3.5 Scalar Types

Discussion: These need to be simple_expressions rather than more general expressions because ranges appear in membership tests and other contexts where expression .. expression would be ambiguous. 

Discussion: In Ada 95, constraints only appear within subtype_indications; things that look like constraints that appear in type declarations are called something else like real_range_specifications.

{AI05-0299-1} We say "the expected type is ..." or "the type is expected to be ..." depending on which reads better. They are fundamentally equivalent, and both feed into the type resolution rules of subclause 8.6.

In some cases, it doesn't work to use expected types. For example, in the above rule, we say that the “type of the range shall resolve to ...” rather than “the expected type for the range is ...”. We then use “expected type” for the bounds. If we used “expected” at both points, there would be an ambiguity, since one could apply the rules of 8.6 either on determining the type of the range, or on determining the types of the individual bounds. It is clearly important to allow one bound to be of a universal type, and the other of a specific type, so we need to use “expected type” for the bounds. Hence, we used “shall resolve to” for the type of the range as a whole. There are other situations where “expected type” is not quite right, and we use “shall resolve to” instead. 

Implementation Note: Note that in some machine architectures intermediates in an expression (particularly if static), and register-resident variables might accommodate a wider range. The base range does not include the values of this wider range that are not assignable without overflow to memory-resident objects.

Ramification: The base range of an enumeration type is the range of values of the enumeration type. 

Reason: If the representation supports infinities, the base range is nevertheless restricted to include only the representable finite values, so that 'Base'First and 'Base'Last are always guaranteed to be finite.

To be honest: By a "value that can be assigned without overflow" we don't mean to restrict ourselves to values that can be represented exactly. Values between machine representable values can be assigned, but on subsequent reading, a slightly different value might be retrieved, as (partially) determined by the number of digits of precision of the type. 

Ramification: Only range_constraints (explicit or implicit) impose conditions on the values of a scalar subtype. The other scalar_constraints, digits_constraints and delta_constraints impose conditions on the subtype denoted by the subtype_mark in a subtype_indication, but don't impose a condition on the values of the subtype being defined. Therefore, a scalar subtype is not called constrained if all that applies to it is a digits_constraint. Decimal subtypes are subtle, because a digits_constraint without a range_constraint nevertheless includes an implicit range_constraint.

Ramification: Evaluating S'First never raises Constraint_Error.

Ramification: Evaluating S'Last never raises Constraint_Error.

Discussion: The formal parameter names are italicized because they cannot be used in calls — see 6.4. Such a specification cannot be written by the user because an attribute_reference is not permitted as the designator of a user-defined function, nor can its formal parameters be anonymous. 

Ramification: S'Succ for a modular integer subtype wraps around if the value of Arg is S'Base'Last. S'Succ for a signed integer subtype might raise Constraint_Error if the value of Arg is S'Base'Last, or it might return the out-of-base-range value S'Base'Last+1, as is permitted for all predefined numeric operations.

Ramification: S'Pred for a modular integer subtype wraps around if the value of Arg is S'Base'First. S'Pred for a signed integer subtype might raise Constraint_Error if the value of Arg is S'Base'First, or it might return the out-of-base-range value S'Base'First–1, as is permitted for all predefined numeric operations.

Implementation Note: If the machine supports negative zeros for signed integer types, it is not specified whether " 0" or "–0" should be returned for negative zero. We don't have enough experience with such machines to know what is appropriate, and what other languages do. In any case, the implementation should be consistent. 

Implementation Note: For an enumeration type T that has “holes” (caused by an enumeration_representation_clause), T'Wide_Image should raise Program_Error if the value is one of the holes (which is a bounded error anyway, since holes can be generated only via uninitialized variables and similar things). 

To be honest: Leading zeros are present in the exponent only if necessary to make the exponent at least two digits.

Reason: This image is intended to conform to that produced by Text_IO.Float_IO.Put in its default format. 

Implementation Note: The rounding direction is specified here to ensure portability of output results. 

Reason: This image is intended to conform to that produced by Text_IO.Fixed_IO.Put. 

Implementation Note: The rounding direction is specified here to ensure portability of output results. 

Implementation Note: For a machine that supports negative zeros, it is not specified whether " 0.000" or "–0.000" is returned. See corresponding comment above about integer types with signed zeros. 

Implementation defined: The sequence of characters of the value returned by S'Wide_Image when some of the graphic characters of S'Wide_Wide_Image are not defined in Wide_Character.

Implementation defined: The sequence of characters of the value returned by S'Image when some of the graphic characters of S'Wide_Wide_Image are not defined in Character.

Discussion: It's not crystal clear that Range_Check is appropriate here, but it doesn't seem worthwhile to invent a whole new check name just for this weird case, so we decided to lump it in with Range_Check. 

To be honest: {8652/0096} {AI95-00053-01} A sequence of characters corresponds to the result of S'Wide_Wide_Image if it is the same ignoring case. Thus, the case of an image of a nongraphic character does not matter. For example, Character'Wide_Wide_Value("nul") does not raise Constraint_Error, even though Character'Wide_Wide_Image returns "NUL" for the nul character. 

Discussion: We considered allowing 'Value to return a representable but out-of-range value without a Constraint_Error. However, we currently require (see 4.9) in an assignment_statement like "X := <numeric_literal>;" that the value of the numeric-literal be in X's base range (at compile time), so it seems unfriendly and confusing to have a different range allowed for 'Value. Furthermore, for modular types, without the requirement for being in the base range, 'Value would have to handle arbitrarily long literals (since overflow never occurs for modular types). 

This paragraph was deleted.{8652/0096} {AI95-00053-01}

Reason: S'Wide_Value is subtly different from S'Wide_Wide_Value for enumeration subtypes since S'Wide_Image might produce a different sequence of characters than S'Wide_Wide_Image if the enumeration literal uses characters outside of the predefined type Wide_Character. That is why we don't just define S'Wide_Value in terms of S'Wide_Wide_Value for enumeration subtypes. S'Wide_Value and S'Wide_Wide_Value for numeric subtypes yield the same result given the same sequence of characters. 

Reason: {AI95-00285-01} S'Value is subtly different from S'Wide_Wide_Value for enumeration subtypes; see the discussion under S'Wide_Value. 

Proof: {AI95-00285-01} The permission is really only necessary for Wide_Wide_Value, because Value and Wide_Value are defined in terms of Wide_Wide_Value, and because the behavior of Wide_Wide_Image, Wide_Image, and Image is already unspecified for things like infinities and NaNs. 

Reason: This is to allow implementations to define full support for IEEE arithmetic. See also the similar permission for Get in A.10.9. 

Reason: The part about requiring an explicit expression is to disallow omitting the value for this aspect, which would otherwise be allowed by the rules of 13.1.1.

This is a representation aspect in order to disallow specifying it on a derived type that has inherited primitive subprograms; that is necessary as the sizes of out parameters could be different whether or not a Default_Value is specified (see 6.4.1).

Aspect Description for Default_Value: Default value for a scalar subtype.

Reason: This overrides the 13.1.1 rule that says that a boolean aspect with a value True cannot be changed.

Discussion: This paragraph addresses an issue that came up with Ada 83, where for fixed point types, the end points of the range specified in the type definition were not necessarily within the base range of the type. However, it was later clarified (and we reconfirm it in 3.5.9, “Fixed Point Types”) that the First and Last attributes reflect the true bounds chosen for the type, not the bounds specified in the type definition (which might be outside the ultimately chosen base range).

S'Base is no longer defined for nonscalar types. One conceivable existing use of S'Base for nonscalar types is S'Base'Size where S is a generic formal private type. However, that is not generally useful because the actual subtype corresponding to S might be a constrained array or discriminated type, which would mean that S'Base'Size might very well overflow (for example, S'Base'Size where S is a constrained subtype of String will generally be 8 * (Integer'Last + 1)). For derived discriminated types that are packed, S'Base'Size might not even be well defined if the first subtype is constrained, thereby allowing some amount of normally required “dope” to have been squeezed out in the packing. Hence our conclusion is that S'Base'Size is not generally useful in a generic, and does not justify keeping the attribute Base for nonscalar types just so it can be used as a prefix.

The attribute S'Base for a scalar subtype is now permitted anywhere a subtype_mark is permitted. S'Base'First .. S'Base'Last is the base range of the type. Using an attribute_definition_clause, one cannot specify any subtype-specific attributes for the subtype denoted by S'Base (the base subtype).

The attribute S'Range is now allowed for scalar subtypes.

The attributes S'Min and S'Max are now defined, and made available for all scalar types.

The attributes S'Succ, S'Pred, S'Image, S'Value, and S'Width are now defined for real types as well as discrete types.

Wide_String versions of S'Image and S'Value are defined. These are called S'Wide_Image and S'Wide_Value to avoid introducing ambiguities involving uses of these attributes with string literals. 

We now use the syntactic category range_attribute_reference since it is now syntactically distinguished from other attribute references.

The definition of S'Base has been moved here from 3.3.3 since it now applies only to scalar types.

More explicit rules are provided for nongraphic characters. 

{AI95-00285-01} The attributes Wide_Wide_Image, Wide_Wide_Value, and Wide_Wide_Width are new. Note that Wide_Image and Wide_Value are now defined in terms of Wide_Wide_Image and Wide_Wide_Value, but the image of types other than characters have not changed. 

{AI95-00285-01} The Wide_Image and Wide_Value attributes are now defined in terms of Wide_Wide_Image and Wide_Wide_Value, but the images of numeric types have not changed. 

{AI05-0181-1} Correction: Soft hyphen (code point 173) is nongraphic in ISO/IEC 10646:2011 (and also in the 2003 version of that standard). Thus, we have given it the language-defined name soft_hyphen. This changes the result of Character'Image (and all of the related types and Image attributes) for this character, and changes the behavior of Character'Value (and all of the related types and Value attributes) for this character, and (in unusual circumstances), changes the result for Character'Width (and all of the related types and Width attributes). The vast majority of programs won't see any difference, as they are already prepared to handle nongraphic characters.

{AI05-0182-1} Correction: Added an Implementation Permissions to let Wide_Wide_Value, Wide_Value, and Value accept strings in the form of literals containing nongraphic characters and "Hex_hhhhhhhh" for Latin-1 and graphic characters. These were required to raise Constraint_Error in Ada 2005. Since these attributes aren't very useful, implementations were inconsistent as to whether these were accepted, and since code that would care why the attribute failed seems unlikely, this should not be a problem in practice. 

{AI05-0228-1} The new aspect Default_Value allows defining implicit initial values (see 3.3.1) for scalar types.

{AI12-0124-1} Corrigendum: An object can be now used as the prefix of the Image attribute (as well as Wide_Image and Wide_Wide_Image), a convenience feature already present in some implementations.