Question

I want to define global array (used in other functions) based on input from main(); (concretely array size). The extern keyword didn’t help.

#include <iostream>
    using namespace std;

void gen_sieve_primes(void);

int main() {
    int MaxNum;
    cin >> MaxNum;
    int *primes = new int[MaxNum];
    delete[] primes;
    return 0;
}
//functions where variable MaxNum is used

Answer 1

You declare it outside of main:

int maxNum;
int main() {
...
}

Ideally, you don’t do this at all. Globals are rarely useful, and hardly ever (or rather: never) needed.