Ada Reference Manual (Ada 2022)

3.8.1 Variant Parts and Discrete Choices

A record type with a variant_part specifies alternative lists of components. Each variant defines the components for the value or values of the discriminant covered by its discrete_choice_list.

Syntax

variant_part ::=
case discriminant_direct_name is
variant
{variant}
end case;

variant ::=
when discrete_choice_list =>
component_list

discrete_choice_list ::= discrete_choice {'|' discrete_choice}

discrete_choice ::=
choice_expression | discrete_subtype_indication | range | others

Name Resolution Rules

The discriminant_direct_name shall resolve to denote a discriminant (called the discriminant of the variant_part) specified in the known_discriminant_part of the full_type_declaration that contains the variant_part. The expected type for each discrete_choice in a variant is the type of the discriminant of the variant_part.

Legality Rules

The discriminant of the variant_part shall be of a discrete type.

The choice_expressions, subtype_indications, and ranges given as discrete_choices in a variant_part shall be static. The discrete_choice others shall appear alone in a discrete_choice_list, and such a discrete_choice_list, if it appears, shall be the last one in the enclosing construct.

A discrete_choice is defined to cover a value in the following cases:

A discrete_choice that is a choice_expression covers a value if the value equals the value of the choice_expression converted to the expected type.

A discrete_choice that is a subtype_indication covers all values (possibly none) that belong to the subtype and that satisfy the static predicates of the subtype (see 3.2.4).

A discrete_choice that is a range covers all values (possibly none) that belong to the range.

The discrete_choice others covers all values of its expected type that are not covered by previous discrete_choice_lists of the same construct.

A discrete_choice_list covers a value if one of its discrete_choices covers the value.

The possible values of the discriminant of a variant_part shall be covered as follows:

If the discriminant is of a static constrained scalar subtype then, except within an instance of a generic unit, each non-others discrete_choice shall cover only values in that subtype that satisfy its predicates, and each value of that subtype that satisfies its predicates shall be covered by some discrete_choice (either explicitly or by others);

If the type of the discriminant is a descendant of a generic formal scalar type, then the variant_part shall have an others discrete_choice;

Otherwise, each value of the base range of the type of the discriminant shall be covered (either explicitly or by others).

Two distinct discrete_choices of a variant_part shall not cover the same value.

Static Semantics

If the component_list of a variant is specified by null, the variant has no components.

The discriminant of a variant_part is said to govern the variant_part and its variants. In addition, the discriminant of a derived type governs a variant_part and its variants if it corresponds (see 3.7) to the discriminant of the variant_part.

Dynamic Semantics

A record value contains the values of the components of a particular variant only if the value of the discriminant governing the variant is covered by the discrete_choice_list of the variant. This rule applies in turn to any further variant that is, itself, included in the component_list of the given variant.

When an object of a discriminated type T is initialized by default, Constraint_Error is raised if no discrete_choice_list of any variant of a variant_part of T covers the value of the discriminant that governs the variant_part. When a variant_part appears in the component_list of another variant V, this test is only applied if the value of the discriminant governing V is covered by the discrete_choice_list of V.

The elaboration of a variant_part consists of the elaboration of the component_list of each variant in the order in which they appear.

Examples

Example of record type with a variant part:

type Device is (Printer, Disk, Drum);
type State is (Open, Closed);

type Peripheral(Unit : Device := Disk) is
record
Status : State;
case Unit is
when Printer =>
Line_Count : Integer range 1 .. Page_Size;
when others =>
Cylinder : Cylinder_Index;
Track : Track_Number;
end case;
end record;

Examples of record subtypes:

subtype Drum_Unit is Peripheral(Drum);
subtype Disk_Unit is Peripheral(Disk);

Examples of constrained record variables:

Writer : Peripheral(Unit => Printer);
Archive : Disk_Unit;