22 Localization library [localization]

22.4 Standard locale categories [locale.categories]

22.4.1 The ctype category [category.ctype]

22.4.1.4 Class template codecvt [locale.codecvt]

22.4.1.4.2 codecvt virtual functions [locale.codecvt.virtuals]

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; 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;

Requires: (from<=from_end && to<=to_end) well-defined and true; state initialized, if at the beginning of a sequence, or else equal to the result of converting the preceding characters in the sequence.

Effects: Translates characters in the source range [from, from_end), placing the results in sequential positions starting at destination to. Converts no more than (from_end-from) source elements, and stores no more than (to_end-to) destination elements.

Stops if it encounters a character it cannot convert. It always leaves the from_next and to_next pointers pointing one beyond the last element successfully converted. If returns noconv, internT and externT are the same type and the converted sequence is identical to the input sequence [from, from_next). to_next is set equal to to, the value of state is unchanged, and there are no changes to the values in [to, to_end).

A codecvt facet that is used by basic_filebuf ([file.streams]) shall have the property that if

do_out(state, from, from_end, from_next, to, to_end, to_next)

would return ok, where from != from_end, then

do_out(state, from, from + 1, from_next, to, to_end, to_next)

shall also return ok, and that if

do_in(state, from, from_end, from_next, to, to_end, to_next)

would return ok, where to != to_end, then

do_in(state, from, from_end, from_next, to, to + 1, to_next)

shall also return ok.237Note: As a result of operations on state, it can return ok or partial and set from_next == from and to_next != to.  — end note ]

Remarks: Its operations on state are unspecified. [ Note: This argument can be used, for example, to maintain shift state, to specify conversion options (such as count only), or to identify a cache of seek offsets.  — end note ]

Returns: An enumeration value, as summarized in Table [tab:localization.convert.result.values.out.in].

Table 71do_in/do_out result values
ValueMeaning
ok completed the conversion
partial not all source characters converted
error encountered a character in [from, from_end) that it could not convert
noconv internT and externT are the same type, and input sequence is identical to converted sequence

A return value of partial, if (from_next==from_end), indicates that either the destination sequence has not absorbed all the available destination elements, or that additional source elements are needed before another destination element can be produced.

result do_unshift(stateT& state, externT* to, externT* to_end, externT*& to_next) const;

Requires: (to <= to_end) well defined and true; state initialized, if at the beginning of a sequence, or else equal to the result of converting the preceding characters in the sequence.

Effects: Places characters starting at to that should be appended to terminate a sequence when the current stateT is given by state.238 Stores no more than (to_end-to) destination elements, and leaves the to_next pointer pointing one beyond the last element successfully stored.

Returns: An enumeration value, as summarized in Table [tab:localization.convert.result.values.unshift].

Table 72do_unshift result values
ValueMeaning
ok completed the sequence
partial space for more than to_end-to destination elements was needed to terminate a sequence given the value of state
error an unspecified error has occurred
noconv no termination is needed for this state_type

int do_encoding() const noexcept;

Returns: -1 if the encoding of the externT sequence is state-dependent; else the constant number of externT characters needed to produce an internal character; or 0 if this number is not a constant.239

bool do_always_noconv() const noexcept;

Returns: true if do_in() and do_out() return noconv for all valid argument values. codecvt<char, char, mbstate_t> returns true.

int do_length(stateT& state, const externT* from, const externT* from_end, size_t max) const;

Requires: (from<=from_end) well-defined and true; state initialized, if at the beginning of a sequence, or else equal to the result of converting the preceding characters in the sequence.

Effects: The effect on the state argument is “as if” it called do_in(state, from, from_end, from, to, to+max, to) for to pointing to a buffer of at least max elements.

Returns: (from_next-from) where from_next is the largest value in the range [from, from_end] such that the sequence of values in the range [from, from_next) represents max or fewer valid complete characters of type internT. The specialization codecvt<char, char, mbstate_t>, returns the lesser of max and (from_end-from).

int do_max_length() const noexcept;

Returns: The maximum value that do_length(state, from, from_end, 1) can return for any valid range [from, from_end) and stateT value state. The specialization codecvt<char, char, mbstate_t>::do_max_length() returns 1.

Informally, this means that basic_filebuf assumes that the mappings from internal to external characters is 1 to N: a codecvt facet that is used by basic_filebuf must be able to translate characters one internal character at a time.

Typically these will be characters to return the state to stateT().

If encoding() yields -1, then more than max_length() externT elements may be consumed when producing a single internT character, and additional externT elements may appear at the end of a sequence after those that yield the final internT character.