The following source code could check whether your input is an integer:
#include <iostream>
#include <string>
#include <ctype.h>
#include <stdlib.h>
using namespace std;
int main () {
int n, ans;
string instr;
cout << "Enter an integer: ";
cin >> instr;
cout << instr.c_str() << endl;
const char *c_instr = instr.c_str();
for( int i=0; i<instr.length(); i++) { // check input
if( !isnumber( c_instr ) ) {
cout << "Input error: MUST be a positive integer.\n";
exit(1);
}
}
n = atoi( c_instr ); // convert to number
ans = n + 1;
cout << ans;
return 0;
}