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
{evaluation
(Val) [partial]} {Constraint_Error
(raised by failure of run-time check)} This
function returns a value of the type of S whose position number equals
the value of
Arg.
{Range_Check
[partial]} {check,
language-defined (Range_Check)} 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.
Ramification: By the overload resolution
rules, a formal parameter of type universal_integer allows an
actual parameter of any integer type.
Reason: We considered allowing S'Val
for a signed integer subtype S to return an out-of-range value, but since
checks were required for enumeration and modular types anyway, the allowance
didn't seem worth the complexity of the rule.
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.
{Program_Error
(raised by failure of run-time check)} This
is particularly important for enumeration types with noncontiguous internal
codes specified by an
enumeration_representation_clause.
Implementation Advice: Program_Error
should be raised for the evaluation of S'Pos for an enumeration type,
if the value of the operand does not correspond to the internal code
for any enumeration literal of the type.
Reason: We say Program_Error here, rather
than Constraint_Error, because the main reason for such values is uninitialized
variables, and the normal way to indicate such a use (if detected) is
to raise Program_Error. (Other reasons would involve the misuse of low-level
features such as Unchecked_Conversion.)
30 Indexing and loop iteration use values
of discrete types.
31
{predefined
operations (of a discrete type) [partial]} 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 Section 4.
32 As for all types, objects of a discrete
type have Size and Address attributes (see
13.3).
33 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
Extensions to Ada 83
{
extensions to Ada 83}
The
attributes S'Succ, S'Pred, S'Width, S'Image, and S'Value have been generalized
to apply to real types as well (see
3.5, “
Scalar
Types”).
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