C++: vector size vs. vector capacity

#include <iostream>
#include <vector>

using namespace std;

//////// vector size vs. vector capacity
//////// Size: the number of items currently in the vector
//////// Capacity: how many items can be fit in the vector before it is "full". Once full, adding new items will result in a new, larger block of memory being allocated and the existing items being copied to it
//////// source: https://stackoverflow.com/a/6296965/9868445

int main(){

  vector <int> a;

  a.push_back(1);
  a.push_back(2);
  a.push_back(3);
  cout << a.size() << endl;
  cout << a.capacity() << endl;
  a.pop_back();
  a.pop_back();
  a.pop_back();
  cout << a.size() << endl;
  cout << a.capacity() << endl;
  a.push_back(1);
  a.push_back(2);
  a.push_back(3);
  a.push_back(4);
  cout << a.size() << endl;
  cout << a.capacity() << endl;
  a.push_back(5);
  cout << a.size() << endl;
  cout << a.capacity() << endl;

  return 0;
}