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Showing content from https://en.cppreference.com/w/cpp/language/../error/error_code/../../locale/num_put/put.html below:

std::num_put<CharT,OutputIt>::put, std::num_put<CharT,OutputIt>::do_put - cppreference.com

1) Public member function, calls the protected virtual member function do_put of the most derived class.

Writes characters to the output sequence

which represent the value of

, formatted as requested by the formatting flags

and the

and

facets of the locale imbued in the stream

. This function is called by all formatted output stream operators, such as

.

Conversion occurs in four stages:

• I/O format flags are obtained, as if by

fmtflags basefield = (str.flags() & std::ios_base::basefield);

fmtflags uppercase = (str.flags() & std::ios_base::uppercase);

fmtflags floatfield = (str.flags() & std::ios_base::floatfield);

fmtflags showpos = (str.flags() & std::ios_base::showpos);

fmtflags showbase = (str.flags() & std::ios_base::showbase);

fmtflags showpoint = (str.flags() & std::ios_base::showpoint);

• If the type of val is bool:
  • If boolalpha == 0, then converts val to type int and performs integer output.
  • If boolalpha != 0, obtains std::use_facet<std::numpunct<CharT>>(str.getloc()).truename() if val == true or std::use_facet<std::numpunct<CharT>>(str.getloc()).falsename() if val == false, and outputs each successive character c of that string to out with *out++ = c. No further processing is done in this case, the function returns out.
• If the type of val is an integer type, the first applicable choice of the following is selected:
  • If basefield == oct, will use conversion specifier %o.
  • If basefield == hex && !uppercase, will use conversion specifier %x.
  • If basefield == hex, will use conversion specifier %X.
  • If the type of val is signed, will use conversion specifier %d.
  • If the type of val is unsigned, will use conversion specifier %u.
• For integer types, length modifier is added to the conversion specification if necessary: l for long and unsigned long, ll for long long and unsigned long long(since C++11).
• If the type of val is a floating-point type, the first applicable choice of the following is selected:

• If floatfield == std::ios_base::scientific && !uppercase, will use conversion specifier %e.
• If floatfield == std::ios_base::scientific, will use conversion specifier %E.

• If !uppercase, will use conversion specifier %g.
• Otherwise, will use conversion specifier %G.

Also:

• If the type of val is long double, the length modifier L is added to the conversion specifier.
• If the type of val is a floating-point type and floatfield != (ios_base::fixed | ios_base::scientific)(since C++11), the precision modifier is added and set to str.precision(). Otherwise, no precision is specified.

• For both integer and floating-point types, if showpos is set, the modifier + is prepended.
• For integer types, if showbase is set, the modifier # is prepended.
• For floating-point types, if showpoint is set, the modifier # is prepended.
• If the type of val is void*, will use conversion specifier %p
• A narrow character string is created as if by a call to std::printf(spec, val) in the "C" locale, where spec is the chosen conversion specifier.

• Every character c obtained in Stage 1, other than the decimal point '.', is converted to CharT by calling std::use_facet<std::ctype<CharT>>(str.getloc()).widen(c).
• For arithmetic types, the thousands separator character, obtained from std::use_facet<std::numpunct<CharT>>(str.getloc()).thousands_sep(), is inserted into the sequence according to the grouping rules provided by std::use_facet<std::numpunct<CharT>>(str.getloc()).grouping().
• Decimal point characters ('.') are replaced by std::use_facet<std::numpunct<CharT>>(str.getloc()).decimal_point().

• The adjustment flag is obtained as if by std::fmtflags adjustfield = (flags & (std::ios_base::adjustfield)) and examined to identify padding location, as follows:
  • If adjustfield == std::ios_base::left, will pad after.
  • If adjustfield == std::ios_base::right, will pad before.
  • If adjustfield == std::ios_base::internal and a sign character occurs in the representation, will pad after the sign.
  • If adjustfield == std::ios_base::internal and Stage 1 representation began with 0x or 0X, will pad after the x or X.
  • Otherwise, will pad before.
• If str.width() is non-zero (e.g. std::setw was just used) and the number of CharT's after Stage 2 is less than str.width(), then copies of the fill character are inserted at the position indicated by padding to bring the length of the sequence to str.width().

In any case, str.width(0) is called to cancel the effects of std::setw.

Every successive character c from the sequence of CharT's from Stage 3 is output as if by *out++ = c.

out

The leading zero generated by the conversion specification #o (resulting from the combination of std::showbase and std::oct for example) is not counted as a padding character.

When formatting a floating point value as hexfloat (i.e., when floatfield == (std::ios_base::fixed | std::ios_base::scientific)), the stream's precision is not used; instead, the number is always printed with enough precision to exactly represent the value.

Output a number using the facet directly, and demonstrate user-defined facet:

#include <iostream>
#include <locale>
 
// this custom num_put outputs squares of all integers (except long long)
struct squaring_num_put : std::num_put<char>
{
    iter_type do_put(iter_type out, std::ios_base& str,
                     char_type fill, long val) const
    {
        return std::num_put<char>::do_put(out, str, fill, val * val);
    }
 
    iter_type do_put(iter_type out, std::ios_base& str,
                     char_type fill, unsigned long val) const
    {
        return std::num_put<char>::do_put(out, str, fill, val * val);
    }
};
 
int main()
{
    auto& facet = std::use_facet<std::num_put<char>>(std::locale());
    facet.put(std::cout, std::cout, '0', 2.71);
    std::cout << '\n';
 
    std::cout.imbue(std::locale(std::cout.getloc(), new squaring_num_put));
    std::cout << 6 << ' ' << -12 << '\n';
}

Output:

An implementation of operator<< for a user-defined type.

Output:

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

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