Pointers - Is it safe to point to invalid memory?

[Steven Schveighoffer]

On Monday, 18 August 2025 at 15:26:21 UTC, monkyyy wrote:
> On Monday, 18 August 2025 at 07:32:13 UTC, Steven Schveighoffer 
> wrote:
>> 2. You can store data in the lower bits of an aligned pointer 
>> (after all, this is just an interior pointer).
>
> Will the gc count this as a reference while scaning in any case?

As long as it's stored as a pointer, it is treated as a pointer.

I'll try my hand at ascii art.

Let's say you have an integer 0x4321 allocated in memory at 
address 0x100:

```
          -------------------------
Address: | 100 | 101 | 102 | 103 |
          -------------------------
    Data: |  1  |  2  |  3  |  4  |
          -------------------------
```

A pointer to the integer points at address 0x100

```
          -------------------------
Address: | 100 | 101 | 102 | 103 |
          -------------------------
    Data: |  1  |  2  |  3  |  4  |
          -------------------------
             ^ ptr
```

Now, you add a 1 to the pointer value to have it pointing at 
0x101, it looks like this:

```
          -------------------------
Address: | 100 | 101 | 102 | 103 |
          -------------------------
    Data: |  1  |  2  |  3  |  4  |
          -------------------------
                   ^ ptr
```

This is still pointing at the integer. So the GC believes the 
integer memory block is still valid because there's a pointer 
interior to the block

Add 2, and you get a pointer to the `3` byte, add 3 and you get a 
pointer to the `4` byte. Add 4, and now it's pointing at the next 
integer, and so now it's no longer pointing at the integer, and 
this becomes invalid.

So you have 2 bits of space you can use to store a value from 0 
to 3, and still have it be a valid pointer.

-Steve