Problem in using the delete operator for an array of objects allocated in heap using a FOR loop instead of using the delete[] array_of_objects keyword

Question

#include <iostream>

class ArrayClass
{
private:
    int *m_Arr;
    int i;

public:
    int Size;
    ArrayClass() : Size(5), i(0), m_Arr(nullptr)
    {
        std::cout << "The constructor was called." << std::endl;
    }
    ~ArrayClass()
    {
        std::cout << "The destructor was called." << std::endl;
        delete[] m_Arr;
    }
    void InitArr()
    {
        m_Arr = new int[Size];
    }
    void Setter(int &var)
    {
        *(m_Arr + i) = var;
        i++;
    }
    int *Getter() const
    {
        return m_Arr;
    }
};

int main()
{
    ArrayClass *instance = new ArrayClass[2];
    std::cout << instance << std::endl;
    for (int j = 0; j < 2; j++)
    {
        (instance + j)->InitArr();
        for (int i = 1; i <= (instance + j)->Size; i++)
        {
            (instance + j)->Setter(i);
        }
        int *Get = (instance + j)->Getter();
        std::cout << "The numbers are:" << std::endl;
        for (int i = 0; i < (instance + j)->Size; i++)
        {
            std::cout << *(Get + i) << std::endl;
        }
    }
    for (int l = 0; l < 2; l++)
    {
        delete (instance + l);
    }
    std::cin.get();
    return 0;
}

I am a beginner in C++, and have ran into a problem, I am trying to use the delete keyword, using the for loop, instead of delete[] instance. I am just trying to keep this method of deleting the instance array in heap, using the pointer arithmetic. But the delete is not doing the work, when I run the program, the destructor code snippet is not printed out on the cmd, the program just terminates, not respecting the line std::cin.get();

Only, doing this works. delete[] instance;.
But, I want this method to be used,

for (int l = 0; l < 2; l++)
    {
        delete (instance + l);
    }

So, can you please rectify the mistake.
Or, is it that the method I am trying to use, even valid in C++

Answer 1

Unfortunately method you want to use isn't valid in C++.

You need to use delete to free memory for object created with new, and delete[] to free memory for array created with new[]. Simple mnemonic - for each new one delete, for each new[] one delete[].

The reason for this that when you create something with new or new[] you allocate a bit more memory than you expect, additional memory is allocated for technical information for delete or delete[] operator. This information specify amount of memory that was allocated and which should be later deleted with operator delete or delete[]. And when you use delete for each element of array allocated with new[] the delete operator looks in a wrong place for technical information and this can lead to undefined behavior.

This is simple explanation, for more precise I advice you to look on cppreference.com, but for the beginning I believe this explanation is enough