4.3 Character class determination for wide characters
Amendment 1 to ISO C90 defines functions to classify wide
characters. Although the original ISO C90 standard already defined
the type wchar_t, no functions operating on them were defined.
The general design of the classification functions for wide characters
is more general. It allows extensions to the set of available
classifications, beyond those which are always available. The POSIX
standard specifies how extensions can be made, and this is already
implemented in the GNU C library implementation of the localedef
program.
The character class functions are normally implemented with bitsets,
with a bitset per character. For a given character, the appropriate
bitset is read from a table and a test is performed as to whether a
certain bit is set. Which bit is tested for is determined by the
class.
For the wide character classification functions this is made visible.
There is a type classification type defined, a function to retrieve this
value for a given class, and a function to test whether a given
character is in this class, using the classification value. On top of
this the normal character classification functions as used for
char objects can be defined.
— Data type: wctype_t
The wctype_t can hold a value which represents a character class.
The only defined way to generate such a value is by using the
wctype function.
The wctype returns a value representing a class of wide
characters which is identified by the string property. Beside
some standard properties each locale can define its own ones. In case
no property with the given name is known for the current locale
selected for the LC_CTYPE category, the function returns zero.
The properties known in every locale are:
"alnum"
"alpha"
"cntrl"
"digit"
"graph"
"lower"
"print"
"punct"
"space"
"upper"
"xdigit"
This function is declared in wctype.h.
To test the membership of a character to one of the non-standard classes
the ISO C standard defines a completely new function.
— Function: int iswctype (wint_t wc, wctype_t desc)
This function returns a nonzero value if wc is in the character
class specified by desc. desc must previously be returned
by a successful call to wctype.
This function is declared in wctype.h.
To make it easier to use the commonly-used classification functions,
they are defined in the C library. There is no need to use
wctype if the property string is one of the known character
classes. In some situations it is desirable to construct the property
strings, and then it is important that wctype can also handle the
standard classes.
— Function: int iswalnum (wint_t wc)
This function returns a nonzero value if wc is an alphanumeric
character (a letter or number); in other words, if either iswalpha
or iswdigit is true of a character, then iswalnum is also
true.
This function can be implemented using
iswctype (wc, wctype ("alnum"))
It is declared in wctype.h.
— Function: int iswalpha (wint_t wc)
Returns true if wc is an alphabetic character (a letter). If
iswlower or iswupper is true of a character, then
iswalpha is also true.
In some locales, there may be additional characters for which
iswalpha is true—letters which are neither upper case nor lower
case. But in the standard "C" locale, there are no such
additional characters.
This function can be implemented using
iswctype (wc, wctype ("alpha"))
It is declared in wctype.h.
— Function: int iswcntrl (wint_t wc)
Returns true if wc is a control character (that is, a character that
is not a printing character).
This function can be implemented using
iswctype (wc, wctype ("cntrl"))
It is declared in wctype.h.
— Function: int iswdigit (wint_t wc)
Returns true if wc is a digit (e.g., `0' through `9').
Please note that this function does not only return a nonzero value for
decimal digits, but for all kinds of digits. A consequence is
that code like the following will not work unconditionally for
wide characters:
n = 0;
while (iswdigit (*wc))
{
n *= 10;
n += *wc++ - L'0';
}
This function can be implemented using
iswctype (wc, wctype ("digit"))
It is declared in wctype.h.
— Function: int iswgraph (wint_t wc)
Returns true if wc is a graphic character; that is, a character
that has a glyph associated with it. The whitespace characters are not
considered graphic.
This function can be implemented using
iswctype (wc, wctype ("graph"))
It is declared in wctype.h.
— Function: int iswlower (wint_t wc)
Returns true if wc is a lower-case letter. The letter need not be
from the Latin alphabet, any alphabet representable is valid.
This function can be implemented using
iswctype (wc, wctype ("lower"))
It is declared in wctype.h.
— Function: int iswprint (wint_t wc)
Returns true if wc is a printing character. Printing characters
include all the graphic characters, plus the space (`') character.
This function can be implemented using
iswctype (wc, wctype ("print"))
It is declared in wctype.h.
— Function: int iswpunct (wint_t wc)
Returns true if wc is a punctuation character.
This means any printing character that is not alphanumeric or a space
character.
This function can be implemented using
iswctype (wc, wctype ("punct"))
It is declared in wctype.h.
— Function: int iswspace (wint_t wc)
Returns true if wc is a whitespace character. In the standard
"C" locale, iswspace returns true for only the standard
whitespace characters:
L' '
space
L'\f'
formfeed
L'\n'
newline
L'\r'
carriage return
L'\t'
horizontal tab
L'\v'
vertical tab
This function can be implemented using
iswctype (wc, wctype ("space"))
It is declared in wctype.h.
— Function: int iswupper (wint_t wc)
Returns true if wc is an upper-case letter. The letter need not be
from the Latin alphabet, any alphabet representable is valid.
This function can be implemented using
iswctype (wc, wctype ("upper"))
It is declared in wctype.h.
— Function: int iswxdigit (wint_t wc)
Returns true if wc is a hexadecimal digit.
Hexadecimal digits include the normal decimal digits `0' through
`9' and the letters `A' through `F' and
`a' through `f'.
This function can be implemented using
iswctype (wc, wctype ("xdigit"))
It is declared in wctype.h.
The GNU C library also provides a function which is not defined in the
ISO C standard but which is available as a version for single byte
characters as well.
— Function: int iswblank (wint_t wc)
Returns true if wc is a blank character; that is, a space or a tab.
This function is a GNU extension. It is declared in wchar.h.
Published under the terms of the GNU General Public License
