[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 of a class (in an elaborated-type-specifier ([dcl.type.elab]) or class-head ([class]), respectively).

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

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

(2.1)
the constant-expression shall be an integral constant expression
(2.2)
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(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 1: 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 2: / Translation unit #1: struct S { int x; } s, *p = &s; / Translation unit #2: struct alignas(16) S; / ill-formed, no diagnostic required: definition of S lacks alignment extern S* p; — end example]

[Example 3:

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 4: 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]

9 Declarations [dcl]

9.13 Attributes [dcl.attr]

9.13.2 Alignment specifier [dcl.align]