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Showing content from https://en.cppreference.com/w/cpp/language/../error/error_code/../../../cpp/locale/num_get.html below:

std::num_get - cppreference.com

Class std::num_get encapsulates the rules for parsing string representations of numeric values. Specifically, types bool, unsigned short, unsigned int, long, unsigned long, long long, unsigned long long(since C++11), float, double, long double, and void* are supported. The standard formatting input operators (such as cin >> n;) use the std::num_get facet of the I/O stream's locale to parse the text representations of the numbers.

Inheritance diagram

If a std::num_get specialization is not guaranteed to be provided by the standard library (see below), the behaviors of its get() and do_get() are not guaranteed as specified.

[edit] Specializations

The standard library is guaranteed to provide the following specializations (they are required to be implemented by any locale object):

std::num_get<char> creates narrow string parsing of numbers std::num_get<wchar_t> creates wide string parsing of numbers

In addition, the standard library is also guaranteed to provide every specialization that satisfies the following type requirements:

[edit] Nested types Type Definition char_type CharT iter_type InputIt [edit] Data members [edit] Member functions constructs a new num_get facet
(public member function) invokes do_get
(public member function) [edit] Protected member functions destructs a num_get facet
(protected member function) parses a number from an input stream
(virtual protected member function) [edit] Example 
#include <iostream>
#include <iterator>
#include <locale>
#include <sstream>
#include <string>
 
int main()
{
    std::string de_double = "1.234.567,89";
    std::string us_double = "1,234,567.89";
 
    // parse using streams
    std::istringstream de_in(de_double);
    de_in.imbue(std::locale("de_DE.UTF-8"));
    double f1;
    de_in >> f1;
 
    std::istringstream us_in(de_double);
    us_in.imbue(std::locale("en_US.UTF-8"));
    double f2;
    us_in >> f2;
 
    std::cout << "Parsing " << de_double << " as double gives " << std::fixed
              << f1 << " in de_DE locale and " << f2 << " in en_US\n";
 
    // use the facet directly
    std::istringstream s3(us_double);
    s3.imbue(std::locale("en_US.UTF-8"));
 
    auto& f = std::use_facet<std::num_get<char>>(s3.getloc());
    std::istreambuf_iterator<char> beg(s3), end;
    double f3;
    std::ios::iostate err;
    f.get(beg, end, s3, err, f3);
 
    std::cout << "parsing " << us_double
              << " as double using raw en_US facet gives " << f3 << '\n';
}

Output:

Parsing 1.234.567,89 as double gives 1234567.890000 in de_DE locale and 1.234000 in en_US
parsing 1,234,567.89 as double using raw en_US facet gives 1234567.890000
[edit] 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 427 C++98 num_get was guaranteed to accept any CharT that
meets the requirements for a character on which
any of the iostream components can be instantiated only guarantees to accept char,
wchar_t and other implementation-
defined character types LWG 2392 C++98 only character type CharT could be
guaranteed to be accepted by num_get can guarantee to accept implementation-
defined character container types [edit] See also defines numeric punctuation rules
(class template) [edit] formats numeric values for output as character sequence
(class template) [edit] extracts formatted data
(public member function of std::basic_istream<CharT,Traits>) [edit]

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