28 Localization library [localization]

28.4 Standard locale categories [locale.categories]

28.4.2 The ctype category [category.ctype]

28.4.2.5 Class template codecvt [locale.codecvt]

28.4.2.5.1 General [locale.codecvt.general]

namespace std { class codecvt_base { public: enum result { ok, partial, error, noconv }; }; template<class internT, class externT, class stateT> class codecvt : public locale::facet, public codecvt_base { public: using intern_type = internT; using extern_type = externT; using state_type = stateT; explicit codecvt(size_t refs = 0); result out( stateT& state, const internT* from, const internT* from_end, const internT*& from_next, externT* to, externT* to_end, externT*& to_next) const; result unshift( stateT& state, externT* to, externT* to_end, externT*& to_next) const; result in( stateT& state, const externT* from, const externT* from_end, const externT*& from_next, internT* to, internT* to_end, internT*& to_next) const; int encoding() const noexcept; bool always_noconv() const noexcept; int length(stateT&, const externT* from, const externT* end, size_t max) const; int max_length() const noexcept; static locale::id id; protected: ~codecvt(); virtual result do_out( stateT& state, const internT* from, const internT* from_end, const internT*& from_next, externT* to, externT* to_end, externT*& to_next) const; virtual result do_in( stateT& state, const externT* from, const externT* from_end, const externT*& from_next, internT* to, internT* to_end, internT*& to_next) const; virtual result do_unshift( stateT& state, externT* to, externT* to_end, externT*& to_next) const; virtual int do_encoding() const noexcept; virtual bool do_always_noconv() const noexcept; virtual int do_length(stateT&, const externT* from, const externT* end, size_t max) const; virtual int do_max_length() const noexcept; }; }
The class codecvt<internT, externT, stateT> is for use when converting from one character encoding to another, such as from wide characters to multibyte characters or between wide character encodings such as UTF-32 and EUC.
The stateT argument selects the pair of character encodings being mapped between.
The specializations required in Table 106 ([locale.category]) convert the implementation-defined native character set.
codecvt<char, char, mbstate_­t> implements a degenerate conversion; it does not convert at all.
The specialization codecvt<char16_­t, char8_­t, mbstate_­t> converts between the UTF-16 and UTF-8 encoding forms, and the specialization codecvt <char32_­t, char8_­t, mbstate_­t> converts between the UTF-32 and UTF-8 encoding forms.
codecvt<wchar_­t, char, mbstate_­t> converts between the native character sets for ordinary and wide characters.
Specializations on mbstate_­t perform conversion between encodings known to the library implementer.
Other encodings can be converted by specializing on a program-defined stateT type.
Objects of type stateT can contain any state that is useful to communicate to or from the specialized do_­in or do_­out members.