3.5.2 Character Types
Static Semantics
An enumeration type is said to
be a
character type if at least one of its enumeration literals
is a
character_literal.
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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
10646:2020 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.
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The predefined
type Wide_Character is a character type whose values correspond to the
65536 code points of the ISO/IEC 10646:2020 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.
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The predefined type Wide_Wide_Character
is a character type whose values correspond to the 2147483648 code points
of the ISO/IEC 10646:2020 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.
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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.
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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.
Original Paragraphs
4 and 5 were deleted.
NOTE 1 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.
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.
Examples
Example of a character
type:
type Roman_Digit is ('I', 'V', 'X', 'L', 'C', 'D', 'M');
Inconsistencies With Ada 83
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
The presence of Wide_Character
in package Standard means that an expression such as
'a' = 'b'
is ambiguous in Ada 95, whereas in Ada 83 both
literals could be resolved to be of type Character.
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
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.
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
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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.
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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.
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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.
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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
Wording Changes from Ada 95
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Characters are now defined in terms of the entire ISO/IEC 10646:2003
character set.
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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
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Correction: Removed the position numbers of nongraphic characters
from the text, as it is wrong and thus misleading.
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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.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
