The numpunct facet controls the symbol used as the decimal point, the insertion of optional thousands separators, and the strings used for the textual representation of Boolean values. Table 14.7 lists the members of numpunct.

numpunct takes a character type charT as the template argument. The characters returned from decimal_point() and thousand_sep() are of this type, and the functions truename() and falsename() return a basic_string<charT>. The two instantiations numpunct<char> and numpunct<wchar_t> are required.

Because long numbers are hard to read without intervening characters, the standard facets for numeric formatting and numeric parsing support thousands separators. Often, the digits representing an integer are grouped into triples. For example, one million is written like this:

    1,000,000

Unfortunately, it is not used everywhere exactly like that. For example, in German a period is used instead of a comma. Thus, a German would write one million like this:

    1.000.000

This difference is covered by the thousands_sep() member. But this is not sufficient because in some countries digits are not put into triples. For example, in Nepal people would write

    10.00.000

using even different numbers of digits in the groups. This is where the string returned from the function grouping() comes in. The number stored at index i gives the number of digits in the ith group, where counting starts with zero for the rightmost group. If there are fewer characters in the string than groups, the size of the last specified group is repeated. To create unlimited groups, you can use the value numeric_limits<char>: :max() or, if there is no group at all, the empty string.Table 14.8 lists some examples of the formatting of one million.

Note that normal digits are usually not very useful. For example, the string "2" specifies groups of 50 digits for ASCII encoding because the character '2' has the integer value 50 in the ASCII character set.

The num_put facet is used for textual formatting of numbers. It is a template class that takes two template arguments: the type charT of the characters to be produced and the type OutIt of an output iterator to the location at which the produced characters are written. The output iterator defaults to ostreambuf_iterator<charT>. The num_put facet provides a set of functions, all called put() and differing only in the last argument. You can use the facet as follows:

    std::locale      loc;
    OutIt            to = ...;
    std: : ios_base& fmt = ...;
    charT            fill = ...;
    T                value = ...;


    //get numeric output facet of the loc locale 
    const std::num_put<charT,OutIt>& np 
     = std::use_facet<std::num_put<charT,OutIt>(loc);


    //write value with numeric output facet
    np.put(to, fmt, fill, value);

These statements would produce a textual representation of the value value using characters of type charT written to the output iterator to. The exact format is determined from the formatting flags stored in fmt, where the character fill is used as a fill character. The put() function returns an iterator pointing immediately after the last character written.

The facet num_put provides member functions that take objects of types bool, long, unsigned long, double, long double, and void* as the last argument. It does not provide member functions, for example, for short or int. This is no problem because corresponding values of built-in types are promoted to supported types if necessary.

The standard requires that the two instantiations num_put<char> and num_put<wchar_t> are stored in each locale (both using the default for the second template argument). In addition, the C++ standard library supports all instantiations that take a character type as the first template argument and an output iterator type as the second. Of course, it is not required that all of these instantiations are stored in each locale because this would be an infinite amount of facets.

The facet num_get is used to parse textual representations of numbers. Corresponding to the facet num_put, it is a template that takes two template arguments: the character type charT and an input iterator type InIt, which defaults to istreambuf_iterator<charT>. It provides a set of get() functions that differ only in the last argument. You can use the facet as follows:^[4]

    std::locale      loc;               // locale
    InIt             beg = ...;         // begin of input sequence
    InIt             end = ...;         // end of input sequence
    std::ios_base&   fmt = ...;     // stream which defines input format
    std::ios_base::iostate err;         // state after call
    T             value;                // value after successful call


    //get numeric input facet of the loc locale
    const std::num_get<charT,InIt>& ng
    = std::use_facet<std::num_get<charT,InIt> > (loc);


    // read value with numeric input facet
    ng.get(beg, end, fmt, err, value);

These statements attempt to parse a numeric value corresponding to the type T from the sequence of characters between beg and end. The format of the expected numeric value is defined by the argument fmt. If the parsing fails, err is modified to contain the value ios_base: :failbit. Otherwise, ios_base: :goodbit is stored in err and the parsed value in value. The value of value is modified only if the parsing is successful. get() returns the second parameter (end) if the sequence was used completely. Otherwise, it returns an iterator pointing to the first character that could not be parsed as part of the numeric value.

The facet num_get supports functions to read objects of the types bool, long, unsigned short, unsigned int, unsigned long, float, double, long double, and void*. There are some types for which there is no corresponding function in the num_put facet; for example, unsigned short. This is because writing a value of type unsigned short produces the same result as writing a value of type unsigned short promoted to an unsigned long. However, reading a value as type unsigned long and then converting it to unsigned short may yield a different value than reading it as type unsigned short directly.

The standard requires that the two instantiations num_get<char> and num_get<wchar_t> be stored in each locale (both using the default for the second template argument). In addition, the C++ standard library supports all instantiations that take a character type as the first template argument and an input iterator type as the second. As with num_put, not all supported instantiations are required to be present in all locale objects.

The facet time_get is a template that takes a character type charT and an input iterator type InIt as template arguments. The input iterator type defaults to istreambuf_iterator<charT>. Table 14.10 lists the members defined for the time_get facet. All of these members, except date_order(), parse the string and store the results in the tm object pointed to by the argument t. If the string could not be parsed correctly, either an error is reported (for example, by modifying the argument err) or an unspecified value is stored. This means that a time produced by a program can be parsed reliably but user input cannot. With the argument fmt, other facets used during parsing are determined. Whether other flags from fmt have any influence on the parsing is not specified.

All functions return an iterator that has the position immediately after the last character read. The parsing stops if parsing is complete or if an error occurs (for example, because a string could not be parsed as a date).

A function reading the name of a weekday or a month reads both abbreviated names and full names. If the abbreviation is followed by a letter, which would be legal for a full name, the function attempts to read the full name. If this fails, the parsing fails, even though an abbreviated name was already parsed successfully.

Whether a function that is parsing a year allows two-digit years is unspecified. The year that is assumed for a two-digit year, if it is allowed, is also unspecified.

date_order() returns the order in which the day, month, and year appear in a date string. This is necessary for some dates because the order cannot be determined from the string representing a date. For example, the first day in February in the year 2003 may be printed either as 3/2/1 or as 1/2/3. Class time_base, which is the base class of the facet time_get, defines an enumeration called dateorder for possible date order values. Table 14.11 lists these values.

The standard requires that the two instantiations time_get<char> and time_get<wchar_t> are stored in each locale. In addition, the C++ standard library supports all instantiations that take char or wchar_t as the first template argument, and a corresponding input iterator as the second. All of these instantiations are not required to be stored in each locale object.

The facet time_put is used for formatting times and dates. It is a template that takes as arguments a character type charT and an optional output iterator type Out It. The latter defaults to type ostreambuf_iterator (see page 665).

The facet time_put defines two functions called put(), which are used to convert the date information stored in an object of type tm into a sequence of characters written to an output iterator. Table 14.12 lists the members of the facet time_put.

Both functions write their results to the output iterator to and return an iterator pointing immediately after the last character produced. The argument fmt is of type ios_base and is used to access other facets and potentially additional formatting information. The character fill is used when a space character is needed and for filling. The argument t points to an object of type tm that is storing the date to be formatted.

The version of put() that takes two characters as the last two arguments formats the date found in the tm object to which t refers, interpreting the argument cvt like a conversion specifier to strftime(). This put() function does only one conversion; namely, the one specified by the cvt character. This function is called by the other put() function for each conversion specifier found. For example, using 'X' as the conversion specifier results in the time that is stored in *t being written to the output iterator. The meaning of the argument mod is not defined by the standard. It is intended to be used as a modifier to the conversion as found in several implementations of the strftime() function.

The version of put() that takes a string defined by the range [cbeg,cend) to guide the conversion behaves very much like strftime(). It scans the string and writes every character that is not part of a conversion specification to the output iterator to. If it encounters a conversion specification introduced by the character %, it extracts an optional modifier and a conversion specifier. The function continues by calling the other version of put(), using the conversion specifier and the modifier as the last two arguments. After processing a conversion specification, put() continues to scan the string.

Note that this facet is somewhat unusual because it provides a nonvirtual member function; namely, the function put(), which uses a string as the conversion specification. This function cannot be overridden in classes derived from time_put. Only the other put() function can be overridden.

The standard requires that the two instantiations time_put<char> and time_put<wchar_t> are stored in each locale. In addition, the C++ standard library supports all instantiations that take char or wchar_t as the first template argument and a corresponding output iterator as the second. There is no guaranteed support for instantiations using a type other than char or wchar_t as the first template argument. Also, it is not guaranteed that any instantiations other than time_put<char> and time_put<wchar_t> be stored in locale objects by default.

Monetary values are printed differently depending on the context. The formats used in different cultural communities differ widely. Examples of the varying details are the placement of the currency symbol (if present at all), the notation for negative or positive values, the use of national or international currency symbols, and the use of thousands separators. To provide the necessary flexibility, the details of the format are factored into the facet moneypunct.

The facet moneypunct is a template that takes as arguments a character type charT and a Boolean value that defaults to false. The Boolean value indicates whether local (false) or international (true) currency symbols are to be used. Table 14.13 lists the members of the facet moneypunct.

moneypunct derives from the class money_base. This base class defines an enumeration called part, which is used to form a pattern for monetary values. The class also defines a type called pattern (which is actually a type definition for char [4]). This type is used to store four values of type part that form a pattern describing the layout of a monetary value. Table 14.14 lists the five possible parts that can be placed in a pattern.

moneypunct defines two functions that return patterns: the function neg_format() for negative values and the function pos_format() for non-negative values. In a pattern, each of the parts sign, symbol, and value is mandatory, and one of the parts none and space has to appear. This does not mean, however, that there is really a sign or a currency symbol printed. What is printed at the positions indicated by the parts depends on the values returned from other members of the facet and on the formatting flags passed to the functions for formatting.

Only the value always appears. Of course, it is placed at the position where the part value appears in the pattern. The value has exactly frac_digits() fractional digits, with decimal_point() used as the decimal point (unless there are no fractional digits, in which case no decimal point is used).

When reading monetary values, thousand separators are allowed but not required in the input. When present they are checked for correct placements according to grouping(). If grouping () is empty, no thousand separators are allowed. The character used for the thousands separator is the one returned from thousands_sep(). The rules for the placement of the thousands separators are identical to the rules for numeric formatting (see page 705). When monetary values are printed, thousands separators are always inserted according to the string returned from grouping(). When monetary values are read, thousands separators are optional unless the grouping string is empty. The correct placement of thousands separators is checked after all other parsing is successful.

The parts space and none control the placement of spaces. space is used at a position where at least one space is required. During formatting, if ios_base::internal is specified in the format flags, fill characters are inserted at the position of the space or the none part. Of course, filling is done only if the minimum width specified is not used with other characters. The character used as the space character is passed as the argument to the functions for the formatting of monetary values. If the formatted value does not contain a space, none can be placed at the last position. space and none may not appear as the first part in a pattern, and space may not be the last part in a pattern.

Signs for monetary values may consist of more than one character. For example, in certain contexts parentheses around a value are used to indicate negative values. At the position where the sign part appears in the pattern, the first character of the sign appears. All other characters of the sign appear at the end after all other components. If the string for a sign is empty, no character indicating the sign appears. The character that is to be used as a sign is determined with the function positive_sign() for non-negative values and negative_sign() for negative values.

At the position of the symbol part, the currency symbol appears. The symbol is present only if the formatting flags used during formatting or parsing have the ios_base::showbase flag set. The string returned from the function curr_symbol() is used as the currency symbol. The currency symbol is a local symbol to be used to indicate the currency if the second template argument is false (the default). Otherwise, an international currency symbol is used.

Table 14.15 illustrates all of this, using the value $-1234.56 as an example. Of course, this means that frac_digits() returns 2. In addition, a width of 0 is always used.

The standard requires that the instantiations moneypunct<char>, moneypunct<wchar_t>, moneypunct<char, true>, and moneypunct<wchar_t, true> are stored in each locale. The C++ standard library does not support any other instantiation.

The facet money_put is used to format monetary values. It is a template that takes a character type charT as the first template argument and an output iterator OutIt as the second. The output iterator defaults to ostreambuf _iterator<charT>. The two member functions put() produce a sequence of characters corresponding to the format specified by a moneypunct facet. The value to be formatted is either passed as type long double or as type basic_string<charT>. You can use the facet as follows:

//get monetary output facet of the loc locale
    const std::money_put<charT,OutIt>& mp
    = std::use_facet<std::money_put<charT,OutIt> >(loc);


    // write value with monetary output facet
    mp.put(to, intl, frat, fill, value);

The argument to is an output iterator of type OutIt to which the formatted string is written. put() returns an object of this type pointing immediately after the last character produced. The argument intl indicates whether a local or an international currency symbol is to be used. fmt is used to determine formatting flags, such as the width to be used and the moneypunct facet defining the format of the value to be printed. Where a space character has to appear, the character fill is inserted.

The argument value has type long double or type basic_string<charT>. This is the value that is formatted. If the argument is a string, this string may consist only of decimal digits with an optional leading minus sign. If the first character of the string is a minus sign, the value is formatted as a negative value. After it is determined that the value is negative, the minus sign is discarded. The number of fractional digits in the string is determined from the member function frac_digits() of the moneypunct facet.

The standard requires that the two instantiations money_put<char> and money_put<wchar_t> are stored in each locale. In addition, the C++ standard library supports all instantiations that take char or wchar_t as the first template argument and a corresponding output iterator as the second. All of these instantiations are not required to be stored in each locale object.

The facet money_get is used for parsing of monetary values. It is a template class that takes a character type charT as the first template argument and an input iterator type InIt as the second. The second template argument defaults to istreambuf_iterator<charT>. This class defines two member functions called get() that try to parse a character and, if the parse is successful, store the result in a value of type long double or of type basic_string<charT>. You can use the facet as follows:

//get monetary input facet of the loc locale
    const std::money_get<charT,InIt>& mg 
     = std::use_facet<std::money_get<charT,InIt> >(loc);


    //read value with monetary input facet
    mg.get(beg, end, intl, fmt, err, val);

The character sequence to be parsed is defined by the sequence between beg and end. The parsing stops as soon as either all elements of the used pattern are read or an error is encountered. If an error is encountered, the ios_base::failbit is set in err and nothing is stored in val. If parsing is successful, the result is stored in the value of types long double or basic_string that is passed by reference as argument val.

The argument intl is a Boolean value that selects a local or an international currency string. The moneypunct facet defining the format of the value to be parsed is retrieved using the locale object imbued by the argument fmt. For parsing a monetary value, the pattern returned from the member neg_format() of the moneypunct facet is always used.

At the position of none or space, the function that is parsing a monetary value consumes all available space, unless none is the last part in a pattern. Trailing spaces are not skipped. The get() functions return an iterator that points after the last character that was consumed.

The standard requires that the two instantiations money_get<char> and money_get<wchar_t> be stored in each locale. In addition, the C++ standard library supports all instantiations that take char or wchar_t as the first template argument and a corresponding input iterator as the second. All of these instantiations are not required to be stored in each locale object.

The facet ctype is a template class parameterized with a character type. Three kinds of functions are provided by the class ctype<charT>:

Table 14.16 lists the members defined for the facet ctype.

The function is(beg,end, vec) is used to store a set of masks in an array. For each of the characters in the range between beg and end, a mask with the attributes corresponding to the character is stored in the array pointed to by vec. This is useful to avoid virtual function calls for the classification of characters if there are lots of characters to be classified.

The function widen() can be used to convert a character of type char from the native character set to the corresponding character in the character set used by a locale. Thus, it makes sense to widen a character even if the result is also of type char. For the opposite direction, the function narrow() can be used to convert a character from the character set used by the locale to a corresponding char in the native character set, provided there is such a char. For example, the following code converts the decimal digits from char to wchar_t:

    std::locale loc;
    char narrow[] = "0123456789";
    wchar_t wide [10];


    std::use_facet<std::ctype<wchar_t> >(loc).widen(narrow, narrow+10,
                                                    wide);

Class ctype derives from the class ctype_base. This class is used only to define an enumeration called mask. This enumeration defines values that can be combined to form a bitmask used for testing character properties. The values defined in ctype_base are shown in Table 14.17. The functions for character classification all take a bitmask as an argument, which is formed by combinations of the values defined in ctype_base. To create bitmasks as needed, you can use the operators for bit manipulation (|, &,^, and ~). A character matches this mask if it is any of the characters identified by the mask.

For better performance of the character classification functions, the facet ctype is specialized for the character type char. This specialization does not delegate the functions dealing with character classification (is(), scan_is(), and scan_not()) to corresponding virtual functions. Instead, these functions are implemented inline using a table lookup. For this case additional members are provided (Table 14.18).

Manipulating the behavior of these functions for specific locales is done with a corresponding table of masks that is passed as a constructor argument:

// create and initialize the table
    std::ctype_base::mask mytable[std::ctype<char>::table_size] = {
         ...
    };


    // use the table for the  ctype<char>facet ct
                 std::ctype<char> ct(mytable, false);

This code constructs a ctype<char> facet that uses the table mytable to determine the character class of a character. More precisely, the character class of the character c is determined by

    mytable[static_cast<unsigned char>(c)]

The static member table_size is a constant defined by the library implementation and gives the size of the lookup table. This size is at least 256 characters. The second optional argument to the constructor of ctype<char> indicates whether the table should be deleted if the facet is destroyed. If it is true, the table passed to the constructor is released by using delete [] when the facet is no longer needed.

The member function table() is a protected member function that returns the table that is passed as the first argument to the constructor. The static protected member function classic_table() returns the table that is used for character classification in the classic C locale.

Convenient use of the ctype facets is provided by predefined global functions. Table 14.19 lists all of the global functions.

For example, the following expression determines whether the character c is a lowercase letter in the locale loc:

    std::islower(c,loc)

It returns a corresponding value of type bool.

The following expression returns the character c converted to an uppercase letter, if c is a lowercase letter in the locale loc:

    std::toupper(c,loc)

If c is not a lowercase letter, the first argument is returned unmodified.

The expression

    std::islower(c,loc)

is equivalent to the following expression:

    std::use_facet<std::ctype<char> >(loc).is(std::ctype_base::lower,c)

This expression calls the member function is() of the facet ctype<char>. is() determines whether the character c fulfills any of the character properties that are passed as the bitmask in the first argument. The values for the bitmask are defined in the class ctype_base. See page 502 and page 669 for examples of the use of these convenience functions.

The global convenience functions for character classification correspond to C functions that have the same name but only the first argument. They are defined in <cctype> and <ctype.h>, and always use the current global C locale.^[4] Their use is even more convenient:

    if (std::isdigit(c)) {
        ...
    }

However, by using them you can't use different locales in the same program. Also, you can't use a user-defined ctype facet using the C function. See page 497 for an example that demonstrates how to use these C functions to convert all characters of a string to uppercase letters.

It is important to note that the C++ convenience functions should not be used in code sections where performance is crucial. It is much faster to obtain the corresponding facet from the locale and use the functions on this object directly. If a lot of characters are to be classified according to the same locale, this can be improved even more, at least for non-char characters. The function is(beg,end,vec) can be used to determine the masks for typical characters: This function determines for each character in the range [beg,end)amask that describes the properties of the character. The resulting mask is stored in vec at the position corresponding to the character's position. This vector can then be used for fast lookup of the characters.

The facet codecvt is used to convert between internal and external character encoding. For example, it can be used to convert between Unicode and EUC (Extended UNIX Code), provided the implementation of the C++ standard library supports a corresponding facet.

This facet is used by the class basic_filebuf to convert between the internal representation and the representation stored in a file. The class basic_filebuf <charT,traits> (see page 627) uses the instantiation codecvt<charT,char,typename traits::state_type> to do so. The facet used is taken from the locale stored with basic_filebuf. This is the major application of the codecvt facet. Only rarely is it necessary to use this facet directly.

In Section 14.1, some basics of character encodings are introduced. To understand codecvt, you need to know that there are two approaches for the encoding of characters: One is character encodings that use a fixed number of bytes for each character (wide-character representation), and the other is character encodings that use a varying number of bytes per character (multibyte representation).

It is also necessary to know that multibyte representations use so-called shift states for space efficient representation of characters. The correct interpretation of a byte is possible only with the correct shift state at this position. This in turn can be determined only by walking through the whole sequence of multibyte characters (see Section 14.1, for more details).

The codecvt<> facet takes three template arguments:

The intermediate state may consist of incomplete wide characters or the current shift state. The C++ standard makes no restriction about what is stored in the objects representing the state.

The internal representation always uses a representation with a fixed number of bytes per character. Mainly the two types char and wchar_t are intended to be used within a program. The external representation may be a representation that uses a fixed size or a multibyte representation. When a multibyte representation is used, the second template argument is the type used to represent the basic units of the multibyte encoding. Each multibyte character is stored in one or more objects of this type. Normally, the type char is used for this.

The third argument is the type used to represent the current state of the conversion. It is necessary, for example, if one of the character encodings is a multibyte encoding. In this case, the processing of a multibyte character might be terminated because the source buffer is drained or the destination buffer is full while one character is being processed. If this happens, the current state of the conversion is stored in an object of this type.

Similar to the other facets, the standard requires support for only very few conversions. Only the following two instantiations are supported by the C++ standard library:

The C++ standard does not specify the exact semantics of the second conversion. The only natural thing to do, however, is to split each wchar_t into sizeof(wchar_t) objects of type char for the conversion from wchar_t to char, and to assemble a wchar_t from the same amount of chars when converting in the opposite direction. Note that this conversion is very different from the conversion between char and wchar_t done by the widen() and narrow() member functions of the ctype facet: While the codecvt functions use the bits of multiple chars to form one wchar_t (or vice versa), the ctype functions convert a character in one encoding to the corresponding character in another encoding (if there is such a character).

Like the ctype facet, codecvt derives from a base class used to define an enumeration type. This class is named codecvt.base, and it defines an enumeration called result. The values of this enumeration are used to indicate the results of codecvt's members. The exact meanings of the values depend on the member function used. Table 14.20 lists the member functions of the codecvt facet.

The function in() converts an external representation to an internal representation. The argument s is a reference to a stateT. At the beginning, this argument represents the shift state used when the conversion is started. At the end, the final shift state is stored there. The shift state passed in can differ from the initial state if the input buffer to be converted is not the first buffer being converted. The arguments fb (from begin) and fe (from end) are of type const internT*, and represent the beginning and the end of the input buffer. The arguments tb (to begin) and te (to end) are of type externT*, and represent the beginning and the end of the output buffer. The arguments

fn (from next, of type const externT*&) and tn (to next, of type internT*&) are references used to return the end of the sequence converted in the input buffer and the output buffer respectively. Either buffer may reach the end before the other buffer reaches the end. The function returns a value of type codecvt_base:: result, as indicated in Table 14.21.

If ok is returned the function made some progress. If fn == fe holds, this means that the whole input buffer was processed and the sequence between tb and tn contains the result of the conversion. The characters in this sequence represent the characters from the input sequence, potentially with a finished character from a previous conversion. If the argument s passed to in() was not the initial state, a partial character from a previous conversion that was not completed could have been stored there.

If partial is returned, either the output buffer was full before the input buffer could be drained or the input buffer was drained when a character was not yet complete (for example, because the last byte in the input sequence was part of an escape sequence switching between shift states). If fe == fn, the input buffer was drained. In this case, the sequence between tb and tn contains all characters that were converted completely but the input sequence terminated with a partially converted character. The necessary information to complete this character's conversion during a subsequent conversion is stored in the shift state s. If fe ! = fn, the input buffer was not completely drained. In this case, te == tn holds; thus, the output buffer is full. The next time the conversion is continued, it should start with fn.

The return value noconv indicates a special situation. That is, no conversion was necessary to convert the external representation to the internal representation. In this case, fn is set to fb and tn is set to tb. Nothing is stored in the destination sequence because everything is already stored in the input sequence.

If error is returned, that means a source character that could not be converted was encountered. There are several reasons why this can happen. For example, the destination character set has no representation for a corresponding character, or the input sequence ends up with an illegal shift state. The C++ standard does not define any method that can be used to determine the cause of the error more precisely.

The function out() is equivalent to the function in(), except that it converts in the opposite direction. That is, it converts an internal representation to an external representation. The meanings of the arguments and the values returned are the same; only the types of the arguments are swapped. That is, tb and te now have the type const internT*, and fb and fe now have the type const externT*. The same applies to fn and tn.

The function unshift() inserts characters necessary to complete a sequence when the current state of the conversion is passed as the argument s. This normally means that a shift state is switched to the initial switch state. Only the external representation is terminated. Thus, the arguments tb and tf are of type externT*, and tn is of type externT&*. The sequence between tb and te defines the output buffer in which the characters are stored. The end of the result sequence is stored in tn. unshift() returns a value as shown in Table 14.22.

The function encoding() returns some information about the encoding of the external representation. If encoding() returns −1, the conversion is state dependent. If encoding() returns 0, the number of externTs needed to produce an internal character is not constant. Otherwise, the number of externTs need to produce an internT is returned. This information can be used to provide appropriate buffer sizes.

The function always_noconv() returns true if the functions in() and out() never perform a conversion. For example, the standard implementation of codecvt<char, char, mbstate_t> does no conversion, and thus, always_noconv() returns true for this facet. However, this only holds for the codecvt facet from the "C" locale. Other instances of this facet may actually do a conversion.

The function length() returns the number of externTs from the sequence between fb and fe necessary to produce max characters of type internT. If there are fewer than max complete internT characters in the sequence between fb and fe, the number of externTs used to produce a maximum number of internTs from the sequence is returned.