std::num_get::get

// num_get example
#include <iostream>       // std::cin, std::cout, std::ios
#include <locale>         // std::locale, std::num_get, std::use_facet
#include <iterator>       // std::istreambuf_iterator
#include <string>         // std::string

int main ()
{
  std::locale loc;
  std::ios::iostate state;
  float mypi,yourpi;

  std::string number ("3.14159");
  // get from string:
  std::use_facet<std::num_get<char,std::string::iterator> > (loc).get
    (number.begin(), number.end(), std::cin, state, mypi);

  std::cout << "Please, enter PI: ";
  // get from istream:
  std::use_facet<std::num_get<char> >(loc).get
    (std::cin, std::istreambuf_iterator<char>(), std::cin, state, yourpi);

  if ( (mypi-yourpi>0.01) || (mypi-yourpi<-0.01) )
    std::cout << "Wrong!\n";
  else
    std::cout << "Right!\n";

  return 0;
}