[dcl.align] (original) (raw)

9 Declarations [dcl.dcl]

9.12 Attributes [dcl.attr]

9.12.2 Alignment specifier [dcl.align]

When the alignment-specifier is of the formalignas( 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-specifiersappertaining 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 analignment-specifier, any non-defining declaration of that entity shall either specify equivalent alignment or have noalignment-specifier.

Conversely, if any declaration of an entity has analignment-specifier, every defining declaration of that entity shall specify an equivalent alignment.

No diagnostic is required if declarations of an entity have different alignment-specifiersin 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 Tcan 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

]