How to make fields inaccessible (unreadable and unachangeable) outside of the structure?

[curiousprogramma08]

```d
struct QueueNode
{

     private int data;
     private QueueNode *next = null;

     this(int data)
     {
         this.data = data;
     }
}
```
I also tried to write it like this too:

```d
struct QueueNode
{

     private:
     int data;
     QueueNode *next = null;

     public:
     this(int data)
     {
         this.data = data;
     }
}
```
(I will use readonly ```@property``` as only way to read them)

But ```data``` and ```next``` can be changed and can be read from 
outside of the structure. What to do to prevent fields from being 
read and changed from outside the structure?