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3.8 Record Types
1
A record
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
components.
Syntax
2
record_type_definition
::= [[
abstract]
tagged] [
limited]
record_definition
3
record_definition
::=
record
component_list
end record
|
null record
4
component_list
::=
component_item {
component_item}
| {
component_item}
variant_part
|
null;
5/1
component_item
::= component_declaration |
aspect_clause
6
component_declaration
::=
defining_identifier_list :
component_definition [:=
default_expression];
Name Resolution Rules
7
The expected type for the
default_expression, if any, in a
component_declaration is the type
of the component.
Legality Rules
8
A default_expression
is not permitted if the component is of a limited type.
9
Each
component_declaration
declares a
component of the record type. Besides components declared
by
component_declarations, the components
of a record type include any components declared by
discriminant_specifications
of the record type declaration. The identifiers of all components of
a record type shall be distinct.
10
Within a type_declaration,
a name that denotes a component,
protected subprogram, or entry of the type is allowed only in the following
cases:
11
- A name
that denotes any component, protected subprogram, or entry is allowed
within a representation item that occurs within the declaration of the
composite type.
12
- A name
that 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.
13
If the name of the current instance of a type
(see
8.6) is used to define the constraint
of a component, then it shall appear as a
direct_name
that is the
prefix of an
attribute_reference
whose result is of an access type, and the
attribute_reference
shall appear alone.
Static Semantics
14
The
component_definition
of a
component_declaration defines
the (nominal) subtype of the component. If the reserved word
aliased
appears in the
component_definition,
then the component is aliased (see
3.10).
15
If the
component_list
of a record type is defined by the reserved word
null and there
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.
Dynamic Semantics
16
The elaboration of a
record_type_definition
creates the record type and its first subtype, and consists of the elaboration
of the
record_definition.
The
elaboration of a
record_definition
consists of the elaboration of its
component_list,
if any.
17
The elaboration of a
component_list
consists of the elaboration of the
component_items
and
variant_part, if any, in the
order in which they appear.
The elaboration of a
component_declaration consists of
the elaboration of the
component_definition.
18/1
Within
the definition of a composite type, if a
component_definition
or
discrete_subtype_definition (see
9.5.2) includes a
name
that denotes a discriminant of the type, or that is an
attribute_reference
whose
prefix denotes the current
instance of the type, the expression containing the
name
is called a
per-object expression, and the
constraint
or
range being defined is called
a
per-object constraint.
For 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
constraint
or
range of the
subtype_indication
or
discrete_subtype_definition is
not a per-object constraint, then the
subtype_indication
or
discrete_subtype_definition is
elaborated. On the other hand, if the
constraint
or
range 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.
18.1/1
When a per-object constraint
is elaborated (as part of creating an object), each per-object expression
of the constraint is evaluated. For other expressions, the values determined
during the elaboration of the
component_definition
or
entry_declaration are used. Any
checks associated with the enclosing
subtype_indication
or
discrete_subtype_definition are
performed, including the subtype compatibility check (see
3.2.2),
and the associated subtype is created.
19
55 A component_declaration
with several identifiers is equivalent to a sequence of single component_declarations,
as explained in 3.3.1.
20
56 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).
21
57 The subtype defined
by a component_definition (see 3.6)
has to be a definite subtype.
22
58 If a record type does
not have a variant_part, then the
same components are present in all values of the type.
23
59 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).
24
60 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.
25
61 A component of a record
can be named with a selected_component.
A value of a record can be specified with a record_aggregate,
unless the record type is limited.
Examples
26
Examples of
record type declarations:
27
type Date is
record
Day : Integer range 1 .. 31;
Month : Month_Name;
Year : Integer range 0 .. 4000;
end record;
28
type Complex is
record
Re : Real := 0.0;
Im : Real := 0.0;
end record;
29
Examples of record
variables:
30
Tomorrow, Yesterday : Date;
A, B, C : Complex;
31
-- both components of A, B, and C are implicitly initialized to zero
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