10 Declarations [dcl.dcl]

10.6 Attributes [dcl.attr]

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 ([dcl.enum])). An alignment-specifier with an ellipsis is a pack expansion.

When the alignment-specifier is of the form alignas( constant-expression ):

An alignment-specifier of the form alignas( type-id ) has the same effect as alignas(​alignof( type-id )) ([expr.alignof]).

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]


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
  extern unsigned char c[sizeof(double)];           // error: different alignment in declaration

end example]