std::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.

cpp/locale/locale/facet

std-num get-inheritance.svg

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.

#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