How to set the alignment in a platform independent way? - c++

Question

In the latest draft of the c++11 standard, chapter 3.11 talks about the alignment.
Later, the chapter 7.6.1 defines how to define an aligned structures (or variables?)
If I define a structure like this :
alignas(16) struct A
{
int n;
unsigned char[ 1020 ];
};
does it means that all instances of the class A are going to be aligned to 16 bytes?
Or, do I have to do it like in the next code?
struct A
{
char data[300];
};
alignas(16) A a;
If both examples are wrong, how to do it properly?
PS I am not looking for a compiler dependent solution.

Answer 1

Alignment is first and foremost a property of types.
It can be overridden for a type with alignas; alignas can also be used to assign a new alignment value to a specific object.
So, both examples are valid, and will have the semantics that you've presumed.
[n3290: 3.11/1]: Object types have alignment requirements (3.9.1,
3.9.2) which place restrictions on the addresses at which an object of
that type may be allocated. An alignment is an implementation-defined
integer value representing the number of bytes between successive
addresses at which a given object can be allocated. An object type
imposes an alignment requirement on every object of that type;
stricter alignment can be requested using the alignment specifier
(7.6.2).
[n3290: 7.6.2/1]: 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, the formal parameter of a catch
clause (15.3), or a variable declared with the register storage class
specifier. An alignment-specifier may also be applied to the
declaration of a class or enumeration type. An alignment-specifier
with an ellipsis is a pack expansion (14.5.3).
[n3290: 7.6.2/2]: When the alignment-specifier is of the form
alignas( assignment-expression ):
the assignment-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.