25 Localization library [localization]

25.4 Standard locale categories [locale.categories]

25.4.1 The ctype category [category.ctype]

25.4.1.4 Class template codecvt [locale.codecvt]

25.4.1.4.2 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;
1
#
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.
2
#
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).
3
#
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
 ]
4
#
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
 ]
5
#
Returns: An enumeration value, as summarized in Table 66.
Table 66 — do_­in/do_­out result values
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;
6
#
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.
7
#
Effects: Places characters starting at to that should be appended to terminate a sequence when the current stateT is given by state.237
Stores no more than (to_­end - to) destination elements, and leaves the to_­next pointer pointing one beyond the last element successfully stored.
8
#
Returns: An enumeration value, as summarized in Table 67.
Table 67 — do_­unshift result values
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;
9
#
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.238
bool do_always_noconv() const noexcept;
10
#
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;
11
#
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.
12
#
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.
13
#
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;
14
#
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.
236)
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.
237)
Typically these will be characters to return the state to stateT().
238)
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.