Ada Resource Association
News and resources for the Ada programming language
Ada Reference ManualLegal Information
Contents   Index   References   Search   Previous   Next 

3.8.1 Variant Parts and Discrete Choices

1
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

2
variant_part ::= 
   case discriminant_direct_name is
       variant
      {variant}
   end case;
3
variant ::= 
   when discrete_choice_list =>
      component_list
4
discrete_choice_list ::= discrete_choice {| discrete_choice}
5
discrete_choice ::= expression | discrete_range | others

Name Resolution Rules

6
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

7
The discriminant of the variant_part shall be of a discrete type. 
8
The expressions and discrete_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.
9
A discrete_choice is defined to cover a value in the following cases: 
10
11
12
13
A discrete_choice_list covers a value if one of its discrete_choices covers the value.
14
The possible values of the discriminant of a variant_part shall be covered as follows: 
15
16
17
18
Two distinct discrete_choices of a variant_part shall not cover the same value.

Static Semantics

19
If the component_list of a variant is specified by null, the variant has no components.
20
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

21
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.
22
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

23
Example of record type with a variant part: 
24
type Device is (Printer, Disk, Drum);
type State  is (Open, Closed);
25
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;
26
Examples of record subtypes: 
27
subtype Drum_Unit is Peripheral(Drum);
subtype Disk_Unit is Peripheral(Disk);
28
Examples of constrained record variables: 
29
Writer   : Peripheral(Unit  => Printer);
Archive  : Disk_Unit;

Contents   Index   References   Search   Previous   Next 
Ada-Europe Sponsored by Ada-Europe