C++ create an array of size of a vector and then copy vector into C style array

VC++ is giving an error on below code:

#include <vector>
#include <iostream>

int main()
{
    std::vector<int> a;
    a.push_back(10);
    a.push_back(20);
    a.push_back(30);
    int arr[a.size()];
    std::copy(a.begin(), a.end(), arr);
    for(int index = 0 ; index < a.size(); index++)
    {
        std::cout << " The value is " << arr[index] << std::endl;
    }
}

It errors out at the integer array declaration stating that the value of variable 'a' cannot be used as constant?

How can we resolve the issue where my objective is to transfer content of vector into a 'C' style array?