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.
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;
};
— 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:
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 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 ]
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