C++ - How am I misusing ignore with istringstream?

Question

I plan in main code to ask user to input phone number in form of [phone], which will be sent to my PhoneNumber constructor then sent to setPhoneNumber for breaking it up, setting my private variables, and analysis for errors. Additionally, I wanted to write my setPhoneNumber code so it accounts for user errors in the original input. My PhoneNumber.h code is:

// PhoneNumber.h
#ifndef PHONENUMBER_H
#define PHONENUMBER_H

#include <string>

class PhoneNumber {
   private:
      short areaCode;
      short exchange;
      short line;
   public:
      PhoneNumber(std::string number);
      void setPhoneNumber(std::string number);
      void printPhoneNumber() const;
};
#endif

And here is my PhoneNumber.cpp code

// PhoneNumber.cpp
#include <iostream>
#include <iomanip>
#include <string>
#include <sstream>
#include <cctype>
#include "PhoneNumber.h"

PhoneNumber::PhoneNumber(std::string number) {
   setPhoneNumber(number);
}

void PhoneNumber::setPhoneNumber(std::string number) {
   bool areaCodeDone = false;
   bool exchangeDone = false;
   int length = number.length();
   
   std::istringstream iss(number);
   for (int i = 0; i < length; i++) {
      if (! areaCodeDone) {
         if (! std::isdigit(number[i])) {
            std::string str;
            iss >> std::ignore();
         }
         else {
            iss >> std::setw(3) >> areaCode;
            areaCodeDone = true;
         }
      }
      else if (! exchangeDone) {
         if (! std::isdigit(number[i])) {
            iss >> std::ignore();
         }
         else {
            iss >> std::setw(3) >> exchange;
            exchangeDone = true;
         }
      }
      else {
         if (! std::isdigit(number[i])) {
            iss >> std::ignore();
         }
         else {
            if (length - i < 4) {
               throw std::invalid_argument("Something wrong with phone number entry.");
            }
            else {
               iss >> std::setw(4) >> line;
            }
         }
      }
   }
}

The errors I get are on my lines with std::ignore, but I don't know how I am using it incorrectly. Error from g++ compiler is:

PhoneNumber.cpp:23:32: error: no match for call to ‘(const std::_Swallow_assign) ()’

iss >> std::ignore();

Answer 1

std::ignore() has a very different purpose than what you are hoping to accomplish. Instead of

iss >> std::ignore();

use

iss.ignore();

You can see the documentation of std::istream::ignore() and example usage at https://en.cppreference.com/w/cpp/io/basic_istream/ignore.

Answer 2

std::ignore doesn't do what you think it does: it is used for tuples.

Instead, what you looking for is actually called std::istream::ignore(). You would use it like this instead:

iss.ignore();

For more information on std::istream::ignore(), you can read this question.