How to properly declare a class member pointer to an array without allocating array

Question

I am new to C++ and trying to create my first Octree structure.

Common way to store children is simply to store 8 pointers in each node.
like so:

class Octant
{
    Octant *child0;
    Octant *child1;
    Octant *child2;
    //...
}

This means each node will contain 8 pointers, witch is 64 bytes per node.
And also each leaf will store 8 null pointers.
To avoid memory wasting I want to store only one pointer to the first child
and allocate the array only during subdividing a leaf.

I tried this:

Octant *child[8]; // actually the same as 8 pointers + 1 pointer to an array
Octant child[8]; // One pointer but I have to allocate children in each Octant memory

so I did something like this:

class Octant
{
private:
    Octant *child; // Store only one pointer in each Octant

public:
    Octant();
    void subdivide();
    void doStuff(int i){ std::cout << i << " Child exists and do some stuff \n"; }
    //...
};
Octant::Octant() {

    child = nullptr; // leaves have no allocated children
    //...
}
void Octant::subdivide() {

    child = new Octant[8]; // I can allocate it wherever I want

    child[0].doStuff(0);
    child[1].doStuff(1);
    child[2].doStuff(2);
    child[3].doStuff(3);
    //...
}

It works fine, but there is a little problem during debugging.
My code "thinks" that the child is a pointer to one Octant not an array
For example if i do something like sizeof(child) it will return sizeof(pointer) (8 bytes)

So my question is it ok to use pointers like I did?
Or is there another "proper" way to do such stuff in C++?
Is there a way to convert a pointer to pointer[8] and does it make any sense?

Answer 1

My code "thinks" that the child is a pointer to one Octant not an array..

Your code thinks right. child is in fact a pointer to Octant. Just see how you've declared it:

`Octant *child;`//this declared(and defines) child as a pointer to a non-const Octant object

The above defines an object named child which is a pointer to a non-const Octant object. Note that child has not been initialized in the above declaration. If you want to initailze it you could have just written:

Octant *child = nullptr; //uses in class initializer

Or

Even initialize it in the constructor initializer list :

//----------------vvvvvvvvvvvvvv----->use constructor initializer list to initialize child
Octant::Octant(): child(nullptr)
{
    
}

Now, the value of &child and &child[0] is the same.

Moreover, when you wrote:

child = new Octant[8]; //child points to the first element of the dynamically allocated array of size 8 

In the above assignment, you're assigning to child a pointer that points to the first element of the dynamically allocated array of size 8 and not 9.

---

The following is also possible:

//----------------vvvvvvvvvvvvvvvvvvvvvvv---->use constructor initializer list to initializer child
Octant::Octant(): child(new Octant[10]()) {

    
}

The question was how to declare an "array pointer" without allocation but maybe it does not make any sense.

Looking at your above quoted comment, it looks like you want to avoid dynamic allocation and declare a pointer to an array which can be done as shown below:

Octant arr[8]{}; //arr is an array of size 8 with elements of type Octant 
Octant (*ptr)[8] = &arr;//ptr is a pointer to an array of size 8 with elements of type Octant