Casting arrays

[Dennis]

On Thursday, 30 May 2019 at 01:14:57 UTC, solidstate1991 wrote:
> /**
>  * Safely casts one type of an array to another.
>  */
> T[] reinterpretCast(T, U)(ref U[] input) @trusted{
> 	T[] _reinterpretCast() @system{
> 		return cast(T[])(cast(void[])input);
> 	}
> 	if ((U.sizeof * input.length) % T.sizeof == 0){
> 		return _reinterpretCast();
> 	} else {
> 		throw new Exception("Cannot cast safely!");
> 	}
> }

Don't do this, that function can not be trusted. For casts of 
basic types, the compiler already has such runtime checks:

```
int[] a = [3, 4, 5];
auto b = cast(double[]) a;
```

"An array of size 12 does not align on an array of size 8, so 
`int` cannot be cast to `double`"

You allowing it for ANY type as long as alignment is okay invites 
all kinds of safety violations.

```
void main() @safe
{
     char*[1] a = [new char('\xCC')];
     auto slice = a[];
     auto b = reinterpretCast!(int*)(slice);
     writefln("%08X", *b[0]);
}
```
https://run.dlang.io/is/QVNlEv

Prints:
743B50CC

The 74 3B 50 are bytes out of bounds. There's more to array 
casting than alignment for it to be safe.