Object types have alignment requirements 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.
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
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:
double d; };
char c; };
D is the type of a complete
object, it will have a subobject of type
so it must be aligned appropriately for a
appears as a subobject of another object that also has
as a virtual base class, the
subobject might be part of a different subobject, reducing the alignment requirements
D subobject. —end
example] The result of the
operator reflects the alignment requirement of the type in the complete-object
An extended alignment is represented by an alignment greater
alignof(std::max_align_t). It is implementation-defined whether any
extended alignments are supported and the contexts in which they are supported.
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]
Alignments are represented as values of the type
Valid alignments include only those values returned by an
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.
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.
The alignment requirement of a complete type can be queried using an
alignof expression. Furthermore, the types
char shall have the weakest alignment
requirement. [Note: This enables the character types to
be used as the underlying type for an aligned memory area. —end
Comparing alignments is meaningful and provides the obvious results:
[Note: The runtime pointer alignment function can be used to obtain an aligned pointer within a buffer; the aligned-storage templates in the library can be used to obtain aligned storage. —end note]
If a request for a specific extended alignment in a specific context is not supported by an implementation, the program is ill-formed. Additionally, a request for runtime allocation of dynamic storage for which the requested alignment cannot be honored shall be treated as an allocation failure.