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.
Syntax
2
extension_aggregate ::=
(
ancestor_part with record_component_association_list)
3
ancestor_part ::= expression |
subtype_mark
Name Resolution Rules
4/2
The expected type for an
extension_aggregate
shall be a single type that is a record extension.
If
the
ancestor_part is an
expression,
it is expected to be of any tagged type.
Legality Rules
5/2
If the ancestor_part is
a subtype_mark, it shall denote a specific
tagged subtype. If the ancestor_part is an
expression, it shall not be dynamically tagged.
The type of the extension_aggregate shall
be derived from the type of the ancestor_part,
through one or more record extensions (and no private extensions).
Static Semantics
6
For the
record_component_association_list
of an
extension_aggregate, the only components
needed are those of the composite value defined by the aggregate
that are not inherited from the type of the
ancestor_part,
plus any inherited discriminants if the
ancestor_part
is a
subtype_mark that denotes an unconstrained
subtype.
Dynamic Semantics
7
For the evaluation of an
extension_aggregate,
the
record_component_association_list is evaluated.
If the
ancestor_part is an
expression,
it is also evaluated; if the
ancestor_part
is a
subtype_mark, the components of the value
of the aggregate not given by the
record_component_association_list
are initialized by default as for an object of the ancestor type. Any
implicit initializations or evaluations are performed in an arbitrary
order, except that the
expression for a discriminant
is evaluated prior to any other evaluation or initialization that depends
on it.
8
If the type
of the
ancestor_part has discriminants that
are not inherited by the type of the
extension_aggregate,
then, unless the
ancestor_part is a
subtype_mark
that denotes an unconstrained subtype, a check is made that each discriminant
of the ancestor has the value specified for a corresponding discriminant,
either in the
record_component_association_list,
or in the
derived_type_definition for some
ancestor of the type of the
extension_aggregate.
Constraint_Error is raised if this check fails.
9
8 If all components of the value of the
extension_aggregate are determined by the
ancestor_part, then the record_component_association_list
is required to be simply null record.
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
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