How to properly use cin.peek()

Question

This function is supposed to read a fraction and place it in an array. If the user enters '0' the function is supposed to exit. I am trying to do this using the cin.peek() function but execution always goes into the if statement and doesn't allow the user to exit.

How should I properly code this (I am open to not using peek(), I thought it was the simplest way of doing it.)

Thanks!

void enterFrac(Fraction* fracs[], int& index)
    {
        int n, d;
        char c, slash;
        cout << "Enter fractions (end by entering a 0): ";
        c = cin.peek();

        if ( c != '0')
        {
            cin >> n >> slash >> d;
            Fraction* f = new Fraction();
            f->num = n;
            f->den = d;
            fracs[index] = f;
            index++;
        }
    }

This test of peek() works however:

#include <iostream>
using namespace std;

int main () {
  char c;
  int n;
  char str[256];

  cout << "Enter a number or a word: ";
  c=cin.peek();

  if ( (c >= '0') && (c <= '9') )
  {
    cin >> n;
    cout << "You have entered number " << n << endl;
  }
  else
  {
    cin >> str;
    cout << " You have entered word " << str << endl;
  }

  return 0;
}

Answer 1

There are two issues with your use of std::istream::peek():

1. This function access the next character and does not skip leading whitespace. You probably want to skip leading whitespace before determining what the next character is, e.g., using the manipulator std::ws: (std::cin >> std::ws).peek().
2. The result from std::istream::peek() is not a char. Instead, it is an std::char_traits<char>::int_type (which is a fancy spelling of int). The result may, e.g., be std::char_traits<char>::eof() and if the value of '0' happens to be negative (I'm not aware of any platform where it is; however, e.g., the funny character from my name 'ü' is a negative value on platforms where char is signed) you wouldn't get the correct result, either. That is, you normally compare the result of std::istream::peek() against the result of std::char_traits<char>::to_int_type(), i.e., you'd use something like this: std::cin.peek() == std::char_traits<char>::to_int_type('0')

That said, your program doesn't check whether it could successfully read the nominator and the denominator, separated by a slash. You always want to verify that reading was successful, e.g., using something like

if ((std::cin >> nominator >> slash >> denominator) && slash == '/') {
    ...
}

Just for entertainment, you can create a manipulator for testing that a character is a slash, indeed:

std::istream& slash(std::istream& in) {
    if ((in >> std::ws).peek() != std::char_traits<char>::to_int_type('/')) {
        in.setstate(std::ios_base::failbit);
    }
    return in;
}

This way, you'd encapsulate the test for slash. If you need to use this in multiple places this is quite handy.