c++ vector size

#include <vector>

int main() {
  std::vector<int> myVector = { 666, 1337, 420 };
  
  size_t size = myVector.size(); // 3
  
  myVector.push_back(399); // Add 399 to the end of the vector
  
  size = myVector.size(); // 4
}

#include <vector>

int main () {
    std::vector<int> v;
    auto size = v.size();
}

#include <iostream>
#include <vector>
using namespace std;

int main() {
  //vector element size
  const int size = 4; 
  //vector with int data type
  //all elements are equal to 4
  vector<int> myVect (size, 4);

  for (int i=0; i<size; i++) {
    cout << "Vector index(" << i <<") is: "<< myVect[i] << endl; 
  }
  return 0;
}

// create a vector with 20 integer elements
std::vector<int> arr(20);

for(int x = 0; x < 20; ++x)
   arr[x] = x;

#include <vector>

auto n = 20
// create a vector with n=20 integer elements
std::vector<int> arr(n);