Check out example codes for "how to declare a vector in c++". It will help you in understanding the concepts better.

Code Example 1

// CPP program to initialize a vector from 
// another vector. 
#include <bits/stdc++.h> 
using namespace std; 
  
int main() 
{ 
    vector<int> vect1{ 10, 20, 30 }; 
  
    vector<int> vect2(vect1.begin(), vect1.end()); 
  
    for (int x : vect2) 
        cout << x << " "; 
  
    return 0; 
}

Code Example 2

// CPP program to initialize a vector from 
// an array. 
#include <bits/stdc++.h> 
using namespace std; 
  
int main() 
{ 
    int arr[] = { 10, 20, 30 }; 
    int n = sizeof(arr) / sizeof(arr[0]); 
  
    vector<int> vect(arr, arr + n); 
  
    for (int x : vect) 
        cout << x << " "; 
  
    return 0; 
}

Code Example 3

//Note: any data type can be used in template.
std::vector<typename T> vectorName;

Code Example 4

// CPP program to create an empty vector 
// and push values one by one. 
#include <bits/stdc++.h> 
using namespace std; 
  
int main() 
{ 
    // Create an empty vector 
    vector<int> vect;  
     
    vect.push_back(10); 
    vect.push_back(20); 
    vect.push_back(30); 
  
    for (int x : vect) 
        cout << x << " "; 
  
    return 0; 
}

Code Example 5

vector<int> vec;
//Creates an empty (size 0) vector
 

vector<int> vec(4);
//Creates a vector with 4 elements.

/*Each element is initialised to zero.
If this were a vector of strings, each
string would be empty. */

vector<int> vec(4, 42);

/*Creates a vector with 4 elements.
Each element is initialised to 42. */


vector<int> vec(4, 42);
vector<int> vec2(vec);

/*The second line creates a new vector, copying each element from the
vec into vec2. */

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