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3.5.2 Character Types

Static Semantics

1
An enumeration type is said to be a character type if at least one of its enumeration literals is a character_literal.
2/5
{AI95-00285-01} {AI05-0181-1} {AI05-0262-1} {AI05-0266-1} {AI12-0263-1} The predefined type Character is a character type whose values correspond to the 256 code points of Row 00 (also known as Latin-1) of the ISO/IEC 10646:2017 2011 Basic Multilingual Plane (BMP). Each of the graphic characters of Row 00 of the BMP has a corresponding character_literal in Character. Each of the nongraphic characters of Row 00 has a corresponding language-defined name, which is not usable as an enumeration literal, but which is usable with the attributes Image, Wide_Image, Wide_Wide_Image, Value, Wide_Value, and Wide_Wide_Value; these names are given in the definition of type Character in A.1, “The Package Standard”, but are set in italics.
2.a/5
Discussion: {AI05-0262-1} {AI12-0263-1} Code point is defined in ISO/IEC 10646:2017 2011.
3/5
{AI95-00285-01} {AI05-0262-1} {AI12-0263-1} The predefined type Wide_Character is a character type whose values correspond to the 65536 code points of the ISO/IEC 10646:2017 2011 Basic Multilingual Plane (BMP). Each of the graphic characters of the BMP has a corresponding character_literal in Wide_Character. The first 256 values of Wide_Character have the same character_literal or language-defined name as defined for Character. Each of the graphic_characters has a corresponding character_literal.
4/5
{AI95-00285-01} {AI05-0262-1} {AI12-0263-1} The predefined type Wide_Wide_Character is a character type whose values correspond to the 2147483648 code points of the ISO/IEC 10646:2017 2011 character set. Each of the graphic_characters has a corresponding character_literal in Wide_Wide_Character. The first 65536 values of Wide_Wide_Character have the same character_literal or language-defined name as defined for Wide_Character.
5/3
{AI95-00285-01} {AI05-0262-1} The characters whose code point is larger than 16#FF# and which are not graphic_characters have language-defined names which are formed by appending to the string "Hex_" the representation of their code point in hexadecimal as eight extended digits. As with other language-defined names, these names are usable only with the attributes (Wide_)Wide_Image and (Wide_)Wide_Value; they are not usable as enumeration literals.
5.a/2
Reason: {AI95-00285-01} The language-defined names are not usable as enumeration literals to avoid "polluting" the name space. Since Wide_Character and Wide_Wide_Character are defined in Standard, if the language-defined names were usable as enumeration literals, they would hide other nonoverloadable declarations with the same names in use-d packages.]}
5.b/2
{AI95-00285-01}

Implementation Permissions

6/2
{AI95-00285-01}
Original Paragraphs 4 and 5 were deleted. 

Implementation Advice

7/2
{AI95-00285-01}
NOTES
8
31  The language-defined library package Characters.Latin_1 (see A.3.3) includes the declaration of constants denoting control characters, lower case characters, and special characters of the predefined type Character. 
8.a
To be honest: The package ASCII does the same, but only for the first 128 characters of Character. Hence, it is an obsolescent package, and we no longer mention it here. 
9/3
32  {AI05-0299-1} A conventional character set such as EBCDIC can be declared as a character type; the internal codes of the characters can be specified by an enumeration_representation_clause as explained in subclause 13.4

Examples

10
Example of a character type: 
11
type Roman_Digit is ('I', 'V', 'X', 'L', 'C', 'D', 'M');

Inconsistencies With Ada 83

11.a
The declaration of Wide_Character in package Standard hides use-visible declarations with the same defining identifier. In the unlikely event that an Ada 83 program had depended on such a use-visible declaration, and the program remains legal after the substitution of Standard.Wide_Character, the meaning of the program will be different. 

Incompatibilities With Ada 83

11.b
The presence of Wide_Character in package Standard means that an expression such as 
11.c
'a' = 'b'
11.d
is ambiguous in Ada 95, whereas in Ada 83 both literals could be resolved to be of type Character.
11.e
The change in visibility rules (see 4.2) for character literals means that additional qualification might be necessary to resolve expressions involving overloaded subprograms and character literals. 

Extensions to Ada 83

11.f
The type Character has been extended to have 256 positions, and the type Wide_Character has been added. Note that this change was already approved by the ARG for Ada 83 conforming compilers.
11.g
The rules for referencing character literals are changed (see 4.2), so that the declaration of the character type need not be directly visible to use its literals, similar to null and string literals. Context is used to resolve their type. 

Inconsistencies With Ada 95

11.h/2
{AI95-00285-01} Ada 95 defined most characters in Wide_Character to be graphic characters, while Ada 2005 uses the categorizations from ISO-10646:2003. It also provides language-defined names for all nongraphic characters. That means that in Ada 2005, Wide_Character'Wide_Value will raise Constraint_Error for a string representing a character_literal of a nongraphic character, while Ada 95 would have accepted it. Similarly, the result of Wide_Character'Wide_Image will change for such nongraphic characters.
11.i/3
{AI95-00395-01} {AI05-0005-1} {AI05-0262-1} The language-defined names FFFE and FFFF were replaced by a consistent set of language-defined names for all nongraphic characters with code points greater than 16#FF#. That means that in Ada 2005, Wide_Character'Wide_Value("FFFE") will raise Constraint_Error while Ada 95 would have accepted it. Similarly, the result of Wide_Character'Wide_Image will change for the position numbers 16#FFFE# and 16#FFFF#. It is very unlikely that this will matter in practice, as these names do not represent usable characters.
11.j/2
{AI95-00285-01} {AI95-00395-01} Because of the previously mentioned changes to the Wide_Character'Wide_Image of various character values, the value of attribute Wide_Width will change for some subtypes of Wide_Character. However, the new language-defined names were chosen so that the value of Wide_Character'Wide_Width itself does not change.
11.k/2
{AI95-00285-01} The declaration of Wide_Wide_Character in package Standard hides use-visible declarations with the same defining identifier. In the (very) unlikely event that an Ada 95 program had depended on such a use-visible declaration, and the program remains legal after the substitution of Standard.Wide_Wide_Character, the meaning of the program will be different. 

Extensions to Ada 95

11.l/2
{AI95-00285-01} The type Wide_Wide_Character is new. 

Wording Changes from Ada 95

11.m/2
{AI95-00285-01} Characters are now defined in terms of the entire ISO/IEC 10646:2003 character set.
11.n/3
{AI95-00285-01} {AI05-0248-1} We dropped the Implementation Advice for nonstandard interpretation of character sets; an implementation can do what it wants in a nonstandard mode, so there isn't much point to any advice. 

Wording Changes from Ada 2005

11.o/3
{AI05-0181-1} Correction: Removed the position numbers of nongraphic characters from the text, as it is wrong and thus misleading.
11.p/3
{AI05-0262-1} Changed "code position" to "code point" consistently throughout the standard, as ISO/IEC 10646:2011 prefers "code point" and we are referring to the definition in that Standard. This change also reduces confusion between "code point" and "position number"; while these have the same values for the predefined character types, there is no required relationship for other character types.

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