Bulls and Cows game in C++

bulls_and_cows.cpp

#include <iostream>
#include <vector>
#include <stdexcept>
#include <ctime>
#include <cstdlib>
#include <cctype>

using namespace std;

constexpr short int number_of_digits{ 4 };
constexpr short int upper_limit{ 10 };

void init_digits(vector<char>& digits);
void get_chars(vector<char>& chars);
void check_input(vector<char>& digits, vector<char>& chars, short int& bulls, short int& cows);

int main() {
    vector<char> digits(number_of_digits);
    vector<char> chars(number_of_digits);
    
    char answer;
    do {
        short int bulls{ 0 }, cows{ 0 };
        unsigned int tries{ 1 };
        
        init_digits(digits);
        
        /*
         // <DEBUG>
         cout << "-- Secret chars: ";
         for (short int i{ 0 }; i < number_of_digits; i++) {
         cout << digits[i];
         }
         cout << " --" << endl;
         // </DEBUG>
         */
        
        while(true) {
            cout << "Try to guess the 4 chars (0-9): ";
            
            try {
                get_chars(chars);
            } catch(runtime_error& e) {
                cerr << e.what() << endl;
                continue;
            }
            
            check_input(digits, chars, bulls, cows);
            
            if (bulls == 4) {
                cout << "You win with " << tries << " tries!" << endl;
                break;
            }
            
            cout << bulls << " bulls and " << cows << " cows." << endl;
            tries++;
        }
        
        cout << endl << "Do you want to play again [y/N]? ";
        cin >> answer;
    } while(tolower(answer) == 'y');
    
    return 0;
}

void init_digits(vector<char>& digits) {
    srand(unsigned(time(NULL)));
    
    vector<bool> checks(10, false);
    for (short int i{ 0 }; i < number_of_digits; i++) {
        short int d = (rand() % upper_limit);
        
        if (checks[d]) {
            i--;
            continue; // Skip duplicated digit
        }
        
        digits[i] = '0' + d;
        checks[d] = true;
    }
}

void get_chars(vector<char>& chars) {
    vector<bool> checks(10, false);
    for (short int i{ 0 }; i < number_of_digits; i++) {
        char c;
        cin >> c;
        
        if (!isnumber(c)) {
            string s{c};
            s = "Error: invalid char \"" + s + "\"!";
            throw runtime_error(s);
        }
        
        unsigned short d = c - '0';
        
        if (checks[d]) {
            string s{c};
            s = "Error: digit \"" + s + "\" is duplicated!";
            throw runtime_error(s);
        }
        
        chars[i] = c;
        checks[d] = true;
    }
}

void check_input(vector<char>& digits, vector<char>& chars, short int& bulls, short int& cows) {
    bulls = cows = 0; // Double-checking initial value.
    
    for (short int i{ 0 }; i < number_of_digits; i++) {
        for (short int j{ 0 }; j < number_of_digits; j++) {
            if (digits[i] == chars[j]) {
                if (i == j) {
                    bulls++;
                    break;
                } else {
                    cows++;
                }
            }
        }
    }
}

Will it not count multiple cows(useless addition) ?
I tried to make for and I had to track the indicies of bulls and skip them. at first I used the same logic as yours.