3.5.1 Enumeration Types
1
[
{enumeration type}
An
enumeration_type_definition
defines an enumeration type.]
Syntax
2
enumeration_type_definition ::=
(
enumeration_literal_specification {,
enumeration_literal_specification})
3
enumeration_literal_specification ::= defining_identifier |
defining_character_literal
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defining_character_literal ::= character_literal
Legality Rules
5
[The defining_identifiers
and defining_character_literals listed in
an enumeration_type_definition shall be distinct.]
5.a
Proof: This is a ramification of the
normal disallowance of homographs explicitly declared immediately in
the same declarative region.
Static Semantics
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{enumeration literal}
Each
enumeration_literal_specification
is the explicit declaration of the corresponding
enumeration literal:
it declares a parameterless function, whose defining name is the
defining_identifier
or
defining_character_literal, and whose result
type is the enumeration type.
6.a
Reason: This rule defines the profile
of the enumeration literal, which is used in the various types of conformance.
6.b
Ramification: The parameterless function
associated with an enumeration literal is fully defined by the enumeration_type_definition;
a body is not permitted for it, and it never fails the Elaboration_Check
when called.
7
Each enumeration literal corresponds to a distinct
value of the enumeration type, and to a distinct position number.
{position
number (of an enumeration value) [partial]} The
position number of the value of the first listed enumeration literal
is zero; the position number of the value of each subsequent enumeration
literal is one more than that of its predecessor in the list.
8
[The predefined order relations between values of
the enumeration type follow the order of corresponding position numbers.]
9
[
{overloaded (enumeration
literal) [partial]} If the same
defining_identifier
or
defining_character_literal is specified
in more than one
enumeration_type_definition,
the corresponding enumeration literals are said to be
overloaded.
At any place where an overloaded enumeration literal occurs in the text
of a program, the type of the enumeration literal has to be determinable
from the context (see
8.6).]
Dynamic Semantics
10
{elaboration (enumeration_type_definition)
[partial]} {constrained
(subtype)} {unconstrained
(subtype)} The elaboration of an
enumeration_type_definition
creates the enumeration type and its first subtype, which is constrained
to the base range of the type.
10.a
Ramification: The first subtype of a
discrete type is always constrained, except in the case of a derived
type whose parent subtype is Whatever'Base.
11
When called, the parameterless function associated
with an enumeration literal returns the corresponding value of the enumeration
type.
12
24 If an enumeration literal occurs in
a context that does not otherwise suffice to determine the type of the
literal, then qualification by the name of the enumeration type is one
way to resolve the ambiguity (see
4.7).
Examples
13
Examples of enumeration
types and subtypes:
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type Day is (Mon, Tue, Wed, Thu, Fri, Sat, Sun);
type Suit is (Clubs, Diamonds, Hearts, Spades);
type Gender is (M, F);
type Level is (Low, Medium, Urgent);
type Color is (White, Red, Yellow, Green, Blue, Brown, Black);
type Light is (Red, Amber, Green); -- Red and Green are overloaded
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type Hexa is ('A', 'B', 'C', 'D', 'E', 'F');
type Mixed is ('A', 'B', '*', B, None, '?', '%');
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subtype Weekday is Day range Mon .. Fri;
subtype Major is Suit range Hearts .. Spades;
subtype Rainbow is Color range Red .. Blue; -- the Color Red, not the Light
Wording Changes from Ada 83
16.a
The syntax rule for defining_character_literal
is new. It is used for the defining occurrence of a character_literal,
analogously to defining_identifier. Usage
occurrences use the name or selector_name
syntactic categories.
16.b
We emphasize the fact that an enumeration literal
denotes a function, which is called to produce a value.
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