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 ]
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