How to declare and initialize a global array in C++?

In Java, I can declare global (within the scope of a file) arrays and then initialize them in the main function.

public static int[] arr;
public static void main(String[] args) {
    arr = new int[N]; // N is possibly given as i/o.
    ...
}

This allows me to have this array is a global variable. Can I do the same thing in C++?

I tried the following code block in C++:

#include <bits/stdc++.h>
using namespace std;
int N;
vector<int> cows(N);
int main() {
	cin >> N;
	
	for (auto& elem : cows) {
		elem = 2;
	}
	cout << cows.size() << '\n';
}

This failed and it told me that the size of my array is 0. However, when I move the initialization of the vector into main, like this:

#include <bits/stdc++.h>
using namespace std;
int N;
int main() {
	cin >> N;
	vector<int> cows(N);
	for (auto& elem : cows) {
		elem = 2;
	}
	cout << cows.size() << '\n';
}

Now, the program gives me that the size of the array is indeed N (and not 0). However, now this vector is only avaiable in main and isn't global. Can I make the vector cows global, like you can do in Java?

The problem is that in your first example,both N as well as cows are global variables which means that N will be at least zero initialized(due to static initialization) which in turn means that cows is defined to be a vector with no elements(that is, a vector of size 0).

To solve this, you can use std::vector::resize to resize the vector after asking the user the size as shown below:

int N;  //at least zero initialized(static initialization)
vector<int> cows(N); //you can rewrite this as just vector<int> cows;
int main() {
    cin >> N;
    //resize the vector passing the input size 
    cows.resize(N);
    
    for (auto& elem : cows) {
        elem = 2;
    }
    cout << cows.size() << '\n';
}

OTOH the second case works because you directly create a vector of size N where N is the size given by the user unlike the first case where it was zero initialized.