how to declare size of vector in c++ code example

Example 1: 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
}

Example 2: finding the size of vector in c++

vector<int> a;
//to directly find the size of the vector;
//use  a.size(;

cout <<" " << a.size();

Example 3: c++ create vector of size

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

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

Example 4: declare vector of size n in c++

#include <vector>

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