3.2.2 Subtype Declarations
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A subtype_declaration
declares a subtype of some previously declared type, as defined by a
subtype_indication.
Syntax
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subtype_declaration ::=
subtype defining_identifier is subtype_indication;
3/2
subtype_indication ::= [
null_exclusion]
subtype_mark [
constraint]
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subtype_mark ::= subtype_name
5
constraint ::= scalar_constraint |
composite_constraint
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scalar_constraint ::=
range_constraint |
digits_constraint |
delta_constraint
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composite_constraint ::=
index_constraint |
discriminant_constraint
Name Resolution Rules
8
A
subtype_mark shall resolve
to denote a subtype.
The type
determined by
a
subtype_mark is the type of the subtype
denoted by the
subtype_mark.
Dynamic Semantics
9
The
elaboration of a
subtype_declaration consists
of the elaboration of the
subtype_indication.
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:
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- The constraint
is first elaborated.
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- A
check is then made that the constraint is
compatible with the subtype denoted by the subtype_mark.
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The condition imposed by a
constraint
is the condition obtained after elaboration of the
constraint.
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.
The
exception Constraint_Error is raised if any check of compatibility fails.
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4 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
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Examples of subtype
declarations:
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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 1 .. K; --
see 3.2.1
subtype Square
is Matrix(1 .. 10, 1 .. 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
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