4.3.2 Extension Aggregates
1
[An
extension_aggregate
specifies a value for a type that is a record extension by specifying
a value or subtype for an ancestor of the type, followed by associations
for any components not determined by the
ancestor_part.]
Language Design Principles
1.a
The model underlying this syntax is that a record
extension can also be viewed as a regular record type with an ancestor
"prefix." The
record_component_association_list
corresponds to exactly what would be needed if there were no ancestor/prefix
type. The
ancestor_part
determines the value of the ancestor/prefix.
Syntax
2
3
Name Resolution Rules
4/2
{
AI95-00287-01}
{expected type (extension_aggregate)
[partial]} The expected type for an
extension_aggregate
shall be a single
nonlimited type that is
a record extension.
{expected type (extension_aggregate
ancestor expression) [partial]} If the
ancestor_part
is an
expression,
it is expected to be of any
nonlimited tagged
type.
4.a
Reason: We could have made the expected
type T'Class where T is the ultimate ancestor of the type
of the aggregate, or we could have made it even more specific than that.
However, if the overload resolution rules get too complicated, the implementation
gets more difficult and it becomes harder to produce good error messages.
Legality Rules
5/2
5.a/2
Reason: {
AI95-00306-01}
The expression cannot be dynamically tagged to
prevent implicit "truncation" of a dynamically-tagged value
to the specific ancestor type. This is similar to the rules in 3.9.2.
Static Semantics
6
Dynamic Semantics
7
8
8.a
Ramification: Corresponding and specified
discriminants are defined in
3.7. The rules
requiring static compatibility between new discriminants of a derived
type and the parent discriminant(s) they constrain ensure that at most
one check is required per discriminant of the ancestor expression.
9
10
9 If the
ancestor_part
is a
subtype_mark,
then its type can be abstract. If its type is controlled, then as the
last step of evaluating the aggregate, the Initialize procedure of the
ancestor type is called, unless the Initialize procedure is abstract
(see
7.6).
Examples
11
Examples of extension
aggregates (for types defined in 3.9.1):
12
Painted_Point'(Point with Red)
(Point'(P) with Paint => Black)
13
(Expression with Left => 1.2, Right => 3.4)
Addition'(Binop with null record)
-- presuming Binop is of type Binary_Operation
Extensions to Ada 83
13.a
{
extensions to Ada 83}
The
extension aggregate syntax is new.
Incompatibilities With Ada 95
13.b/2
{
AI95-00306-01}
{incompatibilities with Ada 95} Amendment
Correction: Eliminated implicit “truncation” of a dynamically
tagged value when it is used as an ancestor expression.
If an aggregate
includes such an expression,
it is illegal in Ada 2005. Such aggregates
are thought to be rare; the problem can be fixed with a type conversion
to the appropriate specific type if it occurs. Wording Changes from Ada 95
13.c/2
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