Program to find 1's complement of a binary number in C++

#include <iostream>
#define size 6
using namespace std;

int main() {
  int i, fail = 0;
  char binary[size + 1], comp[size + 1];
  cout << "Input a " << size << " bit binary number: ";
  cin >> binary;
  for (i = 0; i < size; i++) {
    if (binary[i] == '1') {
      comp[i] = '0';
    }
    else if (binary[i] == '0') {
      comp[i] = '1';
    }
    else {
      cout << "Error! Input the number of assigned bits." << endl;
      fail = 1;
      break;
    }
  }
  comp[size] = '\0';
  if (fail == 0) {
    cout << "The original binary number = " << binary << endl;
    cout << "Ones complement the number = " << comp << endl;
  }
}

Output

Input a 6 bit binary number: 101101
The original binary number = 101101
Ones complement the number = 010010

Input a 6 bit binary number: 10101
Error! Input the number of assigned bits.