std::num_put

Class std::num_put encapsulates the rules for formatting numeric values as strings. Specifically, the types bool, long, unsigned long, long long, unsigned long long(since C++11), double, long double, void*, and of all types implicitly convertible to these (such as int or float) are supported. The standard formatting output operators (such as cout << n;) use the std::num_put facet of the I/O stream's locale to generate text representation of numbers.

cpp/locale/locale/facet

std-num put-inheritance.svg

Inheritance diagram

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

#include <iostream>
#include <iterator>
#include <locale>
#include <string>
 
int main()
{
 double n = 1234567.89;
 std::cout.imbue(std::locale ("de_DE.UTF-8"));
 std::cout << "Direct conversion to string:\n"
 << std::to_string (n) << '\n'
 << "Output using a german locale:\n"
 << std::fixed << n << '\n'
 << "Output using an american locale:\n";
 
 // use the facet directly
 std::cout.imbue(std::locale ("en_US.UTF-8"));
 auto& f = std::use_facet <std::num_put<char>>(std::cout.getloc());
 f.put(std::ostreambuf_iterator <char>(std::cout ), std::cout, ' ', n);
 std::cout << '\n';
}

Possible output:

Direct conversion to string:
1234567.890000
Output using a german locale:
1.234.567,890000
Output using an american locale:
1,234,567.890000

[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_put` 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_put`	can guarantee to accept implementation- defined character container types