Segmentation Fault

[Jose94ji]

So here I have a homework assignment where I was to implement stacks and templates. The program was working fine until I added the s2.pop() at the end of the program. This is where I get my segmentation fault. Any tips on why and how I can fix it? Thanks.

  #include <vector>
#include <string>
#include <iostream>

using namespace std;

template <typename T>
class Stack
{
    vector<T> container;
public:
    Stack(): container() {}
    void push(T x) { container.push_back(x); }
    void pop() { container.pop_back(); }
    T top() { return container.back(); }
    bool empty() { return container.empty(); }
};


main()
{
    Stack<int> s1;

    s1.push(4);
    s1.push(3);
    s1.push(2);
    s1.push(1);
    while (!s1.empty()) {
        cout << s1.top() << endl;
        s1.pop();
    }

    Stack<string> s2;
    s2.push("Yoda said ");
    s2.push("something ");
    s2.push("to write ");
    while (!s2.empty()) {
        cout << s2.top();
        s2.pop();
    }
    //Error occurs here
    s2.pop();
    cout << endl;
}

[Arslan7041]

You are trying the pop the container when it is already empty.

So you may want to change your pop function to first check if the container is empty, and only pop if it is not empty. It should throw an exception otherwise.

[Jose94ji]

so in my pop function

pop() {if (!container.empty(){
    container.pop_back();
}
}

[Arslan7041]

That should work. Fix your braces and formatting though. You have an extra brace floating around in there:

void pop() {
if (!container.empty() )
    container.pop_back();
}

[Jose94ji]

Ok so put that piece a code in and it works!

But now when I try to output the top I get a segmentation fault so.

 #include <vector>
#include <string>
#include <iostream>

using namespace std;

template <typename T>
class Stack
{
    vector<T> container;
public:
    Stack(): container() {}
    void push(T x) { container.push_back(x); }
    void pop() { container.pop_back(); }
    T top() { return container.back(); }
    bool empty() { return container.empty(); }
};


main()
{
    Stack<int> s1;

    s1.push(4);
    s1.push(3);
    s1.push(2);
    s1.push(1);
    while (!s1.empty()) {
        cout << s1.top() << endl;
        s1.pop();
    }

    Stack<string> s2;
    s2.push("Yoda said ");
    s2.push("something ");
    s2.push("to write ");
    while (!s2.empty()) {
        cout << s2.top();
        s2.pop();
    }
    //New error
     cout << s2.top();
    //This was fixed
    s2.pop();
    cout << endl;
}

[Arslan7041]

Its the same problem. By the end of that second while, s2 is empty, therefore there is no "top". You are trying to return something nonexistent.

How will you fix this?

[Jose94ji]

Can I just make a 3rd while that does the same thing as the previous 2?

[Jose94ji]

Did and it works now! Thanks for your help really appreciate it.