[facets.examples] - C++17 → C++20

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-### Program-defined facets <a id="facets.examples">[[facets.examples]]</a>
-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 1*: The locale member function templates verify its type and
-storage class. — *end note*]
-[*Example 1*:
-Traditional global localization is still easy:
-``` cpp
-#include <iostream>
-#include <locale>
-int main(int argc, char** argv) {
-  using namespace std;
-  locale::global(locale(""));           // set the global locale
-                                        // imbue it on all the std streams
-  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 2*:
-Greater flexibility is possible:
-``` cpp
-#include <iostream>
-#include <locale>
-int main() {
-  using namespace std;
-  cin.imbue(locale(""));        // the user's preferred 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 3*:
-Here is an example of the use of locales in a library interface.
-``` cpp
-// file: Date.h
-#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.
-``` cpp
-// file: Date.C
-#include "Date"                 // includes <ctime>
-#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 4*:
-Classifying Japanese characters:
-``` cpp
-// file: <jctype>
-#include <locale>
-namespace My {
-  using namespace std;
-  class JCtype : public locale::facet {
-  public:
-    static locale::id id;       // required for use as a new locale facet
-    bool is_kanji (wchar_t c) const;
-    JCtype() { }
-  protected:
-    ~JCtype() { }
-  };
-}
-// file: filt.C
-#include <iostream>
-#include <locale>
-#include "jctype"               // above
-std::locale::id My::JCtype::id; // the static JCtype member declared above.
-int main() {
-  using namespace std;
-  using wctype = ctype<wchar_t>;
-  locale loc(locale(""),        // the user's preferred locale ...
-         new My::JCtype);       // and a new feature ...
-  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 5*:
-Replacing an existing facet is even easier. The code does not define a
-member `id` because it is reusing the `numpunct<charT>` facet interface:
-``` cpp
-// file: my_bool.C
-#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;
-  // make the user's preferred locale, except for...
-  locale loc(locale(""), new My::BoolNames(""));
-  cout.imbue(loc);
-  cout << boolalpha << "Any arguments today? " << (argc > 1) << endl;
-}
-```
-— *end example*]

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