Expanding a dynamically allocated array

Question

I have allocated an array as follows.

#include <iostream>

int main() {
    const int first_dim = 3;
    const int second_dim = 2;

    // Allocate array and populate with dummy data
    int** myArray = new int*[first_dim];
    for (int i = 0; i < first_dim; i++) {
        myArray[i] = new int[second_dim];
        for (int j = 0; j < second_dim; j++) {
            myArray[i][j] = i*second_dim + j;
            std::cout << "[i = " << i << ", j = " << j << "] Value: " << myArray[i][j] << "\n";
        }
    }

    // De-allocate array
    for (int i = 0; i < first_dim; i++)
        delete[] myArray[i];
    delete[] myArray;
}

Let's say I want to add a 4th element to the first dimension, i.e. myArray[3]. Is this possible?

I've heard that Vectors are so much more efficient for this purpose, but I hardly know what they are and I've never used them before.

Answer 1

Let's say I want to add a 4th element to the first dimension, i.e. myArray[3]. Is this possible?

Yes, but it's a pain in the neck. It basically boils down to allocating a new array, just as your existing code does (hint: put it in the function and make the sizes arguments to that function) and copying compatible elements over.

Edit: One of the things that std::vector does for you is properly de-allocating you memory. In the code you have, failure to allocate one of the arrays along the 2nd dimension will result in a memory leak. A more robust solution would initialize pointers to 0 before performing any allocation. An exception block could then catch the exception and free whatever was partially allocated.

Because this code becomes complex quickly, people resort to allocating a single buffer and addressing using a stride or using a 1D array of 1D arrrays (i.e. std::vector of std::vectors).