Input Validation to Accept Only Int Values - c++

Question

I am designing a program to read several user inputs and the specifications include that they are always positive integers. These values will then be modified to keep only the hundreds value then and used to display a bar chart. The problem I am having is with the program's input validation. Should the user disregard instructions and enter a decimal value instead of an integer, immediately the remainder of the program's output displays without the ability for the user to input any additional values. My attempt at catching this problem is shown below in the if statements.
int main() {
int store1;
int store2;
cout << "Enter today's sales for store 1:" << endl;
cin >> store1;
if(store1 < 0 || ((store1 % 1))!=0)
cout << "Value of sales must be an integer greater than zero" << endl;
cout << "Enter today's sales for store 2:" << endl;
cin >> store2;
if(store2 < 0 || ((store2 % 1))!=0)
cout << "Value of sales must be zero or greater" << endl;
If the user inputs a decimal the program output is:
Enter today's sales for store 1:
400.34
Enter today's sales for store 2:
Enter today's sales for store 3:
Value of sales must be zero or greater
Enter today's sales for store 4:
Enter today's sales for store 5:
SALES BAR CHART
(Each * = $100)
Store 1: ****
Store 2:
Store 3:
Store 4:
Store 5:
Process finished with exit code 0
If I had to guess, the values after the decimal (in this case 3 and 4) were passed to input for store 2 and 3 respectively, and since they were less than 100, an * was not displayed next to the appropriate store. I haven't included my code to get hundreds values and output *'s for readability sake, and because I'm almost positive my problem is related to cin.
To summarize, I need to validate user input so that only a positive integer is accepted through any input method. Any help is greatly appreciated!

Answer 1

You can use strtoll to convert and validate an input string encoding an integer.
Value read from cin is of string type and then it is passed to get_positive_integer() that is a wrapper to strtoll, converting and validating the string, it returns -1 when the string is not at the expected format, else it returns the decoded positive integer. Note the use of an intermediate long long type to easy check for overflow, and the use of getline to read a whole input line without breaking strings at spaces which would pass inputs of first line to next readings.
#include <climits>
#include <cstdlib>
#include <iostream>
#include <string>
using namespace std;
int get_positive_integer(const string& str)
{
const char* s = str.c_str();
char* end;
long long val = strtoll(s, &end, 10);
if (end == s) {
// str was not encoding an integer.
return -1;
}
if (*end) {
// Stopped reading before the end of the string
// the whole string was not an integer.
return -1;
}
if (val > INT_MAX) {
// Too big to fit in an int type
return -1;
}
if (val <= 0) {
// Not a positive integer.
return -1;
}
return val;
}
int main()
{
string input;
cout << "Enter a positive integer:" << endl;
getline(cin, input);
int value = get_positive_integer(input);
if (value < 0) {
cout << "Not a valid positive integer" << endl;
return EXIT_FAILURE;
}
cout << "Value: " << value << endl;
return EXIT_SUCCESS;
}