#include <iostream>
#include <vector>
using namespace std;

void f(unsigned N)
{
    const unsigned stack_array_size=10;
    char stack_array[stack_array_size];
    vector<char> heap_array;
    char *arr=stack_array;
    if(N>stack_array_size)
    {
        heap_array.resize(N);
        arr=&heap_array[0];
    }
    // use arr here
}

int main()
{
    f(20);
}