13.6 Change of Representation
{
AI05-0229-1}
{
AI12-0445-1}
[
A
type_conversion
(see
4.6) can be used to convert between two
different representations of the same array or record. To convert an
array from one representation to another, two array types with matching
component subtypes and convertible index types are required. If one type
has Pack specified and the other does not, then explicit conversion can
be used to pack or unpack an array.
{
AI12-0425-1}
{
AI12-0445-1}
To convert an untagged record from one representation to another, two
record types with a common ancestor type are required. Distinct representations
can then be specified for the record types, and explicit conversion between
the types can be used to effect a change in representation.]
Ramification: {
AI12-0425-1}
The language does not allow implicit copying of by-reference types, so
it also does not allow different representations of related by-reference
types. Similarly, language rules prevent related tagged types from having
different representations of the parent part. Therefore, this technique
cannot be used for tagged or by-reference types.
Examples
Example of change
of representation:
-- Packed_Descriptor and Descriptor are two different types
-- with identical characteristics, apart from their
-- representation
type Descriptor is
record
-- components of a descriptor
end record;
type Packed_Descriptor is new Descriptor;
for Packed_Descriptor use
record
-- component clauses for some or for all components
end record;
-- Change of representation can now be accomplished by explicit type conversions:
D : Descriptor;
P : Packed_Descriptor;
P := Packed_Descriptor(D); -- pack D
D := Descriptor(P); -- unpack P
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
