3.6.1 Index Constraints and Discrete Ranges
1
An index_constraint determines
the range of possible values for every index of an array subtype, and
thereby the corresponding array bounds.
Syntax
2
index_constraint ::= (
discrete_range {,
discrete_range})
3
discrete_range ::= discrete_subtype_indication |
range
Name Resolution Rules
4
The type of a
discrete_range
is the type of the subtype defined by the
subtype_indication,
or the type of the
range.
For
an
index_constraint, each
discrete_range
shall resolve to be of the type of the corresponding index.
Legality Rules
5
An index_constraint shall
appear only in a subtype_indication whose
subtype_mark denotes either an unconstrained
array subtype, or an unconstrained access subtype whose designated subtype
is an unconstrained array subtype; in either case, the index_constraint
shall provide a discrete_range for each index
of the array type.
Static Semantics
6
A
discrete_range
defines a range whose bounds are given by the
range,
or by the range of the subtype defined by the
subtype_indication.
Dynamic Semantics
7
An
index_constraint
is
compatible with an unconstrained array subtype if and only
if the index range defined by each
discrete_range
is compatible (see
3.5) with the corresponding
index subtype.
If any of the
discrete_ranges
defines a null range, any array thus constrained is a
null array,
having no components.
An array value
satisfies
an
index_constraint if at each index position
the array value and the
index_constraint have
the same index bounds.
8
The elaboration of an
index_constraint
consists of the evaluation of the
discrete_range(s),
in an arbitrary order.
The evaluation of a
discrete_range
consists of the elaboration of the
subtype_indication
or the evaluation of the
range.
9
45 The elaboration of a
subtype_indication
consisting of a
subtype_mark followed by an
index_constraint checks the compatibility
of the
index_constraint with the
subtype_mark
(see
3.2.2).
10
46 Even if an array value does not satisfy
the index constraint of an array subtype, Constraint_Error is not raised
on conversion to the array subtype, so long as the length of each dimension
of the array value and the array subtype match. See
4.6.
Examples
11
Examples of array
declarations including an index constraint:
12
Board : Matrix(1 .. 8, 1 .. 8); --
see 3.6
Rectangle : Matrix(1 .. 20, 1 .. 30);
Inverse : Matrix(1 .. N, 1 .. N); --
N need not be static 13
Filter : Bit_Vector(0 .. 31);
14
Example of array declaration
with a constrained array subtype:
15
My_Schedule : Schedule; -- all arrays of type Schedule have the same bounds
16
Example of record
type with a component that is an array:
17
type Var_Line(Length : Natural) is
record
Image : String(1 .. Length);
end record;
18
Null_Line : Var_Line(0); -- Null_Line.Image is a null array
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