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 ([dcl.type.elab]) or class-head (Clause [class]), 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 ([temp.variadic]).
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(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 extern S* p; // diagnostic required
— 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 ]