4.3.1 Record Aggregates
[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.]
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.
Name Resolution Rules
For the association list
of a record_aggregate
“needed components” includes every component of the composite
value, but does not include those in unchosen variant
(see AI83-309). If there are variant
then the value specified for the discriminant that governs them determines
is chosen, and hence which components are needed.
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_part
can be given by static expressions.
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;
This means that for an
association list of an extension_aggregate
only noninherited components are counted to determine the position.
For a derived type (including type extensions), the order of declaration
is defined in 3.4
Types and Classes
”. In particular, all discriminants come first,
regardless of whether they are defined for the parent type or are newly
added to the derived type.
For a named association with one or more component_selector_name
the named component(s);
For a named association with the reserved word
others, all needed components that are not associated with some
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).
For example, "(null
)" 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.
These rules apply to an association with an others
an expression. An others
choice with a <> can match zero
components or several components with different types.
Without these rules, there would be no way to know what was the expected
type for the expression
of the association. Note that some of the rules do not apply to <>
associations, as we do not need to resolve anything. We allow others
=> <> to match no components as this is similar to array aggregates.
That means that (others
=> <>) always represents a default-initialized
record or array value.
AI83-00244 also requires that the expression
shall be legal for each associated component. Ada
95 omitted this wording, as it was thought that all cases of difference
had been eliminated. That probably was true, but Ada 2005 reintroduced
cases where the types match but the legality differs. For example: 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.
type Rec (D : access Integer) is record
F : access Integer;
X : aliased Integer;
R : Rec := (D | F => X'Access); -- Legal for D, illegal for F
There are additional ways
for this to happen; because of cases like the above we require that the
Legality Rules are checked individually for each associated component.
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
The second part of the first sentence ensures
that no needed components are left out, nor specified twice.
If a variant_part
P is nested within a variant
V that is not selected by the discriminant value governing the
enclosing V, then there is no restriction on the discriminant
governing P. Otherwise, the value of the The
value of a
discriminant that governs a variant_part
shall be given by a static expression, or
by a nonstatic expression having a constrained static nominal subtype.
In this latter case of a nonstatic expression, there shall be exactly
of P that covers each value that belongs to the nominal subtype
and satisfies the predicates of the subtype, and there shall be at least
one such value. unless P is nested
within a variant
V that is not selected by the discriminant value governing the
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.
This means that in the nonstatic value case, the
(static) subtype cannot have a null range.
Reason: A discriminant must always have
a defined value, but <> means uninitialized for a discrete type
unless the component has a default value.
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 expression
are evaluated and converted to the subtype of the associated component.
Any constraint elaborations and expression
evaluations (and conversions) occur in an arbitrary order, except that
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. If the value of a discriminant that governs a selected variant_part
P is given by a nonstatic expression,
and the evaluation of that expression
yields a value that does not belong to the nominal subtype of the expression,
then Constraint_Error is raised.
Ramification: The conversion in the first
rule might raise Constraint_Error.
Discussion: This check in the first rule
presumably happened as part of the dependent compatibility check in Ada
The test on the value of the discriminant can only
fail if the value is outside the base range of its type, does not satisfy
the (static) predicates of the subtype (possible when the predicate is
disabled), or is an invalid representation. This is a rule similar to
that used for case statements. As with case statements, this is not a
“check”; it cannot be Suppressed.
We don't need similar language for <>, as we're considering the
value of <> for each individual component. Each component has its
own default expression or its own default initialization (they can be
different for each component; the components even could have different
types), and each one has to be evaluated. So there is no need to repeat
Example of a record
aggregate with positional associations:
(4, July, 1776) -- see 3.8
Examples of record
aggregates with named associations:
(Day => 4, Month => July, Year => 1776)
(Month => July, Day => 4, Year => 1776)
(Disk, Closed, Track => 5, Cylinder => 12) -- see 3.8.1
(Unit => Disk, Status => Closed, Cylinder => 9, Track => 1)
Examples of component associations with several choices:
(Value => 0, Succ|Pred => new
)) -- see 3.10.1
-- The allocator is evaluated twice: Succ and Pred designate different cells
(Value => 0, Succ|Pred => <>) -- see 3.10.1
-- Succ and Pred will be set to null
Examples of record
aggregates for tagged types (see 3.9 and 3.9.1):
Literal'(Value => 0.0)
Painted_Point'(0.0, Pi/2.0, Paint => Red)
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
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.
Extensions to Ada 95
Wording Changes from Ada 95
Incompatibilities With Ada 2005
Corrected wording so that the
rule for discriminants governing variant_part
was not effectively circular. The change makes a few aggregate
where a nonstatic discriminant governs an empty variant_part
illegal. However, most Ada implementations already enforce some version
of the new rule and already reject these aggregate
So it is unlikely that any incompatibility will be noticed in practice.
Extensions to Ada 2005
Fixed the wording so that others
=> <> can be used in place of null record
. This is needed
to avoid a generic contract issue for generic bodies: we do not want
to have to assume the worst to disallow others
if the record type might
be a null record.
We now allow multiple components with anonymous access
types to be specified with a single component association. This is to
be consistent with the capabilities of a named access type.
Extensions to Ada 2012
The value of a discriminant
that governs a variant_part
in an aggregate
can be nonstatic if the nominal subtype of the discriminant expression
is static and all possible values select a single variant.
Wording Changes from Ada 2012
Corrigendum: We explicitly say that the
Legality Rules have to be rechecked for each component individually.
This seems obvious, but as the AARM note 4.3.1
(16.c) appeared to say that this was not necessary, and since we explicitly
state this sort of thing for generic instances, it seemed better to be
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe