Question

I was thinking of ways to make an array larger quickly in C++ and I came up with this:

// set up arr1
int *arr1 = new int[5];

// add data to arr1
arr1[0] = 1;
arr1[1] = 2;
arr1[2] = 3;
arr1[3] = 4;
arr1[4] = 5;

// set up arr2
int *arr2 = new int[10];
arr2 = arr1; // assign arr1 to arr2

// add more values
arr2[5] = 6;
arr2[6] = 7;
arr2[7] = 8;
arr2[8] = 9;
arr2[9] = 10;

Is this even safe? I worry that this will cause some strange behavior and that arr2 is just an int[5] array and you’re now overwriting data that doesn’t belong to it.

Answer 1

The line arr2 = arr1; leaks memory, and all the following arr2[...]= lines invoke undefined behavior as they access the array of 5 ints outside of its bounds.

To do what you wanted to do, replace arr2 = arr1; with std::copy(arr1, arr1+5, arr2); (example program: https://ideone.com/3Rohu)

To do this properly, use std::vector<int>