3.5.5 Operations of Discrete Types
Static Semantics
1
For every discrete
subtype S, the following attributes are defined:
2
S'Pos
S'Pos denotes a function with
the following specification:
3
function S'Pos(Arg : S'Base)
return universal_integer
4
This function returns the position number
of the value of Arg, as a value of type universal_integer.
5
S'Val
S'Val denotes a function with
the following specification:
6
function S'Val(Arg : universal_integer)
return S'Base
7
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.
7.a
Ramification: By the overload resolution
rules, a formal parameter of type universal_integer allows an
actual parameter of any integer type.
7.b
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
8
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.
8.a.1/2
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.
8.a
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.)
9
33 Indexing and loop iteration use values
of discrete types.
10
34
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.
11
35 As for all types, objects of a discrete
type have Size and Address attributes (see
13.3).
12
36 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:
13
S'Val(S'Pos(X)) = X
S'Pos(S'Val(N)) = N
Examples
14
Examples of attributes
of discrete subtypes:
15
--
For the types and subtypes declared in subclause 3.5.1 the following hold: 16
-- Color'First = White, Color'Last = Black
-- Rainbow'First = Red, Rainbow'Last = Blue
17
-- 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
17.a
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”).
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
