build success but run fail for Stack class - c++

Question

So, I am trying to run the class below with
int x = 5;
mystack<int> st;
st.push(x);
However, I keep getting build failure, I can't seem to figure out why.
#ifndef MYSTACK_H
#define MYSTACK_H
#include <vector>
using namespace std;
template<typename T>
class mystack {
private:
vector<T> data;
int size = 0;
public:
void push(T const &);
};
template<typename T>
void mystack<T>::push(T const & elem) {
data[size] = elem;
size++;
}
RUN FAILED ( exit value -1, 073, 741, 819, total time: 1s)
Also completely separate question, how do I throw an underflow? I tried
throw underflow_error();

Answer 1

Initially your vector<T> data is empty. Its size() is 0. You cannot access to any of its elements. That might be the reason of getting an error.
Try using this code:
template<typename T>
void mystack<T>::push(T const & elem) {
data.push_back(elem);
}
It actually will increase you data.size() by 1 every time you push an element.
Don't want to use any vector STL
Wonder you are having vector<T> in code, looks like STL. You can use plain dynamic arrays as an alternative or even static ones.
template<typename T>
class mystack {
private:
T* data;
int size = 0;
int maxSize;
public:
mystack(int maxSize) :maxSize(maxSize) { data = new data[maxSize]; }
~mystack() { delete[] data; }
void push(T const &);
};
template<typename T>
void mystack<T>::push(T const & elem) {
// here you may check if you already reached the maxSize;
data[size++] = elem;
}
Note that in order to fully simulate STL vector behaviour you should consider reallocating the array each time you have size = allocateSize. STL vector makes this every time the number of elements hits the power of 2, it doubles its size.

Answer 2

I propose you to use std::stack instead of implement your own stack using std::vector. If you don't want use STL at all, your should control memory allocation and deletion by yourself. In your example, in the line data[size] = elem;, you assign value to not allocated memory, it is your problem. You can fix it that way, if you don't want to use push_back() method:
template<typename T>
void mystack<T>::push(T const & elem) {
size += 1;
data.resize(size);
data[size] = elem;
}
About std::underflow_error. It designed for arithmetic underflow errors. Anyway, this class has constructor where your should put string, so you need change throw underflow_error(); to throw underflow_error("Error message");.