28 Localization library [localization]

28.3 Locales [locales]

28.3.1 Class locale [locale]

namespace std {
  class locale {
    // types
    class facet;
    class id;
    using category = int;
    static const category   // values assigned here are for exposition only
      none     = 0,
      collate  = 0x010, ctype    = 0x020,
      monetary = 0x040, numeric  = 0x080,
      time     = 0x100, messages = 0x200,
      all = collate | ctype | monetary | numeric | time  | messages;

    // construct/copy/destroy
    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();                  // not virtual
    const locale& operator=(const locale& other) noexcept;
    template<class Facet> locale combine(const locale& other) const;

    // locale operations
    string name() 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;

    // global locale objects
    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<>.
An iostream operator<< might be implemented as:257
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);     // might throw
  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.
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 may 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).
    (This eases upgrading existing extractors ([istream.formatted]).)
— 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.
Note that in the call to put, the stream is implicitly converted to an ostreambuf_­iterator<charT, traits>. Types [locale.types] Type locale​::​category [locale.category]

using category = int;
Valid category values include the locale member bitmask elements collate, ctype, monetary, numeric, time, and messages, each of which represents a single locale category.
In addition, locale member bitmask constant none is defined as zero and represents no category.
And locale member bitmask constant all is defined such that the expression
(collate | ctype | monetary | numeric | time | messages | all) == all
is true, and represents the union of all categories.
Further, the expression (X | Y), where X and Y each represent a single category, represents the union of the two categories.
locale member functions expecting a category argument require one of the category values defined above, or the union of two or more such values.
Such a category value identifies a set of locale categories.
Each locale category, in turn, identifies a set of locale facets, including at least those shown in Table 102.
Table 102: Locale category facets   [tab:locale.category.facets]
Includes facets
collate<char>, collate<wchar_­t>
ctype<char>, ctype<wchar_­t>
codecvt<char, char, mbstate_­t>
codecvt<char16_­t, char8_­t, mbstate_­t>
codecvt<char32_­t, char8_­t, mbstate_­t>
codecvt<wchar_­t, char, mbstate_­t>
moneypunct<char>, moneypunct<wchar_­t>
moneypunct<char, true>, moneypunct<wchar_­t, true>
money_­get<char>, money_­get<wchar_­t>
money_­put<char>, money_­put<wchar_­t>
numpunct<char>, numpunct<wchar_­t>
num_­get<char>, num_­get<wchar_­t>
num_­put<char>, num_­put<wchar_­t>
time_­get<char>, time_­get<wchar_­t>
time_­put<char>, time_­put<wchar_­t>
messages<char>, messages<wchar_­t>
For any locale loc either constructed, or returned by locale​::​classic(), and any facet Facet shown in Table 102, has_­facet<Facet>(loc) is true.
Each locale member function which takes a locale​::​category argument operates on the corresponding set of facets.
An implementation is required to provide those specializations for facet templates identified as members of a category, and for those shown in Table 103.
Table 103: Required specializations   [tab:locale.spec]
Includes facets
collate_­byname<char>, collate_­byname<wchar_­t>
ctype_­byname<char>, ctype_­byname<wchar_­t>
codecvt_­byname<char, char, mbstate_­t>
codecvt_­byname<char16_­t, char8_­t, mbstate_­t>
codecvt_­byname<char32_­t, char8_­t, mbstate_­t>
codecvt_­byname<wchar_­t, char, mbstate_­t>
moneypunct_­byname<char, International>
moneypunct_­byname<wchar_­t, International>
money_­get<C, InputIterator>
money_­put<C, OutputIterator>
numpunct_­byname<char>, numpunct_­byname<wchar_­t>
num_­get<C, InputIterator>, num_­put<C, OutputIterator>
time_­get<char, InputIterator>
time_­get_­byname<char, InputIterator>
time_­get<wchar_­t, InputIterator>
time_­get_­byname<wchar_­t, InputIterator>
time_­put<char, OutputIterator>
time_­put_­byname<char, OutputIterator>
time_­put<wchar_­t, OutputIterator>
time_­put_­byname<wchar_­t, OutputIterator>
messages_­byname<char>, messages_­byname<wchar_­t>
The provided implementation of members of facets num_­get<charT> and num_­put<charT> calls use_­facet<F>(l) only for facet F of types numpunct<charT> and ctype<charT>, and for locale l the value obtained by calling member getloc() on the ios_­base& argument to these functions.
In declarations of facets, a template parameter with name InputIterator or OutputIterator indicates the set of all possible specializations on parameters that meet the Cpp17InputIterator requirements or Cpp17OutputIterator requirements, respectively ([iterator.requirements]).
A template parameter with name C represents the set of types containing char, wchar_­t, and any other implementation-defined character types that meet the requirements for a character on which any of the iostream components can be instantiated.
A template parameter with name International represents the set of all possible specializations on a bool parameter. Class locale​::​facet [locale.facet]

namespace std {
  class locale::facet {
    explicit facet(size_t refs = 0);
    virtual ~facet();
    facet(const facet&) = delete;
    void operator=(const facet&) = delete;
Class facet is the base class for locale feature sets.
A class is a facet if it is publicly derived from another facet, or if it is a class derived from locale​::​facet and contains a publicly accessible declaration as follows:258
static ::std::locale::id id;
Template parameters in this Clause which are required to be facets are those named Facet in declarations.
A program that passes a type that is not a facet, or a type that refers to a volatile-qualified facet, as an (explicit or deduced) template parameter to a locale function expecting a facet, is ill-formed.
A const-qualified facet is a valid template argument to any locale function that expects a Facet template parameter.
The refs argument to the constructor is used for lifetime management.
For refs == 0, the implementation performs delete static_­cast<locale​::​facet*>(f) (where f is a pointer to the facet) when the last locale object containing the facet is destroyed; for refs == 1, the implementation never destroys the facet.
Constructors of all facets defined in this Clause take such an argument and pass it along to their facet base class constructor.
All one-argument constructors defined in this Clause are explicit, preventing their participation in automatic conversions.
For some standard facets a standard “_­byname” class, derived from it, implements the virtual function semantics equivalent to that facet of the locale constructed by locale(const char*) with the same name.
Each such facet provides a constructor that takes a const char* argument, which names the locale, and a refs argument, which is passed to the base class constructor.
Each such facet also provides a constructor that takes a string argument str and a refs argument, which has the same effect as calling the first constructor with the two arguments str.c_­str() and refs.
If there is no “_­byname” version of a facet, the base class implements named locale semantics itself by reference to other facets.
This is a complete list of requirements; there are no other requirements.
Thus, a facet class need not have a public copy constructor, assignment, default constructor, destructor, etc. Class locale​::​id [locale.id]

namespace std {
  class locale::id {
    void operator=(const id&) = delete;
    id(const id&) = delete;
The class locale​::​id provides identification of a locale facet interface, used as an index for lookup and to encapsulate initialization.
Because facets are used by iostreams, potentially while static constructors are running, their initialization cannot depend on programmed static initialization.
One initialization strategy is for locale to initialize each facet's id member the first time an instance of the facet is installed into a locale.
This depends only on static storage being zero before constructors run ([basic.start.static]).
— end note
 ] Constructors and destructor [locale.cons]

locale() noexcept;
Effects: Constructs a copy of the argument last passed to locale​::​global(locale&), if it has been called; else, the resulting facets have virtual function semantics identical to those of locale​::​classic().
This constructor yields a copy of the current global locale.
It is commonly used as a default argument for function parameters of type const locale&.
— end note
explicit locale(const char* std_name);
Effects: Constructs a locale using standard C locale names, e.g., "POSIX".
The resulting locale implements semantics defined to be associated with that name.
Throws: runtime_­error if the argument is not valid, or is null.
Remarks: The set of valid string argument values is "C", "", and any implementation-defined values.
explicit locale(const string& std_name);
Effects: The same as locale(std_­name.c_­str()).
locale(const locale& other, const char* std_name, category);
Effects: Constructs a locale as a copy of other except for the facets identified by the category argument, which instead implement the same semantics as locale(std_­name).
Throws: runtime_­error if the argument is not valid, or is null.
Remarks: The locale has a name if and only if other has a name.
locale(const locale& other, const string& std_name, category cat);
Effects: The same as locale(other, std_­name.c_­str(), cat).
template<class Facet> locale(const locale& other, Facet* f);
Effects: Constructs a locale incorporating all facets from the first argument except that of type Facet, and installs the second argument as the remaining facet.
If f is null, the resulting object is a copy of other.
Remarks: The resulting locale has no name.
locale(const locale& other, const locale& one, category cats);
Effects: Constructs a locale incorporating all facets from the first argument except those that implement cats, which are instead incorporated from the second argument.
Remarks: The resulting locale has a name if and only if the first two arguments have names.
const locale& operator=(const locale& other) noexcept;
Effects: Creates a copy of other, replacing the current value.
Returns: *this. Members [locale.members]

template<class Facet> locale combine(const locale& other) const;
Effects: Constructs a locale incorporating all facets from *this except for that one facet of other that is identified by Facet.
Returns: The newly created locale.
Throws: runtime_­error if has_­facet<Facet>(other) is false.
Remarks: The resulting locale has no name.
string name() const;
Returns: The name of *this, if it has one; otherwise, the string "*". Operators [locale.operators]

bool operator==(const locale& other) const;
Returns: true if both arguments are the same locale, or one is a copy of the other, or each has a name and the names are identical; false otherwise.
template<class charT, class traits, class Allocator> bool operator()(const basic_string<charT, traits, Allocator>& s1, const basic_string<charT, traits, Allocator>& s2) const;
Effects: Compares two strings according to the collate<charT> facet.
Remarks: This member operator template (and therefore locale itself) meets the requirements for a comparator predicate template argument ([algorithms]) applied to strings.
use_facet<collate<charT>>(*this).compare(s1.data(), s1.data() + s1.size(),
                                         s2.data(), s2.data() + s2.size()) < 0
A vector of strings v can be collated according to collation rules in locale loc simply by ([alg.sort], [vector]):
std::sort(v.begin(), v.end(), loc);
— end example
 ] Static members [locale.statics]

static locale global(const locale& loc);
Effects: Sets the global locale to its argument.
Causes future calls to the constructor locale() to return a copy of the argument.
If the argument has a name, does
setlocale(LC_ALL, loc.name().c_str());
otherwise, the effect on the C locale, if any, is implementation-defined.
Remarks: No library function other than locale​::​global() affects the value returned by locale().
See [c.locales] for data race considerations when setlocale is invoked.
— end note
Returns: The previous value of locale().
static const locale& classic();
The "C" locale.
Returns: A locale that implements the classic "C" locale semantics, equivalent to the value locale("C").
Remarks: This locale, its facets, and their member functions, do not change with time.