A C++ program may define facets to be added to a locale and used identically as the built-in facets. To create a new facet interface, C++ programs simply derive from locale::facet a class containing a static member: static locale::id id.
[ Note: The locale member function templates verify its type and storage class. — end note ]
[ Example: Traditional global localization is still easy:
#include <iostream>
#include <locale>
int main(int argc, char** argv) {
using namespace std;
locale::global(locale("")); cin.imbue(locale());
cout.imbue(locale());
cerr.imbue(locale());
wcin.imbue(locale());
wcout.imbue(locale());
wcerr.imbue(locale());
return MyObject(argc, argv).doit();
}
— end example ]
[ Example: Greater flexibility is possible:
#include <iostream>
#include <locale>
int main() {
using namespace std;
cin.imbue(locale("")); cout.imbue(locale::classic());
double f;
while (cin >> f) cout << f << endl;
return (cin.fail() != 0);
}
In a European locale, with input 3.456,78, output is 3456.78. — end example ]
This can be important even for simple programs, which may need to write a data file in a fixed format, regardless of a user's preference.
[ Example: Here is an example of the use of locales in a library interface.
#include <iosfwd>
#include <string>
#include <locale>
class Date {
public:
Date(unsigned day, unsigned month, unsigned year);
std::string asString(const std::locale& = std::locale());
};
std::istream& operator>>(std::istream& s, Date& d);
std::ostream& operator<<(std::ostream& s, Date d);
This example illustrates two architectural uses of class locale.
The first is as a default argument in Date::asString(), where the default is the global (presumably user-preferred) locale.
The second is in the operators << and >>, where a locale “hitchhikes” on another object, in this case a stream, to the point where it is needed.
#include "Date" #include <sstream>
std::string Date::asString(const std::locale& l) {
using namespace std;
ostringstream s; s.imbue(l);
s << *this; return s.str();
}
std::istream& operator>>(std::istream& s, Date& d) {
using namespace std;
istream::sentry cerberos(s);
if (cerberos) {
ios_base::iostate err = goodbit;
struct tm t;
use_facet<time_get<char>>(s.getloc()).get_date(s, 0, s, err, &t);
if (!err) d = Date(t.tm_day, t.tm_mon + 1, t.tm_year + 1900);
s.setstate(err);
}
return s;
}
— end example ]
A locale object may be extended with a new facet simply by constructing it with an instance of a class derived from locale::facet. The only member a C++ program must define is the static member id, which identifies your class interface as a new facet.
[ Example: Classifying Japanese characters:
#include <locale>
namespace My {
using namespace std;
class JCtype : public locale::facet {
public:
static locale::id id; bool is_kanji (wchar_t c) const;
JCtype() { }
protected:
~JCtype() { }
};
}
#include <iostream>
#include <locale>
#include "jctype" std::locale::id My::JCtype::id;
int main() {
using namespace std;
using wctype = ctype<wchar_t>;
locale loc(locale(""), new My::JCtype); wchar_t c = use_facet<wctype>(loc).widen('!');
if (!use_facet<My::JCtype>(loc).is_kanji(c))
cout << "no it isn't!" << endl;
}
The new facet is used exactly like the built-in facets. — end example ]
[ Example: Replacing an existing facet is even easier. The code does not define a member id because it is reusing the numpunct<charT> facet interface:
#include <iostream>
#include <locale>
#include <string>
namespace My {
using namespace std;
using cnumpunct = numpunct_byname<char>;
class BoolNames : public cnumpunct {
protected:
string do_truename() const { return "Oui Oui!"; }
string do_falsename() const { return "Mais Non!"; }
~BoolNames() { }
public:
BoolNames(const char* name) : cnumpunct(name) { }
};
}
int main(int argc, char** argv) {
using namespace std;
locale loc(locale(""), new My::BoolNames(""));
cout.imbue(loc);
cout << boolalpha << "Any arguments today? " << (argc > 1) << endl;
}
— end example ]
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