what is an stack in c++ code example

Example: cpp stack

// Fast DIY Stack
template<class S, const int N> class Stack {
private:
    S arr[N];
    int top_i;

public:
    Stack() : arr(), top_i(-1) {}
    void push (S n) {
        arr[++top_i] = n;
    }
    void pop() {
        top_i--;
    }
    S top() {
        return arr[top_i];
    }
    S bottom() {
        return arr[0];
    }
    int size() {
        return top_i+1;
    }
};