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.236 [ Note: 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 71.
| Value | Meaning |
| 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.
Returns: An enumeration value, as summarized in Table 72.
| Value | Meaning |
| 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;
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.