25 Localization library [localization]

25.3 Locales [locales]

25.3.1 Class locale [locale]

25.3.1.1 locale types [locale.types]

25.3.1.1.2 Class locale​::​facet [locale.facet]

namespace std {
  class locale::facet {
  protected:
    explicit facet(size_t refs = 0);
    virtual ~facet();
    facet(const facet&) = delete;
    void operator=(const facet&) = delete;
  };
}
1
#
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:231 
static ::std::locale::id id;
2
#
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.
3
#
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.
4
#
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.
5
#
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.
231)
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.