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std::num_get<CharT,InputIt>::get, std::num_get<CharT,InputIt>::do_get

(1) 
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, 
               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, 
               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;
(2) 
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, 
                          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;
 
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, 
                          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 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+-", 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 at all 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::scanf with the conversion specifier selected in Stage 1 (until C++11)
The input is parsed as if by std::strtoll for signed integer v, std::strtoull for unsigned integer v, or std::strtold for floating-point v (since C++11)
(until C++17)
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 (since C++17)
  • If the conversion function fails to convert the entire field, the value ​0​ is stored in v
  • If the conversion function results in a positive value too large to fit in the type of v, the most positive representable value is stored in v
  • If the conversion function results in a negative value too large to fit in the type of v, the most negative representable value is stored in v, or zero for unsigned integer types. (until C++17)
(since C++11)
  • 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

In C++98/C++03, if an error occurs, v is left unchanged. In C++11, it is set to a value as described above.

The result of converting a negative number string into an unsigned integer was specified to produce zero until C++17, although some implementations followed the protocol of std::strtoull which negates in the target type, giving ULLONG_MAX for "-1", and so produce the largest value of the target type instead. As of C++17, strictly following std::strtoull is the correct behavior.

Because stage 2 filters out characters such as 'p', 'N' or 'i', the hexadecimal floating-point numbers such as "0x1.23p-10" and the strings "NaN" or "inf" may be rejected by do_get(double) even if they are valid input to strtod: this is LWG #2381.

Example

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

#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;
}

See also

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

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