Ada Reference Manual (Ada 2022) — Legal Information

# 3.8.1 Variant Parts and Discrete Choices

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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

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variant_part ::=
case discriminant_direct_name is
variant
{variant}
end case;
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variant ::=
when discrete_choice_list =>
component_list
4/5
discrete_choice_list ::= discrete_choice {'|' discrete_choice}
5/3
discrete_choice ::=
choice_expression | discrete_subtype_indication | range | others

#### Name Resolution Rules

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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

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The discriminant of the variant_part shall be of a discrete type.
8/3
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.
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A discrete_choice is defined to cover a value in the following cases:
10/3
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.
10.1/4
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).
11/3
A discrete_choice that is a range covers all values (possibly none) that belong to the range.
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The discrete_choice others covers all values of its expected type that are not covered by previous discrete_choice_lists of the same construct.
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A discrete_choice_list covers a value if one of its discrete_choices covers the value.
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The possible values of the discriminant of a variant_part shall be covered as follows:
15/4
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);
16/3
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;
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Otherwise, each value of the base range of the type of the discriminant shall be covered (either explicitly or by others).
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Two distinct discrete_choices of a variant_part shall not cover the same value.

#### Static Semantics

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

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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.
21.1/3
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.
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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

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Example of record type with a variant part:
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type Device is (Printer, Disk, Drum);
type State is (Open, Closed);
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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;
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Examples of record subtypes:
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subtype Drum_Unit is Peripheral(Drum);
subtype Disk_Unit is Peripheral(Disk);
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Examples of constrained record variables:
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Writer : Peripheral(Unit => Printer);
Archive : Disk_Unit;

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