3.8 Record Types
object is a composite object consisting of named components. The value
of a record object is a composite value consisting of the values of the
Name Resolution Rules
Proof: The identifiers of all components
of a record type have to be distinct because they are all declared immediately
within the same declarative region. See Section 8.
Within a type_declaration
denotes a component, protected subprogram, or entry of the type is allowed
only in the following cases:
denotes any component, protected subprogram, or entry is allowed within
an operational or a
representation item that occurs within the declaration of the composite
denotes a noninherited discriminant is allowed within the declaration
of the type, but not within the discriminant_part
If the discriminant is used to define the constraint of a component,
the bounds of an entry family, or the constraint of the parent subtype
in a derived_type_definition,
then its name shall appear alone as a direct_name
(not as part of a larger expression or expanded name). A discriminant
shall not be used to define the constraint of a scalar component.
Reason: The penultimate restriction simplifies
implementation, and allows the outer discriminant and the inner discriminant
or bound to possibly share storage.
Ramification: Other rules prevent such
a discriminant from being an inherited one.
Reason: The last restriction is inherited
from Ada 83. The restriction is not really necessary from a language
design point of view, but we did not remove it, in order to avoid unnecessary
changes to existing compilers.
Discussion: Note that a discriminant
can be used to define the constraint for a component that is of an access-to-composite
The above rules, and a similar one in 6.1
formal parameters, are intended to allow initializations of components
or parameters to occur in a (nearly) an
arbitrary order — whatever order is most efficient (subject to the restrictions of 3.3.1)
since one default_expression
cannot depend on the value of another one. They It
also prevent circularities.
are not allowed to be denoted, except within representation items. However,
of the parent subtype_indication
is allowed to denote a discriminant of the parent.
Reason: This rule allows T'Access or
T'Unchecked_Access, but disallows, for example, a range constraint (1..T'Size).
Allowing things like (1..T'Size) would mean that a per-object constraint
could affect the size of the object, which would be bad.
In this case, the nominal subtype cannot be an
unconstrained discriminated subtype. See 3.6.
If the component_list
of a record type is defined by the reserved word null
are no discriminants, then the record type has no components and all
records of the type are null records
. A record_definition
of null record
is equivalent to record null; end record
This short-hand is available
both for declaring a record type and a record extension — see 3.9.1
definition of a composite type, if a component_definition
) includes a name
that denotes a discriminant of the type, or that is an attribute_reference
denotes the current instance of the type, the expression containing the
a per-object expression
, and the constraint
or range constraint
being defined is called a per-object constraint
the elaboration of a component_definition
of a component_declaration or the discrete_subtype_definition
of an entry_declaration
for an entry family (see 9.5.2)
, if the component subtype is defined by an access_definition
if the constraint
of the subtype_indication
is not a per-object constraint, then the access_definition,
is elaborated. On the other hand, if the constraint
is a per-object constraint, then the elaboration consists of the evaluation
of any included expression that is not part of a per-object expression.
Each such expression is evaluated once unless it
is part of a named association in a discriminant constraint, in which
case it is evaluated once for each associated discriminant.
The evaluation of other expressions
that appear in component_definition
is performed when the type definition is elaborated. The evaluation of
expressions that appear as default_expression
is postponed until an object is created. Expressions in representation
items that appear within a composite type definition are evaluated according
to the rules of the particular representation item.
61 The default_expression
of a record component is only evaluated upon the creation of a default-initialized
object of the record type (presuming the object has the component, if
it is in a variant_part
— see 3.3.1
63 If a record type does not have a variant_part
then the same components are present in all values of the type.
64 A record type is limited if it has the
reserved word limited
in its definition, or if any of its components
are limited (see 7.5
The predefined operations
of a record type include membership tests, qualification, and explicit
conversion. If the record type is nonlimited, they also include assignment
and the predefined equality operators.
Examples of record
type Date is
Day : Integer range 1 .. 31;
Month : Month_Name;
Year : Integer range 0 .. 4000;
type Complex is
Re : Real := 0.0;
Im : Real := 0.0;
Examples of record
Tomorrow, Yesterday : Date;
A, B, C : Complex;
-- both components of A, B, and C are implicitly initialized to zero
Extensions to Ada 83
The syntax rule for component_declaration
is modified to use component_definition
(instead of component_subtype_definition
The effect of this change is to allow the reserved word aliased
before the component_subtype_definition
A short-hand is provided for defining a null
record type (and a null record extension), as these will be more common
for abstract root types (and derived types without additional components).
The syntax rule for record_type_definition
is modified to allow the reserved words tagged
Tagging is new. Limitedness is now orthogonal to privateness. In Ada
83 the syntax implied that limited private was sort of more private than
private. However, limitedness really has nothing to do with privateness;
limitedness simply indicates the lack of assignment capabilities, and
makes perfect sense for nonprivate types such as record types.
Wording Changes from Ada 83
Extensions to Ada 95
Record components can have
an anonymous access type.
Wording Changes from Ada 95
Corrigendum: Improved the description of
the elaboration of per-object constraints.
Corrigendum: Changed representation clauses
to aspect clauses to reflect that they are used for more than just representation.
Defined explicitly limited record type to
use in other rules.
Extensions to Ada 2005
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe