```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?
