3.6.1 Index Constraints and Discrete Ranges

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An index_constraint determines the range of possible values for every index of an array subtype, and thereby the corresponding array bounds.


index_constraint ::=  (discrete_range {, discrete_range})

discrete_range ::= discrete_subtype_indication | range

Name Resolution Rules

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

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

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

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.

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.


43  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).

44  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 of array declarations including an index constraint:

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 

Filter    : Bit_Vector(0 .. 31);

Example of array declaration with a constrained array subtype:

My_Schedule : Schedule;  --  all arrays of type Schedule have the same bounds

Example of record type with a component that is an array:

type Var_Line(Length Natural) is 
       Image : String(1 .. Length); 
   end record;

Null_Line : Var_Line(0);  --  Null_Line.Image is a null array

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