std::num_get::get, std::num_get::do_get - cppreference.com

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# std::num_get::get, std::num_get::do_get

From cppreference.com
< cpp‎ | locale‎ | num 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: iter_type do_get( iter_type in, iter_type end, std::ios_base& str, std::ios_base::iostate& err, bool& v ) const; |  |
iter_type do_get( iter_type in, iter_type end, std::ios_base& str, std::ios_base::iostate& err, long& v ) const; |  |
iter_type do_get( iter_type in, iter_type end, std::ios_base& str, std::ios_base::iostate& err, long long& v ) const; |  |
iter_type do_get( iter_type in, iter_type end, std::ios_base& str, std::ios_base::iostate& err, unsigned short& v ) const; |  |
iter_type do_get( iter_type in, iter_type end, std::ios_base& str, std::ios_base::iostate& err, unsigned int& v ) const; |  |
iter_type do_get( iter_type in, iter_type end, std::ios_base& str, std::ios_base::iostate& err, unsigned long& v ) const; |  |
iter_type do_get( iter_type in, iter_type end, std::ios_base& str, std::ios_base::iostate& err, unsigned long long& v ) const; |  |
iter_type do_get( iter_type in, iter_type end, std::ios_base& str, std::ios_base::iostate& err, float& v ) const; |  |
iter_type do_get( iter_type in, iter_type end, std::ios_base& str, std::ios_base::iostate& err, double& v ) const; |  |
iter_type do_get( iter_type in, iter_type end, std::ios_base& str, std::ios_base::iostate& err, long double& v ) const; |  |
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

#### [edit] 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.

#### [edit] 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 rememebered, 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.

#### [edit] 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)

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` | (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.

### [edit] Return value

`in`

### [edit] 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.

### [edit] 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::istreambuf_iterator<CharT, Traits> end;
                std::use_facet<std::num_get<CharT>>(is.getloc()).get(is, end, is, err, b.x);
            }
        } catch(std::ios_base::failure& error)
        {
            // handle the exception
        }
        return is;
    }

    int main()
    {
        base b;

        std::cin >> b;
    }

### [edit] See also

operator>> | extracts formatted data (public member function of `std::basic_istream`) [edit]

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