6 Basic concepts [basic]

6.11 Alignment [basic.align]

1
#
Object types have alignment requirements ([basic.fundamental], [basic.compound]) 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 ([dcl.align]).
2
#
A fundamental alignment is represented by an alignment less than or equal to the greatest alignment supported by the implementation in all contexts, which is equal to alignof(std​::​max_­align_­t) ([support.types]).
The alignment required for a type might be different when it is used as the type of a complete object and when it is used as the type of a subobject.
[ Example
: 
struct B { long double d; };
struct D : virtual B { char c; };
When D is the type of a complete object, it will have a subobject of type B, so it must be aligned appropriately for a long double.
If D appears as a subobject of another object that also has B as a virtual base class, the B subobject might be part of a different subobject, reducing the alignment requirements on the D subobject.
— end example
 ]
The result of the alignof operator reflects the alignment requirement of the type in the complete-object case.
3
#
An extended alignment is represented by an alignment greater than alignof(std​::​max_­align_­t).
It is implementation-defined whether any extended alignments are supported and the contexts in which they are supported ([dcl.align]).
A type having an extended alignment requirement is an over-aligned type.
[ Note
:
Every over-aligned type is or contains a class type to which extended alignment applies (possibly through a non-static data member).
— end note
 ]
A new-extended alignment is represented by an alignment greater than __STDCPP_­DEFAULT_­NEW_­ALIGNMENT__ ([cpp.predefined]).
4
#
Alignments are represented as values of the type std​::​size_­t.
Valid alignments include only those values returned by an alignof expression for the fundamental types plus an additional implementation-defined set of values, which may be empty.
Every alignment value shall be a non-negative integral power of two.
5
#
Alignments have an order from weaker to stronger or stricter alignments.
Stricter alignments have larger alignment values.
An address that satisfies an alignment requirement also satisfies any weaker valid alignment requirement.
6
#
The alignment requirement of a complete type can be queried using an alignof expression ([expr.alignof]).
Furthermore, the narrow character types ([basic.fundamental]) shall have the weakest alignment requirement.
[ Note
:
This enables the narrow character types to be used as the underlying type for an aligned memory area ([dcl.align]).
— end note
 ]
7
#
Comparing alignments is meaningful and provides the obvious results:
  • (7.1)
    Two alignments are equal when their numeric values are equal.
  • (7.2)
    Two alignments are different when their numeric values are not equal.
  • (7.3)
    When an alignment is larger than another it represents a stricter alignment.
8
#
[ Note
:
The runtime pointer alignment function ([ptr.align]) can be used to obtain an aligned pointer within a buffer; the aligned-storage templates in the library ([meta.trans.other]) can be used to obtain aligned storage.
— end note
 ]
9
#
If a request for a specific extended alignment in a specific context is not supported by an implementation, the program is ill-formed.