#include <iostream>
#include <cassert>

using namespace std;

template<typename T>
class vector
{ protected:
    T * data;
    int capacity;
    int numitems;

  public:
    vector();
    ~vector();
    void resize(int newsize);
    void push_back(T value);
    T back();
    void pop_back();
    T & at(int position);
    int size(); };

template<typename T>
vector<T>::vector()
{ capacity = 0;
  numitems = 0;
  data = nullptr; }

template<typename T>
vector<T>::~vector()
{ delete [] data; }

template<typename T>
void vector<T>::resize(int newsize)
{ if (newsize < capacity)
    return;
  T * newdata = new T[newsize];
  for (int i = 0; i < numitems; i += 1)
    newdata[i] = data[i];
  delete [] data;
  data = newdata;
  capacity = newsize; }

template<typename T>
void vector<T>::push_back(T value)
{ if (numitems >= capacity)
  { int newsize = capacity * 2;
    if (newsize <= capacity)
      newsize = capacity + 1;
    resize(newsize); }
  data[numitems] = value;
  numitems += 1; }

template<typename T>
T vector<T>::back()
{ assert(numitems > 0);
  return data[numitems - 1]; }

template<typename T>
void vector<T>::pop_back()
{ assert(numitems > 0);
  numitems -= 1; }

template<typename T>
T & vector<T>::at(int position)
{ assert(position >= 0);
  assert(position < numitems);
  return data[position]; }

template<typename T>
int vector<T>::size()
{ return numitems; }

int main()
{ vector<int> V;
  vector<string> SV;
  V.push_back(123);
  V.push_back(99);
  V.push_back(7);
  V.push_back(12'345'678);
  V.at(2) = 777777;
  VS.push_back("one");
  VS.push_back("two");
  VS.push_back("three");
  VS.push_back("four");
  VS.push_back("five");
  VS.push_back("six");
  VS.push_back("seven");
  for (int i = 0; i < V.size(); i += 1)
    cout << V.at(i) << "\n";
  for (int i = 0; i < VS.size(); i += 1)
    cout << VS.at(i) << "\n"; }