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3.8 Record Types

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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

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record_type_definition::= [[abstract] tagged] [limited] record_definition

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record_definition::=
    record
       component_list
    end record
  null record

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component_list::=
      component_item {component_item}
   {component_item} variant_part
    null;

5/1

component_item::= component_declaration aspect_clause

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component_declaration::=
   defining_identifier_list component_definition [:= default_expression];
Name Resolution Rules

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The expected type for the default_expression, if any, in a component_declaration is the type of the component.

Legality Rules

8/2
This paragraph was deleted.

9/2
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.

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Within a type_declaration, a name that denotes a component, protected subprogram, or entry of the type is allowed only in the following cases:

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12

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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

13.1/2
If a record_type_definition includes the reserved word limited, the type is called an explicitly limited record type.

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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).

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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

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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.

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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.

18/2
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 (see 3.6) of an entry_declaration (see 9.5.2) for an entry family (see 9.5.2), if the component subtype is defined by an access_definition or if the constraint or range of the subtype_indication or discrete_subtype_definition (see 3.6) is not a per−object constraint, then the access_definition, subtype_indication, or discrete_subtype_definition (see 3.6) 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 (see 3.6) or entry_declaration (see 9.5.2) 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.

     NOTES

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57  A component_declaration with several identifiers is equivalent to a sequence of single component_declarations, as explained in 3.3.1.

20

58  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).

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59  The subtype defined by a component_definition (see 3.6) has to be a definite subtype.

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60  If a record type does not have a variant_part, then the same components are present in all values of the type.

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61  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).

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62  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.

25/2

63  A component of a record can be named with a selected_component. A value of a record can be specified with a record_aggregate.
Examples

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Examples of record type declarations:

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     type Date is
        record
           Day   Integer range .. 31;
           Month Month_Name;
           Year  Integer range .. 4000;
        end record;

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     type Complex is
        record
           Re Real := 0.0;
           Im Real := 0.0;
        end record;

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Examples of record variables:

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     Tomorrow, Yesterday Date;
     A, B, Complex;

31

     −− both components of A, B, and are implicitly initialized to zero