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std::num_put::put, std::num_put::do_put

Defined in header `<locale>`
	(1)
public: iter_type put( iter_type out, std::ios_base& str, char_type fill, bool v ) const;
iter_type put( iter_type out, std::ios_base& str, char_type fill, long v ) const;
iter_type put( iter_type out, std::ios_base& str, char_type fill, long long v ) const;
iter_type put( iter_type out, std::ios_base& str, char_type fill, unsigned long v ) const;
iter_type put( iter_type out, std::ios_base& str, char_type fill, unsigned long long v ) const;
iter_type put( iter_type out, std::ios_base& str, char_type fill, double v ) const;
iter_type put( iter_type out, std::ios_base& str, char_type fill, long double v ) const;
iter_type put( iter_type out, std::ios_base& str, char_type fill, const void* v ) const;
		(2)
protected: virtual iter_type do_put( iter_type out, std::ios_base& str, char_type fill, bool v ) const;
virtual iter_type do_put( iter_type out, std::ios_base& str, char_type fill, long v ) const;
virtual iter_type do_put( iter_type out, std::ios_base& str, char_type fill, long long v ) const;
virtual iter_type do_put( iter_type out, std::ios_base& str, char_type fill, unsigned long ) const;
virtual iter_type do_put( iter_type out, std::ios_base& str, char_type fill, unsigned long long ) const;
virtual iter_type do_put( iter_type out, std::ios_base& str, char_type fill, double v ) const;
virtual iter_type do_put( iter_type out, std::ios_base& str, char_type fill, long double v ) const;
virtual iter_type do_put( iter_type out, std::ios_base& str, char_type fill, const void* v ) const;

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

2) Writes characters to the output sequence out which represent the value of v, formatted as requested by the formatting flags str.flags() and the std::numpunct and std::ctype facets of the locale imbued in the stream str. This function is called by all formatted output stream operators, such as std::cout << n;.

Conversion occurs in four stages.

Stage 1: conversion specifier selection

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 v is bool
- If boolalpha == 0, then converts v to type int and performs integer output.
- If boolalpha != 0 obtains std::use_facet<std::numpunct<charT>>(str.getloc()).truename() if v == true or std::use_facet<std::numpunct<charT>>(str.getloc()).falsename() if v == 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 v is an integer type, the 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 v is signed, will use conversion specifier %d

If the type of v 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.
If the type of v is a floating-point type, the the first applicable choice of the following is selected:

If floatfield == std::ios_base::fixed, will use conversion specifier %f

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

If floatfield == std::ios_base::scientific, will use conversion specifier %E

If floatfield == (std::ios_base::fixed | std::ios_base::scientific) && !uppercase, will use conversion specifier %a

If floatfield == (std::ios_base::fixed | std::ios_base::scientific), will use conversion specifier %A

(since C++11)

If !uppercase, will use conversion specifier %g

otherwise, will use conversion specifier %G

If the type of v is long double, the length modifier L is added to the conversion specifier.
Additionally, if floatfield != (ios_base::fixed | ios_base::scientific), then (since C++11) precision modifier is added, set to str.precision()

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 v is void*, will use conversion specifier %p
A narrow character string is created as if by a call to std::printf(spec, v) in the "C" locale, where spec is the chosen conversion specifier.

Stage 2: locale-specific conversion

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()

Stage 3: padding

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.

Stage 4: output

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

Parameters

out	-	iterator pointing to the first character to be overwritten
str	-	stream to retrieve the formatting information from
fill	-	padding character used when the results needs to be padded to the field width
v	-	value to convert to string and output

Return value

out.

Notes

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. (since C++11)

Example

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 s, std::ios_base& f,
                     char_type fill, long v) const
    {
        return std::num_put<char>::do_put(s, f, fill, v*v );
    }
 
    iter_type do_put(iter_type s, std::ios_base& f,
                     char_type fill, unsigned long v) const
    {
        return std::num_put<char>::do_put(s, f, fill, v*v);
    }
};
 
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:

2.71
36 144

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

#include <iostream>
#include <iterator>
#include <locale>
 
struct base { long x = 10; };
 
template <class CharT, class Traits>
std::basic_ostream<CharT, Traits>&
    operator<< (std::basic_ostream<CharT, Traits>& os, base const& b)
{
    try {
        typename std::basic_ostream<CharT, Traits>::sentry s(os);
        if (s)
        {
            std::ostreambuf_iterator<CharT, Traits> it(os);
            std::use_facet<std::num_put<CharT>>(os.getloc())
                .put(it, os, os.fill(), b.x);
        }
    } catch (...) {
        // set badbit on os and rethrow if required
    }
    return os;
}
 
int main()
{
    base b;
 
    std::cout << b;
}