How to manage millions of game objects with a Slot Map / Object Pool pattern in c++?

I'm developing a game server for a video game called Tibia.

Basically, there can be up to millions of Objects, of which there can be up to thousands of deletes and re-creations as players interact with the game world.

The thing is, the original creators used a Slot Map / Object Pool on which pointers are re-used when an object is removed, this is a huge performance boost since there's no need to do much memory reallocation unless needed.

And of course, I'm trying to accomplish that myself, but I've come into one huge problem with my Slot Map:

Here's just a few explanation of how Slot Map works according to a source I found online:

Object class is the base class for every game object, my Slot Map / object Pool is using this Object class to save every allocated object.

Example:

struct TObjectBlock 
{
     Object Object[36768]; 
};

The way slot map works is that, the server first allocates say: 36768 objects in a list of TObjectBlock stores them a unique ID ObjectID for each Object which can be re-used in a free object list when the server needs to create a new object.

Example:

Object 1 (ID: 555) is deleted, it's ID 555 is put in a free object ID list, an Item creation is requested, ID 555 is reused since it's on the free object list, and there is no need to reallocate another TObjectBlock in the array for further objects.

My problem: How can I use "Player" "Creature" "Item" "Tile" to support this Slot Map ? I don't seem to come up with a solution into this logic problem.

I am using a virtual class to manage all objects:

struct Object
{
    uint32_t ObjectID;
    int32_t posx;
    int32_t posy;
    int32_t posz;
};

Then, I'd create the objects themselves:

struct Creature : Object
{
     char Name[31];
};

struct Player : Creature
{

};

struct Item : Object
{
     uint16_t Attack;
};

struct Tile : Object
{

};

But now if I was to make use of the slot map, I'd have to do something like this:

Object allocatedObject;
allocatedObject.ObjectID = CreateObject(); // Get a free object ID to use
if (allocatedObject.ObjectID != INVALIDOBJECT.ObjectID)
{
      Creature* monster = new Creature();
      // This doesn't make much sense, since I'd have this creature pointer floating around!
      monster.ObjectID = allocatedObject.ObjectID;
}

Pretty much doesn't make much sense to set a whole new object pointer the already allocated object unique ID.

What are my options with this logic?