28 Text processing library [text]
namespace std {
class locale {
public:
class facet;
class id;
using category = int;
static const category
none = 0,
collate = 0x010, ctype = 0x020,
monetary = 0x040, numeric = 0x080,
time = 0x100, messages = 0x200,
all = collate | ctype | monetary | numeric | time | messages;
locale() noexcept;
locale(const locale& other) noexcept;
explicit locale(const char* std_name);
explicit locale(const string& std_name);
locale(const locale& other, const char* std_name, category);
locale(const locale& other, const string& std_name, category);
template<class Facet> locale(const locale& other, Facet* f);
locale(const locale& other, const locale& one, category);
~locale();
const locale& operator=(const locale& other) noexcept;
template<class Facet> locale combine(const locale& other) const;
string name() const;
text_encoding encoding() const;
bool operator==(const locale& other) const;
template<class charT, class traits, class Allocator>
bool operator()(const basic_string<charT, traits, Allocator>& s1,
const basic_string<charT, traits, Allocator>& s2) const;
static locale global(const locale&);
static const locale& classic();
};
}
Class
locale implements a type-safe polymorphic set of facets,
indexed by facet
type.In other words, a facet has a dual role:
in one sense, it's just a class interface;
at the same time, it's an index into a locale's set of facets
.Access to the facets of a
locale is via two function templates,
use_facet<> and
has_facet<>.[
Example 1:
An iostream operator<< can be implemented as:
template<class charT, class traits>
basic_ostream<charT, traits>&
operator<< (basic_ostream<charT, traits>& s, Date d) {
typename basic_ostream<charT, traits>::sentry cerberos(s);
if (cerberos) {
tm tmbuf; d.extract(tmbuf);
bool failed =
use_facet<time_put<charT, ostreambuf_iterator<charT, traits>>>(
s.getloc()).put(s, s, s.fill(), &tmbuf, 'x').failed();
if (failed)
s.setstate(s.badbit);
}
return s;
}
— end example]
In the call to
use_facet<Facet>(loc),
the type argument chooses a facet,
making available all members of the named type
.If
Facet is not present in a locale,
it throws the standard exception
bad_cast.A C++ program can check if a locale implements a particular facet
with the function template
has_facet<Facet>().User-defined facets may be installed in a locale, and
used identically as may standard facets
.[
Note 1:
All locale semantics are accessed via
use_facet<> and has_facet<>,
except that:
- A member operator template
operator()(const basic_string<C, T, A>&, const basic_string<C, T, A>&)
is provided so that a locale can be used as a predicate argument to
the standard collections, to collate strings.
- Convenient global interfaces are provided for
traditional ctype functions such as
isdigit() and isspace(),
so that given a locale object loc
a C++ program can call isspace(c, loc).
—
end note]
Once a facet reference is obtained from a locale object
by calling
use_facet<>,
that reference remains usable,
and the results from member functions of it may be cached and re-used,
as long as some locale object refers to that facet
.In successive calls to a locale facet member function
on a facet object installed in the same locale,
the returned result shall be identical
.A
locale constructed
from a name string (such as
"POSIX"), or
from parts of two named locales, has a name;
all others do not
.Named locales may be compared for equality;
an unnamed locale is equal only to (copies of) itself
.For an unnamed locale,
locale::name() returns the string
"*".Whether there is
one global locale object for the entire program or
one global locale object per thread
is
implementation-defined
.Implementations should provide one global locale object per thread
.If there is a single global locale object for the entire program,
implementations are not required to
avoid data races on it (
[res.on.data.races])
.