C++: Accessing the top of the stack and applying the operations?

Question

#include <iostream>
#include <stack>

using namespace std;
int main ()
{
        stack<int> mystack;
        mystack.push(10);
        mystack.push(20);
        mystack.top() -= 5;
        cout << mystack.top() << endl;
        return 0;
}

Answer 1

This is the definition of the top function

reference& top();

Which means it returns the reference to the top of the stack so following operation will change the value of the top itself

    mystack.top() -= 5;

Hence cout << mystack.top() << endl; will print the value of the top as 15.