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3.2.2 Subtype Declarations

1
A subtype_declaration declares a subtype of some previously declared type, as defined by a subtype_indication.

Syntax

2

subtype_declaration::=
   subtype defining_identifier is subtype_indication;

3/2

{00231AI95−00231−01} subtype_indication::=  [null_exclusion] subtype_mark [constraint]

4

subtype_mark::= subtype_name

4.a

Ramification: Note that name includes attribute_reference; thus, S'Base can be used as a subtype_mark.

4.b

Reason: We considered changing subtype_mark to subtype_name. However, existing users are used to the word "mark," so we're keeping it.

5

constraint::= scalar_constraint composite_constraint

6

scalar_constraint::=
     range_constraint digits_constraint delta_constraint

7

composite_constraint::=
     index_constraint discriminant_constraint
Name Resolution Rules

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A subtype_mark shall resolve to denote a subtype. {determines (a type by a subtype_mark)} The type determined by a subtype_mark is the type of the subtype denoted by the subtype_mark.

8.a

Ramification: Types are never directly named; all subtype_marks denote subtypes −− possibly an unconstrained (base) subtype, but never the type. When we use the term anonymous type we really mean a type with no namable subtypes.
Dynamic Semantics

9
{elaboration (subtype_declaration) [partial]} The elaboration of a subtype_declaration consists of the elaboration of the subtype_indication. {elaboration (subtype_indication) [partial]} The elaboration of a subtype_indication creates a new subtype. If the subtype_indication does not include a constraint, the new subtype has the same (possibly null) constraint as that denoted by the subtype_mark. The elaboration of a subtype_indication that includes a constraint proceeds as follows:

10

11

11.a

Ramification: The checks associated with constraint compatibility are all Range_Checks. Discriminant_Checks and Index_Checks are associated only with checks that a value satisfies a constraint.

12
The condition imposed by a constraint is the condition obtained after elaboration of the constraint. {compatibility (constraint with a subtype) [distributed]} The rules defining compatibility are given for each form of constraint in the appropriate subclause. These rules are such that if a constraint is compatible with a subtype, then the condition imposed by the constraint cannot contradict any condition already imposed by the subtype on its values. {Constraint_Error (raised by failure of run−time check)} The exception Constraint_Error is raised if any check of compatibility fails.

12.a

To be honest: The condition imposed by a constraint is named after it −− a range_constraint imposes a range constraint, etc.

12.b

Ramification: A range_constraint causes freezing of its type. Other constraints do not.

     NOTES

13

 A scalar_constraint may be applied to a subtype of an appropriate scalar type (see 3.5, 3.5.9, and J.3), even if the subtype is already constrained. On the other hand, a composite_constraint may be applied to a composite subtype (or an access−to−composite subtype) only if the composite subtype is unconstrained (see 3.6.1 and 3.7.1).
Examples

14
Examples of subtype declarations:

15/2

     {00433AI95−00433−01} subtype Rainbow   is Color range Red .. Blue;        −−  see 3.2.1
     subtype Red_Blue  is Rainbow;
     subtype Int       is Integer;
     subtype Small_Int is Integer range −10 .. 10;
     subtype Up_To_K   is Column range .. K;            −−  see 3.2.1
     subtype Square    is Matrix(1 .. 10, .. 10);       −−  see 3.6
     subtype Male      is Person(Sex => M);               −−  see 3.10.1
     subtype Binop_Ref is not null Binop_Ptr;             −−  see 3.10
Incompatibilities With Ada 83

15.a

{incompatibilities with Ada 83} In Ada 95, all range_constraints cause freezing of their type. Hence, a type−related representation item for a scalar type has to precede any range_constraints whose type is the scalar type.
Wording Changes from Ada 83

15.b

Subtype_marks allow only subtype names now, since types are never directly named. There is no need for RM83−3.3.2(3), which says a subtype_mark can denote both the type and the subtype; in Ada 95, you denote an unconstrained (base) subtype if you want, but never the type.

15.c

The syntactic category type_mark is now called subtype_mark, since it always denotes a subtype.
Extensions to Ada 95

15.d/2

{00231AI95−00231−01} {extensions to Ada 95} An optional null_exclusion can be used in a subtype_indication. This is described in 3.10