Dynamic Array implementation in C++

Question

I am trying to implement Dynamic Array using C++. However, my resize() function seem to not work properly. There are no errors or warnings. I did some research and tried to look at other implementations found on the internet but was unable to fix the issue. I put my code below.

#include <iostream>

class Array
{
private:
    int* arr;
    int size = 0;
    int capacity = 1;

public:
    Array() { arr = new int[capacity]; }

    Array(int capacity)
        :
        capacity(capacity)
    {
        arr = new int[capacity];
    }

    int length() const { return size; }

    bool is_empty() const { return (length() == 0); }

    int get(int index) const { return arr[index]; }

    void set(int index, int value) { arr[index] = value; }

    void resize()
    {
        capacity *= 2;
        int* temp = new int[capacity];
        for (int i = 0; i < size; i++) { temp[i] = arr[i]; }
        delete[] arr;
        arr = temp;
        for (int i = 0; i < capacity; i++) { arr[i] = 0; }
    }

    void add(int value)
    {
        if (size + 1 >= capacity) { resize(); }
        arr[size++] = value;
    }

    void remove(int index)
    {
        for (int i = index; i < size - 1; i++)
        {
            arr[i] = arr[i + 1];
        }
        size--;
    }

    int& operator[](int index)
    {
        return arr[index];
    }
};

int main()
{
    Array array;

    for (int i = 0; i < 5; i++)
    {
        array.add(i + 1);
    }

    for (int i = 0; i < array.length(); i++)
    {
        std::cout << array.get(i) << " ";
    }
    std::cout << '\t' << array.length() << '\n';

    return 0;
}

The code outputs:

0 0 0 4 5   5

But I expect it to output:

1 2 3 4 5   5

Answer 1

In your resize method you copied over the existing elements from arr

for (int i = 0; i < size; i++) { temp[i] = arr[i]; }

But then later you 0 all of the elements out, effectively clearing the previous data

for (int i = 0; i < capacity; i++) { arr[i] = 0; }

Instead you likely just want to 0 the trailing new elements

for (int i = size; i < capacity; ++i) { arr[i] = 0; }

Answer 2

This loop

for (int i = 0; i < capacity; i++) { arr[i] = 0; }

is incorrect. It sets the first size elements to 0.

In fact this loop is redundant. Instead of this loop you could just write in the statement where the memory is allocated like

int* temp = new int[capacity]();

Also the definition of the function add() is incorrect. It should look like:

void add(int value)
{
    if (size == capacity) { resize(); }
    arr[size++] = value;
}

You need to define explicitly the destructor. For example

~Array()
{
    delete []arr;
}

Also you need either to define explicitly a copy constructor and the copy assignment operator or define them as deleted. For example

Array( const Array & ) = delete;
Array & operator =( const Array & ) = delete;

Otherwise using these member functions can result in undefined behavior.