NAME¶

std::num_get::get,std::num_get::do_get - std::num_get::get,std::num_get::do_get

Synopsis¶

public:

iter_type get( iter_type in, iter_type end,
std::ios_base& str,

std::ios_base::iostate& err, bool& v ) const;
iter_type get( iter_type in, iter_type end,
std::ios_base& str,
std::ios_base::iostate& err, long& v ) const;
iter_type get( iter_type in, iter_type end,
std::ios_base& str, (since C++11)
std::ios_base::iostate& err, long long& v )
const;
iter_type get( iter_type in, iter_type end,
std::ios_base& str,
std::ios_base::iostate& err, unsigned short& v )
const;
iter_type get( iter_type in, iter_type end,
std::ios_base& str,
std::ios_base::iostate& err, unsigned int& v )
const;
iter_type get( iter_type in, iter_type end,
std::ios_base& str,
std::ios_base::iostate& err, unsigned long& v )
const;
iter_type get( iter_type in, iter_type end,
std::ios_base& str, (since C++11)
std::ios_base::iostate& err, unsigned long long&
v ) const;
iter_type get( iter_type in, iter_type end,
std::ios_base& str,
std::ios_base::iostate& err, float& v ) const;
iter_type get( iter_type in, iter_type end,
std::ios_base& str,
std::ios_base::iostate& err, double& v ) const;
iter_type get( iter_type in, iter_type end,
std::ios_base& str,
std::ios_base::iostate& err, long double& v )
const;
iter_type get( iter_type in, iter_type end,
std::ios_base& str,
std::ios_base::iostate& err, void*& v ) const; (1)
protected:

virtual iter_type do_get( iter_type in,
iter_type end, std::ios_base& str,

std::ios_base::iostate& err, bool& v ) const;
virtual iter_type do_get( iter_type in,
iter_type end, std::ios_base& str,
std::ios_base::iostate& err, long& v ) const;
virtual iter_type do_get( iter_type in,
iter_type end, std::ios_base& str, (since C++11)
std::ios_base::iostate& err, long long& v )
const;
virtual iter_type do_get( iter_type in,
iter_type end, std::ios_base& str,
std::ios_base::iostate& err, unsigned short& v )
const;
virtual iter_type do_get( iter_type in,
iter_type end, std::ios_base& str,
std::ios_base::iostate& err, unsigned int& v )
const; (2)
virtual iter_type do_get( iter_type in,
iter_type end, std::ios_base& str,
std::ios_base::iostate& err, unsigned long& v )
const;
virtual iter_type do_get( iter_type in,
iter_type end, std::ios_base& str, (since C++11)
std::ios_base::iostate& err, unsigned long long&
v ) const;
virtual iter_type do_get( iter_type in,
iter_type end, std::ios_base& str,
std::ios_base::iostate& err, float& v ) const;
virtual iter_type do_get( iter_type in,
iter_type end, std::ios_base& str,
std::ios_base::iostate& err, double& v ) const;
virtual iter_type do_get( iter_type in,
iter_type end, std::ios_base& str,
std::ios_base::iostate& err, long double& v )
const;
virtual iter_type do_get( iter_type in,
iter_type end, std::ios_base& str,
std::ios_base::iostate& err, void*& v ) const;

1) Public member function, calls the member function do_get of the most derived
class.
2) Reads characters from the input iterator in and generates the value of the type
of v, taking into account IO stream formatting flags from str.flags(), character
classification rules from std::use_facet<std::ctype<charT>>(str.getloc()), and
numeric punctuation characters from
std::use_facet<std::numpunct<charT>>(str.getloc()). This function is called by all
formatted input stream operators such as std::cin >> n;.

Conversion occurs in three stages

Stage 1: conversion specifier selection¶

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

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

* If the type of v is an integer type, the first applicable choice of the
following five is selected:

If basefield == oct, will use conversion specifier %o
If basefield == hex, will use conversion specifier %X
If basefield == 0, will use conversion specifier %i
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: h for short and unsigned short, l for long and unsigned long, ll for
long long and unsigned long long
* If the type of v is float, will use conversion specifier %g
* If the type of v is double, will use conversion specifier %lg
* If the type of v is long double, will use conversion specifier %Lg
* If the type of v is void*, will use conversion specifier %p
* If the type of v is bool and boolalpha==0, proceeds as if the type of v is long,
except for the value to be stored in v in stage 3.
* If the type of v is bool and boolalpha!=0, the following replaces stages 2 and
3:

* Successive characters obtained from the input iterator in are matched
against the character sequences obtained from
std::use_facet<std::numpunct<charT>>(str.getloc()).falsename() and
std::use_facet<std::numpunct<charT> >(str.getloc()).truename() only as
necessary as to identify the unique match. The input iterator in is
compared to end only when necessary to obtain a character.
* If the target sequence is uniquely matched, v is set to the corresponding
bool value. Otherwise false is stored in v and std::ios_base::failbit is
assigned to err. If unique match could not be found before the input ended
(in==end), err|=std::ios_base::eofbit is executed.

Stage 2: character extraction¶

* If in==end, Stage 2 is terminated immediately, no further characters are
extracted
* The next character is extracted from in as if by char_type ct = *in;

* If the character matches one of
"0123456789abcdefxABCDEFX+-"
(until C++11)
"0123456789abcdefpxABCDEFPX+-"
(since C++11), widened to the locale's char_type as if by
std::use_facet<std::ctype<charT>>(str.getloc()).widen(), it is converted to
the corresponding char.
* If the character matches the decimal point separator
(std::use_facet<std::numpunct<charT>>(str.getloc()).decimal_point())), it
is replaced by '.'.
* If the character matches the thousands separator
(std::use_facet<std::numpunct<charT>>(str.getloc()).thousands_sep()) and
the thousands separation is in use (as determined by
std::use_facet<std::numpunct<charT>>(str.getloc()).grouping().length() !=
0), then if the decimal point '.' has not yet been accumulated, the
position of the character is remembered, but the character is otherwise
ignored. If the decimal point has already been accumulated, the character
is discarded and Stage 2 terminates.
* In any case, the check is made whether the char obtained from the previous
steps is allowed in the input field that would be parsed by std::scanf
given the conversion specifier selected in Stage 1. If it is allowed, it is
accumulated in a temporary buffer and Stage 2 repeats. If it is not
allowed, Stage 2 terminates.

Stage 3: conversion and storage¶

* The sequence of chars accumulated in Stage 2 is converted to a numeric value

The input is parsed as if by std::strtoll for signed integer v,
std::strtoull for unsigned integer v, std::strtof for float v, std::strtod
for double v, or std::strtold for long double v

* If the conversion function fails to convert the entire field, the value 0
is stored in v
* If the type of v is a signed integer type and the conversion function results in
a positive or negative value too large to fit in it, the most positive or
negative representable value is stored in v, respectively
* If the type of v is an unsigned integer type and the conversion function results
in a value that does not fit in it, the most positive representable value is
stored in v.
* In any case, if the conversion function fails std::ios_base::failbit is assigned
to err
* Otherwise, the numeric result of the conversion is stored in v

* If the type of v is bool and boolalpha is not set, then if the value to be
stored is 0, false is stored, if the value to be stored is 1, true is
stored, for any other value std::ios_base::failbit is assigned to err and
true is stored.

* After this, digit grouping is checked. if the position of any of the thousands
separators discarded in Stage 2 does not match the grouping provided by
std::use_facet<std::numpunct<charT>>(str.getloc()).grouping(),
std::ios_base::failbit is assigned to err.
* If Stage 2 was terminated by the test in==end, err|=std::ios_base::eofbit is
executed to set the eof bit.

Return value¶

in

Notes¶

C++98/C++03 specified that v is left unchanged if an error occurs. Such behavior is
corrected by LWG23 and LWG696 which are merged into C++11.

When converting a negative number string into an unsigned integer, some
implementations produce zero (since the value represented by the string is smaller
than what the target type can represent). Such behavior was specified in C++11/14
standard but is corrected by LWG1169.

Because stage 2 filters out characters such as 'N' or 'i', the strings "NaN" or
"inf" is rejected by do_get(double) even if they are valid input to strtod.
Hexadecimal floating-point numbers such "0x1.23p-10" used to be rejected by do_get,
which is a defect fixed by LWG2381.

Example¶

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

// Run this code

#include <iostream>
#include <iterator>
#include <locale>

struct base { long x; };

template <class CharT, class Traits>
std::basic_istream<CharT, Traits>&
operator >>(std::basic_istream<CharT, Traits>& is,
base& b)
{
std::ios_base::iostate err = std::ios_base::goodbit;
try // setting err could throw
{
typename std::basic_istream<CharT, Traits>::sentry s(is);

if (s) // if stream is ready for input
{
std::use_facet<std::num_get<CharT>>(is.getloc()).get(is, {}, is, err, b.x);
}
} catch(std::ios_base::failure& error)
{
// handle the exception
}
return is;
}

int main()
{
base b;

std::cin >> b;
}

Defect reports

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

DR Applied to Behavior as published Correct behavior
LWG 23 C++98 overflowing input resulted in undefined overflow handled
behavior
LWG 696 C++98 the result was unchanged on conversion set to zero
failure
LWG 1169 C++98 overflow handling was inconsistent made consistent with
between floating-point types strtof/strtod
LWG 2381 C++11 do_get did not parse 'p' and 'P' while made 'p' and 'P' parsed
strtod parsed them

See also¶

operator>> extracts formatted data
(public member function of std::basic_istream<CharT,Traits>)