从字符串读取浮点值,精度达到6

我必须从字符串中读取一个精度高达6的flot值,当前代码只读取前6位数字。提前谢谢

template <class T>
bool from_string(T& t,    const std::string& s, 
                 std::ios_base& (*f)(std::ios_base&))
{
  std::istringstream iss(s);
  return !(iss >> f >> t).fail();
}

int main()
{
  int i;
  float f;
 // the third parameter of from_string() should be 
  // one of std::hex, std::dec or std::oct
  if(from_string<int>(i, std::string("ff"), std::hex))
  {
    std::cout << i << std::endl;
  }
  else
  {
    std::cout << "from_string failed" << std::endl;
  }

  if(from_string<float>(f, std::string("1456.909"), std::dec))
  {
    std::cout << f << std::endl;
  }
  else
  {
    std::cout << "from_string failed" << std::endl;
  }
  return 0;
}