std::num_get

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Execution support library (C++26)

Technical specifications

Symbols index

External libraries

 

Localization library

Locales and facets
Locales
has_facet use_facet	locale
Facet category base classes
ctype_base codecvt_base messages_base	time_base money_base
ctype facets
ctype ctype<char> ctype_byname	codecvt codecvt_byname
numeric facets
num_get num_put	numpunct numpunct_byname
collate facets
collate	collate_byname
time facets
time_get time_put	time_get_byname time_put_byname
monetary facets
money_get money_put	moneypunct moneypunct_byname
messages facets
messages	messages_byname

Character classification and conversion
Character classification
isspace isblank (C++11) iscntrl	isupper islower isalpha	ispunct isdigit isxdigit	isalnum isprint isgraph

Character conversions

toupper

tolower

String and stream conversions

wstring_convert (C++11/17/26*)

wbuffer_convert (C++11/17/26*)

Text encoding identifications

text_encoding (C++26)

Unicode conversion facets

codecvt_utf8 (C++11/17/26*)

codecvt_utf16 (C++11/17/26*)

codecvt_utf8_utf16 (C++11/17/26*)

codecvt_mode (C++11/17/26*)

C library locales
LC_ALLLC_COLLATELC_CTYPELC_MONETARYLC_NUMERICLC_TIME	setlocale localeconv lconv

 

std::num_get

Member functions

num_get::num_get

num_get::~num_get

num_get::getnum_get::do_get

 

Defined in header <locale>
template<     class CharT,     class InputIt = std::istreambuf_iterator<CharT> > class num_get;

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.

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:

• CharT is one of
  • char,
  • wchar_t, and
  • any other implementation-defined character container type that meets the requirements for a character on which any of the iostream components can be instantiated; and
• InputIt must meet the requirements of LegacyInputIterator.

Member types

Member type	Definition
char_type	CharT
iter_type	InputIt

Member functions

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

Member objects

static std::locale::id id

id of the locale (public member object)

Protected member functions

(destructor)	destructs a num_get facet (protected member function)
do_get [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

numpunct	defines numeric punctuation rules (class template)
num_put	formats numeric values for output as character sequence (class template)
operator>>	extracts formatted data (public member function of std::basic_istream<CharT,Traits>)