13.2 Packed Types
The Pack aspect having the value True specifies that
storage minimization should be the main criterion when selecting the
representation of a composite type.
Static Semantics
For a full type declaration
of a composite type, the following language-defined representation aspect
may be specified:
Pack
The type of aspect Pack is Boolean. When aspect Pack is True for a type,
the type (or the extension part) is said to be
packed. For a type
extension, the parent part is packed as for the parent type, and specifying
Pack causes packing only of the extension part.
If directly specified, the
aspect_definition
shall be a static expression. If not specified (including by inheritance),
the aspect is False.
Implementation Advice
If a type is packed, then the implementation should
try to minimize storage allocated to objects of the type, possibly at
the expense of speed of accessing components, subject to reasonable complexity
in addressing calculations.
If a packed type has a component that is not of
a by-reference type and has no aliased part, then such a component need
not be aligned according to the Alignment of its subtype; in particular
it need not be allocated on a storage element boundary.
The
recommended level of support for the Pack aspect is:
For a packed record type, the components should
be packed as tightly as possible subject to the Sizes of the component
subtypes, and subject to any
record_representation_clause
that applies to the type; the implementation may, but need not, reorder
components or cross aligned word boundaries to improve the packing. A
component whose Size is greater than the word size may be allocated an
integral number of words.
For a packed array type, if the Size of the component
subtype is less than or equal to the word size, Component_Size should
be less than or equal to the Size of the component subtype, rounded up
to the nearest factor of the word size.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
