std::num_ge...get::do_get(3)

📦 version 2024.07.05, in nixpkgs unstable, opensuse-leap 16.0

🌸 NAME 🌸

okay so bestie, this is std::num_ge...get::do_get(3) from the C++ Standard Libary manual, and it's giving numeric-input main-character energy 💅✨

std::num_get::get, std::num_get::do_get — yep, both of them: std::num_get::get,std::num_get::do_get. periodt.

✨ 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;
          protected:

          virtual iter_type do_get( iter_type in,
          iter_type end, std::ios_base& str,
                                                           (1)

          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;
          virtual iter_type do_get( iter_type in,
          iter_type end, std::ios_base& str,                   (2)

          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. The actual reading era — reads characters from the input iterator in and generates the value of the type of v, taking into account I/O 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 (since C++11).
  • 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 according to the rules of std::scanf. (until C++11)
  • The input is parsed as if by:
    • std::strtoll for signed integer v,
    • std::strtoull for unsigned integer v, (since C++11)
    • 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 💅

okay so these are the little historical receipts, bestie — stuff that used to be broken before the committee fixed it. ngl they're kind of iconic.

  • Before the resolutions of LWG issue 23 and LWG issue 696, v was left unchanged if an error occurs.
  • Before the resolution of LWG issue 221, strings representing hexadecimal integers (e.g. "0xA0") were rejected by do_get(int) even if they are valid input to strtol because stage 2 filters out characters 'X' and 'x'. 💀 imagine rejecting a whole valid hex string, couldn't be stage 2.
  • Before the resolution of LWG issue 1169, converting a negative number string into an unsigned integer might produce zero (since the value represented by the string is smaller than what the target type can represent).
  • Before the resolution of LWG issue 2381, strings representing hexadecimal floating-point numbers with exponents (e.g. "0x1.23p-10") were rejected by do_get(double) even if they are valid input to strtod because stage 2 filters out characters 'P' and 'p'. (since C++11)
  • The strings representing infinity or not-a-number (e.g. "NaN" and "inf") are rejected by do_get(double) even if they are valid input to strtod because stage 2 filters out characters such as 'N' or 'i'.
🫶 lowkey important: that last one is still the behaviour, no LWG issue saving you here. stage 2 is out here filtering 'N' and 'i' and your "NaN" is just not making it through, periodt.

🌸 EXAMPLE 🌸

an implementation of operator>> for a user-defined type. yes she's custom, yes she slays. 👑

// 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;
}

like, the sentry is doing the vibe check on your stream first (is it ready for input?), then use_facet pulls the num_get facet out of the locale and calls .get — and the try/catch is there because setting err could throw. main character error handling fr fr. ✨

🎀 DEFECT REPORTS 🎀

The following behavior-changing defect reports were applied retroactively to previously published C++ standards. (aka: the glow-ups that got applied to old standards after the fact, retroactively, iykyk 💅)

  • LWG 17 — applied to C++98. published: the process of parsing text boolean values was errornous. corrected: corrected.
  • LWG 18 — applied to C++98. published: the overload of get taking bool& value was missing. corrected: added.
  • LWG 23 — applied to C++98. published: overflowing input resulted in undefined behavior. corrected: overflow handled.
  • LWG 154 — applied to C++98. published: the conversion specifier for double was %lg. corrected: changed to %g (same as float).
  • LWG 221 — applied to C++98. published: do_get did not parse 'x' and 'X' while strtol parsed them. corrected: made 'x' and 'X' parsed.
  • LWG 275 — applied to C++98. published: get had an overload taking short& value instead of float&. corrected: corrected.
  • LWG 358 — applied to C++98. published: thousand separators after the decimal point were ignored. corrected: stage 2 is terminated if encountered.
  • LWG 696 — applied to C++98. published: the result was unchanged on conversion failure. corrected: set to zero.
  • LWG 1169 — applied to C++98. published: overflow handling was inconsistent between floating-point types. corrected: made consistent with strtof/strtod.
  • LWG 2381 — applied to C++11. published: do_get did not parse 'p' and 'P' while strtod parsed them. corrected: made 'p' and 'P' parsed.
😭 not LWG 154 changing the specifier and the "corrected" column just being the one word corrected — she said "fixed, next question."

🦋 SEE ALSO 🦋

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

and that's the whole page, bestie!! you made it to the end, so proud of you 🫶💖 this was std::num_ge...get::do_get(3), straight from cppreference.com, dated 2024.06.10 — go forth and parse responsibly, mwah 💅✨

📦 where this page comes from