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.
1.a
Discussion: {
cover a value [distributed]}
Discrete_choice_lists
and
discrete_choices are said to
cover
values as defined below; which
discrete_choice_list
covers a value determines which of various alternatives is chosen. These
are used in
variant_parts,
array_aggregates,
and
case_statements.
Language Design Principles
1.b
The definition of “cover” in this
subclause and the rules about discrete choices are designed so that they
are also appropriate for array aggregates and case statements.
1.c
The rules of this subclause intentionally parallel
those for case statements.
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
{discriminant (of
a variant_part)} 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.
{expected
type (variant_part discrete_choice) [partial]} The
expected type for each
discrete_choice in
a
variant is the type of the discriminant
of the
variant_part.
6.a
Ramification: A full_type_declaration
with a variant_part has to have a (new) known_discriminant_part;
the discriminant of the variant_part cannot
be an inherited discriminant.
Legality Rules
7
The discriminant of the variant_part
shall be of a discrete type.
7.a
Ramification: It shall not be of an access
type, named or anonymous.
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
{cover
a value (by a discrete_choice) [partial]} A
discrete_choice is defined to
cover a value
in the following cases:
10
- A discrete_choice
that is an expression covers a value if the
value equals the value of the expression converted
to the expected type.
11
- A discrete_choice
that is a discrete_range covers all values
(possibly none) that belong to the range.
12
- The discrete_choice
others covers all values of its expected type that are not covered
by previous discrete_choice_lists of the same
construct.
12.a
Ramification: For case_statements,
this includes values outside the range of the static subtype (if any)
to be covered by the choices. It even includes values outside the base
range of the case expression's type, since values of numeric types (and
undefined values of any scalar type?) can be outside their base range.
13
{cover a value (by
a discrete_choice_list) [partial]} 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
- If the discriminant is of a static
constrained scalar subtype, then each non-others discrete_choice
shall cover only values in that subtype, and each value of that subtype
shall be covered by some discrete_choice [(either
explicitly or by others)];
16
- 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;
16.a
Reason: The base range is not known statically
in this case.
17
- Otherwise, each value of the base
range of the type of the discriminant shall be covered [(either explicitly
or by others)].
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
{govern a variant_part}
{govern a variant}
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
{elaboration (variant_part)
[partial]} 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;
Extensions to Ada 83
29.a
{
extensions to Ada 83}
In
Ada 83, the discriminant of a
variant_part
is not allowed to be of a generic formal type. This restriction is removed
in Ada 95; an
others discrete_choice
is required in this case.
Wording Changes from Ada 83
29.b
The syntactic category choice
is removed. The syntax rules for variant,
array_aggregate, and case_statement
now use discrete_choice_list or discrete_choice
instead. The syntax rule for record_aggregate
now defines its own syntax for named associations.
29.c
We have added the term Discrete Choice to the
title since this is where they are talked about. This is analogous to
the name of the subclause "Index Constraints and Discrete Ranges"
in the clause on Array Types.
29.d
The rule requiring that the discriminant denote
a discriminant of the type being defined seems to have been left implicit
in RM83.
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