Why use a dynamic array instead of a regular array?

Question

The following code is used to demonstrate how to insert a new value in a dynamic array:

#include <iostream>

int main()
{
    int* items = new int[5] {1, 2, 3, 4, 5}; // I have 5 items

    for (int i = 0; i < 5; i++)
        std::cout << items[i] << std::endl;

    // oh, I found a new item. I'm going to add it to my collection.
    // I do this by
    // (1) allocating a bigger dynamic array
    // (2) copying the existing elements from the old array to the new array
    // (3) deleting the old array, redirecting its pointer to the new array
    int* items_temp = new int[6];
    for (int i = 0; i < 5; i++)
       items_temp[i] = items[i];
    items_temp[5] = 42;
    delete[] items;
    items = items_temp;

    for (int i = 0; i < 6; i++)
        std::cout << items[i] << std::endl;

    delete[] items;
}

I am confused about the necessity of using it over a regular array. Can't I just do the same thing with a regular array? Basically, you just define a new array with a larger size and move elements in the previous array to this new array. Why is it better to use a dynamic array here?

What if you don't know ahead of time how many elements you will be needing? — UnholySheep
– UnholySheep, Commented Sep 14, 2021 at 14:12
The "normal" way in C++ is to use std::vector for collections that you don't know a size for in advance (or may change in size) and std::array when you do know the size. — crashmstr
– crashmstr, Commented Sep 14, 2021 at 14:13
Use a regular array when the capacity is known a compile time. Use std::vector when the capacity will change or be dynamic. Use dynamic allocated memory when you can't use std::vector. — Thomas Matthews
– Thomas Matthews, Commented Sep 14, 2021 at 14:13
It's not necessary in this code. It is just a demonstration of how you can do it, not a demonstration of situations where you would want to do it (which are irrelevant details in this context). — molbdnilo
– molbdnilo, Commented Sep 14, 2021 at 14:13
Why is it better to use a dynamic array here? -- Once you move from the toy program you posted to something much more complex, you will want to put all of this dynamic array handling in a class -- do you want the real code to keep track of 5, then 6, then up to 100, then back to 1, off to 14, then maybe back to 0, and who knows whatever else for the size changes?. Once you create this class to handle this, then all you're doing is a poor-man's vector class. Thus, you want to drop using the poor-mans vector class and use one that is already established, i.e. std::vector — PaulMcKenzie
– PaulMcKenzie, Commented Sep 14, 2021 at 14:22

Guillaume Racicot · Accepted Answer · 2021-09-14 16:19:57Z

2

You are right, the example you are looking at isn't very good at demonstrating the need for dynamic arrays, but what if instead of going from size 5->6, we had no idea how many items we found until we need to add until the code is actually running?

Regular arrays need to be constructed with their size known at compile time

int foo [5] = { 16, 2, 77, 40, 12071 };

But dynamic arrays can be be assigned as size at runtime

int* Arrary(int size) {
    return new int[size];
}

So if you don't know the size of your array, or it may need to grow/shrink you need to use a dynamic array (or better yet just use a std::vector).

edited Sep 14, 2021 at 16:19

Guillaume Racicot

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answered Sep 14, 2021 at 14:20

Nick Vanderzwet

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Comments

Karen Baghdasaryan · Accepted Answer · 2021-09-14 15:22:18Z

Suppose you want to do what you mentioned a multiple times.

for(int i = 0; i < some_val; ++i)
{
    int val_to_add;
    std::cin >> val_to_add;

    int* new_arr = new int[old_size + 1]; // the value is suppsoed to be big in order to indicate that it takes much memory.
    copy_old_to_new(new_arr, old_arr); //some function which does the copying. 

    new_arr[old_size + 1] = val_to_add;
    delete[] old_arr;
    old_arr = new_arr;
}

Now think about what would happen if we tried to do the same with static arrays.

We wouldn't be able to remove the memory allocated by the old_arr, and the program would use a lot of memory.
We wouldn't be able to construct an array which would be accessible outside the loop, which, obviously, is not intended.

In your example it is not much clear how the usage of dynamic arrays would make use in your intention. So if you feel you could do the same without dynamic arrays, do it without them.

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