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3.8 Record Types -- TOC

(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)
       component_item ::= component_declaration | representation_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)

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 being defined is called a per-object constraint. For the elaboration of a component_definition of a component_declaration, if the constraint of the subtype_indication is not a per-object constraint, then the subtype_indication 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.

NOTES

(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

Subclauses

1. Variant Parts and Discrete Choices

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