Using istringstream like Scanner in Java

Why doesn't a reference to an object of type istringstream retain its last read character position after passing it to one function after another? I'm trying to use it similar to how a Scanner object in Java works.

The goal is to create a function that loops through the characters of a string, which contains a signed decimal e.g. "-14.6", and return a signed integer out of it.

For that I use istringstream on std::string, then I pass the istringstream object to one function after another, where each function reads a specific part of the decimal.

The first function reads two chars, as white spaces, from the example string " -14.6", from the istringstream object. The problem is that the second function, which should start reading the negative sign, starts reading all over again from the beginning of the string, and the position of the istringstream object is not saved.

The code looks like this:

void read_whitespace (std::istringstream &ss);
bool read_sign (std::istringstream &ss);
std::string read_digit (std::istringstream& ss);
void convert_string(const std::string& s);

int main() {
  std::string s = "  -14.6";
  convert_string(s);
  return 0;
}

void convert_string (const std::string& s) {
  std::istringstream ss(s);
  bool sign = false; 
  // This function correctly reads the two white spaces
  read_whitespace(ss); 
  // But _here_ ss starts reading again from the beginning of s
  read_sign(ss);  
  read_digit(ss);
}

void read_whitespace (std::istringstream &ss) {
  char c;
  while (ss.get(c)) {
    if (c == ' ') {
    } else {
      break;
    }
  }
}

bool read_sign (std::istringstream &ss) {
  char c;
  while (ss.get(c)) {
    if (c == '-') {
      return false;
    } else { break; }
  } 
  return true;
}