3.5.5 Operations of Discrete Types
Static Semantics
For every discrete
subtype S, the following attributes are defined:
S'Pos
S'Pos denotes a function with
the following specification:
function S'Pos(Arg : S'Base)
return universal_integer
This function returns the position number
of the value of Arg, as a value of type universal_integer.
S'Val
S'Val denotes a function with
the following specification:
function S'Val(Arg : universal_integer)
return S'Base
This
function returns a value of the type of S whose position number equals
the value of
Arg.
For
the evaluation of a call on S'Val, if there is no value in the base range
of its type with the given position number, Constraint_Error is raised.
For every static
discrete subtype S for which there exists at least one value belonging
to S that satisfies the predicates of S, the following attributes are
defined:
S'First_Valid
S'First_Valid denotes the smallest
value that belongs to S and satisfies the predicates of S. The value
of this attribute is of the type of S.
S'Last_Valid
S'Last_Valid denotes the largest
value that belongs to S and satisfies the predicates of S. The value
of this attribute is of the type of S.
First_Valid and Last_Valid
attribute_references
are always static expressions. Any explicit predicate of S can only have
been specified by a Static_Predicate aspect.
Implementation Advice
For the evaluation of a call on S'Pos for an enumeration
subtype, if the value of the operand does not correspond to the internal
code for any enumeration literal of its type (perhaps due to an uninitialized
variable), then the implementation should raise Program_Error.
This
is particularly important for enumeration types with noncontiguous internal
codes specified by an
enumeration_representation_clause.
36 Indexing and loop iteration use values
of discrete types.
37
The predefined operations
of a discrete type include the assignment operation, qualification, the
membership tests, and the relational operators; for a boolean type they
include the short-circuit control forms and the logical operators; for
an integer type they include type conversion to and from other numeric
types, as well as the binary and unary adding operators – and +,
the multiplying operators, the unary operator
abs, and the exponentiation
operator. The assignment operation is described in
5.2.
The other predefined operations are described in Clause
4.
38 As for all types, objects of a discrete
type have Size and Address attributes (see
13.3).
39 For a
subtype of a discrete type, the result delivered by the attribute Val
might not belong to the subtype; similarly, the actual parameter of the
attribute Pos need not belong to the subtype. The following relations
are satisfied (in the absence of an exception) by these attributes:
S'Val(S'Pos(X)) = X
S'Pos(S'Val(N)) = N
Examples
Examples of attributes
of discrete subtypes:
--
For the types and subtypes declared in subclause 3.5.1 the following hold: -- Color'First = White, Color'Last = Black
-- Rainbow'First = Red, Rainbow'Last = Blue
-- Color'Succ(Blue) = Rainbow'Succ(Blue) = Brown
-- Color'Pos(Blue) = Rainbow'Pos(Blue) = 4
-- Color'Val(0) = Rainbow'Val(0) = White
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