Стек с защитой от ошибок

#include <iostream>
#include <vector>

using namespace  std;

struct stack
{
    int64_t  array[100];
    int cursor=0;
    ////////////
    void push(int64_t n)
    {
        array[cursor++] =n;
    }
    ////////////
    int64_t pop()
    {
        return array[--cursor];
    }
    ////////
    int64_t back()
    {
        return array[cursor-1];
    }
    //////////
    void clear()
    {

        cursor = 0;
    }
    ///////
    uint64_t size()
    {
        return cursor;
    }
};
int main() {
    stack primer;
    string stroka;
    while (cin >> stroka)
        if (stroka == "push")
        {
            int64_t vr;
            cin >> vr;
            primer.push(vr);
            cout << "ok" << endl;
        }
        else if(stroka == "pop")
        {
            if(primer.size() == 0)
            {
                cout << "error" << endl;
            }
            else
            {
                cout << primer.pop() << endl;
            }
        }
        else if(stroka == "back")
        {
            if(primer.size() == 0)
            {
                cout << "error" << endl;
            }
            else
            {
                cout << primer.back() << endl;
            }
        }
        else if(stroka == "size")
        {
            cout << primer.size() << endl;
        }
        else if(stroka == "clear")
        {
            primer.clear();
            cout << "ok" << endl;
        }
        else if(stroka == "exit")
        {
            cout << "bye";
            break;
        }
    return 0;
}