I am not able to make a new array?

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Python

C++14

I want to make a new arraylist of double length than the existing one, but when i try to make a new array of length 2*size(array), it is giving me an error which is
"expression must have a const value".
What should i do?

What I have tried:

int arr[5];
int index;
int n = std::size(arr);

bool insert(int element, int index) {
	if (index<0 && index>n) {
		printf("Enter a valid index:\n");
		return false;
	}
	int const leng = 2*n;

	if (index==n) {
		int new_arr[leng];

	}
	
}