if the constant expression evaluates to an extended alignment and the implementation does not support that alignment in the context of the declaration, the program is ill-formed
if the constant expression evaluates to zero, the alignment specifier shall have no effect
otherwise, the program is ill-formed.
When multiple alignment-specifiers are specified for an entity, the alignment requirement shall be set to the strictest specified alignment.
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 were omitted (including those in other declarations).
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:
struct S { int x; } s, p = &s;
struct alignas(16) S; extern S* p;
— end example ]
[ Example: An aligned buffer with an alignment requirement of A and holding N elements of type T other than char, signed char, or unsigned char 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(); alignas(double) unsigned char c[sizeof(double)]; extern unsigned char c[sizeof(double)]; alignas(float) extern unsigned char c[sizeof(double)];
— end example ]
7.6.3 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 are encouraged to issue a warning if a function marked [[noreturn]] might return. — end note ]
[ Example:
[[ noreturn ]] void f() {
throw "error"; }
[[ noreturn ]] void q(int i) { if (i > 0)
throw "positive";
}
— end example ]
7.6.4 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 ([intro.multithread]) each lvalue-to-rvalue conversion ([conv.lval]) 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:
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]);
}
[[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.
7.6.5 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, an enumeration, or a template specialization.
A name or entity declared without the deprecated attribute can later be re-declared 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 ]
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