13.6 Change of Representation
{
AI05-0229-1}
[
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 need to be
declared with matching component subtypes, and convertible index types.
If one type has Pack specified and the other does not, then explicit
conversion can be used to pack or unpack an array.
To convert a record from one representation to another,
two record types with a common ancestor type need to be declared, with
no inherited subprograms. 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: This technique does not
work if the first type is an untagged type with user-defined primitive
subprograms. It does not work at all for tagged 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
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