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    std::num_get

    From cppreference.com
     
    C++
     
    Text processing library
     
    Localization library
     
    std::num_get
     
    Defined in header <locale>
    template< class CharT, class InputIt = std::istreambuf_iterator<CharT> > class num_get;
    See More

    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.

    cpp/locale/locale/facet

    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.

    Specializations

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

    Defined in header <locale>
    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:

    Nested types

    Type Definition
    char_type CharT
    iter_type InputIt

    Data members

    Member Description
    std::locale::id id [static] the identifier of the facet

    Member functions

    		 constructs a new num_get facet
    (public member function)
    		invokes do_get
    (public member function)

    Protected member functions

    		 destructs a num_get facet
    (protected member function)
    [virtual]
    		 parses a number from an input stream
    (virtual protected member function)

    Example

    Run this code
    #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

    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

    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]