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4.3.1 Record Aggregates
1
[In a record_aggregate,
a value is specified for each component of the record or record extension
value, using either a named or a positional association.]
Syntax
2
record_aggregate
::= (
record_component_association_list)
3
record_component_association_list
::=
record_component_association {,
record_component_association}
|
null record
4
record_component_association
::=
[
component_choice_list => ]
expression
5
component_choice_list
::=
component_selector_name {|
component_selector_name}
|
others
6
{named component association}
A
record_component_association
is a
named component association if it has a
component_choice_list;
{positional component association} otherwise,
it is a
positional component association. Any positional component
associations shall precede any named component associations. If there
is a named association with a
component_choice_list
of
others, it shall come last.
6.a
Discussion: These rules
were implied by the BNF in an early version of the RM9X, but it made
the grammar harder to read, and was inconsistent with how we handle discriminant
constraints. Note that for array aggregates we still express some of
the rules in the grammar, but array aggregates are significantly different
because an array aggregate is either all positional (with a possible
others at the end), or all named.
7
In the record_component_association_list
for a record_aggregate, if there
is only one association, it shall be a named association.
7.a
Reason: Otherwise the
construct would be interpreted as a parenthesized expression. This is
considered a syntax rule, since it is relevant to overload resolution.
We choose not to express it with BNF so we can share the definition of
record_component_association_list
in both record_aggregate and extension_aggregate.
7.b
Ramification: The record_component_association_list
of an extension_aggregate does not
have such a restriction.
Name Resolution Rules
8
{expected type (record_aggregate)
[partial]} The expected type for a
record_aggregate
shall be a single nonlimited record type or record extension.
8.a
Ramification: This rule
is used to resolve whether an aggregate
is an array_aggregate or a record_aggregate.
The presence of a with is used to resolve between a record_aggregate
and an extension_aggregate.
9
{needed component (record_aggregate
record_component_association_list)} For
the
record_component_association_list
of a
record_aggregate, all components
of the composite value defined by the aggregate are
needed[; for
the association list of an
extension_aggregate,
only those components not determined by the ancestor expression or subtype
are needed (see
4.3.2).] Each
selector_name
in a
record_component_association
shall denote a needed component [(including possibly a discriminant)].
9.a
Ramification: For the
association list of a record_aggregate,
``needed components'' includes every component of the composite value,
but does not include those in unchosen variants
(see AI83-309). If there are variants,
then the value specified for the discriminant that governs them determines
which variant is chosen, and hence
which components are needed.
9.b
If an extension defines a new
known_discriminant_part, then all
of its discriminants are needed in the component association list of
an extension aggregate for that type, even if the discriminants have
the same names and types as discriminants of the type of the ancestor
expression. This is necessary to ensure that the positions in the record_component_association_list
are well defined, and that discriminants that govern variant_parts
can be given by static expressions.
10
{expected
type (record_component_association expression) [partial]}
The expected type for the
expression
of a
record_component_association
is the type of the
associated component(s);
{associated
components (of a record_component_association)} the
associated component(s) are as follows:
11
- For a positional association, the
component [(including possibly a discriminant)] in the corresponding
relative position (in the declarative region of the type), counting only
the needed components;
11.a
Ramification: This means
that for an association list of an extension_aggregate,
only noninherited components are counted to determine the position.
12
- For a named association with one or
more component_selector_names,
the named component(s);
13
- For a named association with the reserved
word others, all needed components that are not associated with
some previous association.
Legality Rules
14
If the type of a record_aggregate
is a record extension, then it shall be a descendant of a record type,
through one or more record extensions (and no private extensions).
15
If there are no components needed in a given
record_component_association_list,
then the reserved words null record shall appear rather than a
list of record_component_associations.
15.a
Ramification: For example,
"(null record)" is a record_aggregate
for a null record type. Similarly, "(T'(A) with null record)"
is an extension_aggregate for a
type defined as a null record extension of T.
16
Each record_component_association
shall have at least one associated component, and each needed component
shall be associated with exactly one record_component_association.
If a record_component_association
has two or more associated components, all of them shall be of the same
type.
16.a
Ramification: These rules
apply to an association with an others choice.
16.b
Reason: Without these
rules, there would be no way to know what was the expected type for the
expression of the association.
16.c
Discussion: AI83-00244
also requires that the expression
shall be legal for each associated component. This is because even though
two components have the same type, they might have different subtypes.
Therefore, the legality of the expression,
particularly if it is an array aggregate, might differ depending on the
associated component's subtype. However, we have relaxed the rules on
array aggregates slightly for Ada 95, so the staticness of an applicable
index constraint has no effect on the legality of the array aggregate
to which it applies. See 4.3.3. This was
the only case (that we know of) where a subtype provided by context affected
the legality of an expression.
16.d
Ramification: The rule
that requires at least one associated component for each record_component_association
implies that there can be no extra associations for components that don't
exist in the composite value, or that are already determined by the ancestor
expression or subtype of an extension_aggregate.
16.e
The second part of the first
sentence ensures that no needed components are left out, nor specified
twice.
17
If the components of a variant_part
are needed, then the value of a discriminant that governs the variant_part
shall be given by a static expression.
17.a
Ramification: This expression
might either be given within the aggregate itself, or in a constraint
on the parent subtype in a derived_type_definition
for some ancestor of the type of the aggregate.
Dynamic Semantics
18
{evaluation (record_aggregate)
[partial]} The evaluation of a
record_aggregate
consists of the evaluation of the
record_component_association_list.
19
{evaluation (record_component_association_list)
[partial]} For the evaluation of a
record_component_association_list,
any per-object constraints (see
3.8) for components
specified in the association list are elaborated and any
expressions
are evaluated and converted to the subtype of the associated component.
{implicit subtype conversion (expressions in aggregate)
[partial]} Any constraint elaborations and
expression evaluations (and conversions)
occur in an arbitrary order, except that the
expression
for a discriminant is evaluated (and converted) prior to the elaboration
of any per-object constraint that depends on it, which in turn occurs
prior to the evaluation and conversion of the
expression
for the component with the per-object constraint.
19.a
Ramification: The conversion
in the first rule might raise Constraint_Error.
19.b
Discussion: This check
in the first rule presumably happened as part of the dependent compatibility
check in Ada 83.
20
The expression
of a record_component_association
is evaluated (and converted) once for each associated component.
21
7 For a record_aggregate
with positional associations, expressions specifying discriminant values
appear first since the known_discriminant_part
is given first in the declaration of the type; they have to be in the
same order as in the known_discriminant_part.
Examples
22
Example of a
record aggregate with positional associations:
23
(4, July, 1776) -- see 3.8
24
Examples of record
aggregates with named associations:
25
(Day => 4, Month => July, Year => 1776)
(Month => July, Day => 4, Year => 1776)
26
(Disk, Closed, Track => 5, Cylinder => 12) -- see 3.8.1
(Unit => Disk, Status => Closed, Cylinder => 9, Track => 1)
27
Example of component
association with several choices:
28
(Value => 0, Succ|Pred => new Cell'(0, null, null)) -- see 3.10.1
29
-- The allocator is evaluated twice: Succ and Pred designate different cells
30
Examples of record
aggregates for tagged types (see 3.9 and 3.9.1):
31
Expression'(null record)
Literal'(Value => 0.0)
Painted_Point'(0.0, Pi/2.0, Paint => Red)
Extensions to Ada 83
31.a
{extensions to Ada 83}
Null record aggregates may now be specified, via
"(null record)". However, this syntax is more useful
for null record extensions in extension aggregates.
Wording Changes from Ada 83
31.b
Various AIs have been incorporated
(AI83-00189, AI83-00244, and AI83-00309). In particular, Ada 83 did not
explicitly disallow extra values in a record aggregate. Now we do.
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