Ada Reference Manual (Ada 2022) — Legal Information

# 3.5 Scalar Types

1
Scalar types comprise enumeration types, integer types, and real types. Enumeration types and integer types are called discrete types; each value of a discrete type has a position number which is an integer value. Integer types and real types are called numeric types. All scalar types are ordered, that is, all relational operators are predefined for their values.

#### Syntax

2
range_constraint ::= range range
3
range ::= range_attribute_reference
| simple_expression .. simple_expression
4
A range has a lower bound and an upper bound and specifies a subset of the values of some scalar type (the type of the range). A range with lower bound L and upper bound R is described by “L .. R”. If R is less than L, then the range is a null range, and specifies an empty set of values. Otherwise, the range specifies the values of the type from the lower bound to the upper bound, inclusive. A value belongs to a range if it is of the type of the range, and is in the subset of values specified by the range. A value satisfies a range constraint if it belongs to the associated range. One range is included in another if all values that belong to the first range also belong to the second.

#### Name Resolution Rules

5
For a subtype_indication containing a range_constraint, either directly or as part of some other scalar_constraint, the type of the range shall resolve to that of the type determined by the subtype_mark of the subtype_indication. For a range of a given type, the simple_expressions of the range (likewise, the simple_expressions of the equivalent range for a range_attribute_reference) are expected to be of the type of the range.

#### Static Semantics

6
The base range of a scalar type is the range of finite values of the type that can be represented in every unconstrained object of the type; it is also the range supported at a minimum for intermediate values during the evaluation of expressions involving predefined operators of the type.
7
A constrained scalar subtype is one to which a range constraint applies. The range of a constrained scalar subtype is the range associated with the range constraint of the subtype. The range of an unconstrained scalar subtype is the base range of its type.

#### Dynamic Semantics

8
A range is compatible with a scalar subtype if and only if it is either a null range or each bound of the range belongs to the range of the subtype. A range_constraint is compatible with a scalar subtype if and only if its range is compatible with the subtype.
9
The elaboration of a range_constraint consists of the evaluation of the range. The evaluation of a range determines a lower bound and an upper bound. If simple_expressions are given to specify bounds, the evaluation of the range evaluates these simple_expressions in an arbitrary order, and converts them to the type of the range. If a range_attribute_reference is given, the evaluation of the range consists of the evaluation of the range_attribute_reference.
10
Attributes
11
For every scalar subtype S, the following attributes are defined:
12
S'First
S'First denotes the lower bound of the range of S. The value of this attribute is of the type of S.
13
S'Last
S'Last denotes the upper bound of the range of S. The value of this attribute is of the type of S.
14
S'Range
S'Range is equivalent to the range S'First .. S'Last.
15
S'Base
S'Base denotes an unconstrained subtype of the type of S. This unconstrained subtype is called the base subtype of the type.
16
S'Min
S'Min denotes a function with the following specification:
17
function S'Min(Left, Right : S'Base)
return S'Base
18
The function returns the lesser of the values of the two parameters.
19
S'Max
S'Max denotes a function with the following specification:
20
function S'Max(Left, Right : S'Base)
return S'Base
21
The function returns the greater of the values of the two parameters.
22
S'Succ
S'Succ denotes a function with the following specification:
23
function S'Succ(Arg : S'Base)
return S'Base
24
For an enumeration type, the function returns the value whose position number is one more than that of the value of Arg; Constraint_Error is raised if there is no such value of the type. For an integer type, the function returns the result of adding one to the value of Arg. For a fixed point type, the function returns the result of adding small to the value of Arg. For a floating point type, the function returns the machine number (as defined in 3.5.7) immediately above the value of Arg; Constraint_Error is raised if there is no such machine number.
25
S'Pred
S'Pred denotes a function with the following specification:
26
function S'Pred(Arg : S'Base)
return S'Base
27
For an enumeration type, the function returns the value whose position number is one less than that of the value of Arg; Constraint_Error is raised if there is no such value of the type. For an integer type, the function returns the result of subtracting one from the value of Arg. For a fixed point type, the function returns the result of subtracting small from the value of Arg. For a floating point type, the function returns the machine number (as defined in 3.5.7) immediately below the value of Arg; Constraint_Error is raised if there is no such machine number.
Paragraphs 28 through 37 were moved to 4.10, “Image Attributes”.
37.1/5
S'Wide_Wide_Width
S'Wide_Wide_Width denotes the maximum length of a Wide_Wide_String returned by S'Wide_Wide_Image over all values of the subtype S, assuming a default implementation of S'Put_Image. It denotes zero for a subtype that has a null range. Its type is universal_integer.
38/5
S'Wide_Width
S'Wide_Width denotes the maximum length of a Wide_String returned by S'Wide_Image over all values of the subtype S, assuming a default implementation of S'Put_Image. It denotes zero for a subtype that has a null range. Its type is universal_integer.
39/5
S'Width
S'Width denotes the maximum length of a String returned by S'Image over all values of the subtype S, assuming a default implementation of S'Put_Image. It denotes zero for a subtype that has a null range. Its type is universal_integer.
39.1/2
S'Wide_Wide_Value
S'Wide_Wide_Value denotes a function with the following specification:
39.2/2
function S'Wide_Wide_Value(Arg : Wide_Wide_String)
return S'Base
39.3/2
This function returns a value given an image of the value as a Wide_Wide_String, ignoring any leading or trailing spaces.
39.4/3
For the evaluation of a call on S'Wide_Wide_Value for an enumeration subtype S, if the sequence of characters of the parameter (ignoring leading and trailing spaces) has the syntax of an enumeration literal and if it corresponds to a literal of the type of S (or corresponds to the result of S'Wide_Wide_Image for a nongraphic character of the type), the result is the corresponding enumeration value; otherwise, Constraint_Error is raised.
39.5/3
For the evaluation of a call on S'Wide_Wide_Value for an integer subtype S, if the sequence of characters of the parameter (ignoring leading and trailing spaces) has the syntax of an integer literal, with an optional leading sign character (plus or minus for a signed type; only plus for a modular type), and the corresponding numeric value belongs to the base range of the type of S, then that value is the result; otherwise, Constraint_Error is raised.
39.6/2
For the evaluation of a call on S'Wide_Wide_Value for a real subtype S, if the sequence of characters of the parameter (ignoring leading and trailing spaces) has the syntax of one of the following:
39.7/2
numeric_literal
39.8/2
numeral.[exponent]
39.9/2
.numeral[exponent]
39.10/2
base#based_numeral.#[exponent]
39.11/2
base#.based_numeral#[exponent]
39.12/3
with an optional leading sign character (plus or minus), and if the corresponding numeric value belongs to the base range of the type of S, then that value is the result; otherwise, Constraint_Error is raised. The sign of a zero value is preserved (positive if none has been specified) if S'Signed_Zeros is True.
40
S'Wide_Value
S'Wide_Value denotes a function with the following specification:
41
function S'Wide_Value(Arg : Wide_String)
return S'Base
42
This function returns a value given an image of the value as a Wide_String, ignoring any leading or trailing spaces.
43/5
For the evaluation of a call on S'Wide_Value for an enumeration subtype S, if the sequence of characters of the parameter (ignoring leading and trailing spaces) has the syntax of an enumeration literal and if it corresponds to a literal of the type of S (or corresponds to the result of S'Wide_Image for a value of the type, assuming a default implementation of S'Put_Image), the result is the corresponding enumeration value; otherwise, Constraint_Error is raised. For a numeric subtype S, the evaluation of a call on S'Wide_Value with Arg of type Wide_String is equivalent to a call on S'Wide_Wide_Value for a corresponding Arg of type Wide_Wide_String.
Paragraphs 44 through 51 were moved to Wide_Wide_Value.
52
S'Value
S'Value denotes a function with the following specification:
53
function S'Value(Arg : String)
return S'Base
54
This function returns a value given an image of the value as a String, ignoring any leading or trailing spaces.
55/5
For the evaluation of a call on S'Value for an enumeration subtype S, if the sequence of characters of the parameter (ignoring leading and trailing spaces) has the syntax of an enumeration literal and if it corresponds to a literal of the type of S (or corresponds to the result of S'Image for a value of the type, assuming a default implementation of S'Put_Image), the result is the corresponding enumeration value; otherwise, Constraint_Error is raised. For a numeric subtype S, the evaluation of a call on S'Value with Arg of type String is equivalent to a call on S'Wide_Wide_Value for a corresponding Arg of type Wide_Wide_String.

#### Implementation Permissions

56/2
An implementation may extend the Wide_Wide_Value, Wide_Value, Value, Wide_Wide_Image, Wide_Image, and Image attributes of a floating point type to support special values such as infinities and NaNs.
56.1/3
An implementation may extend the Wide_Wide_Value, Wide_Value, and Value attributes of a character type to accept strings of the form “Hex_hhhhhhhh” (ignoring case) for any character (not just the ones for which Wide_Wide_Image would produce that form — see 3.5.2), as well as three-character strings of the form “'X'”, where X is any character, including nongraphic characters.

#### Static Semantics

56.2/3
For a scalar type, the following language-defined representation aspect may be specified with an aspect_specification (see 13.1.1):
56.3/3
Default_Value
This aspect shall be specified by a static expression, and that expression shall be explicit, even if the aspect has a boolean type. Default_Value shall be specified only on a full_type_declaration.
56.4/5
If a derived type inherits a boolean Default_Value aspect, the aspect may be specified to have any value for the derived type. If a derived type T does not inherit a Default_Value aspect, it shall not specify such an aspect if it inherits a primitive subprogram that has a parameter of type T of mode out.

#### Name Resolution Rules

56.5/3
The expected type for the expression specified for the Default_Value aspect is the type defined by the full_type_declaration on which it appears.
57
NOTE 1 The evaluation of S'First or S'Last never raises an exception. If a scalar subtype S has a nonnull range, S'First and S'Last belong to this range. These values can, for example, always be assigned to a variable of subtype S.
58/5
NOTE 2 For a subtype of a scalar type, the result delivered by the attributes Succ, Pred, and Value can be outside to the subtype; similarly, the actual parameters of the attributes Succ, Pred, and Image can also be outside the subtype.
59/5
NOTE 3 For any value V (including any nongraphic character) of an enumeration subtype S without a specified Put_Image (see 4.10), S'Value(S'Image(V)) equals V, as do S'Wide_Value(S'Wide_Image(V)) and S'Wide_Wide_Value(S'Wide_Wide_Image(V)). None of these expressions ever raise Constraint_Error.

#### Examples

60
Examples of ranges:
61
-10 .. 10
X .. X + 1
0.0 .. 2.0*Pi
Red .. Green -- see 3.5.1
1 .. 0 -- a null range
Table'Range -- a range attribute reference (see 3.6)
62
Examples of range constraints:
63
range -999.0 .. +999.0
range S'First+1 .. S'Last-1

Ada 2005 and 2012 Editions sponsored in part by Ada-Europe