Disallow null references in safe code?

[Andrei Alexandrescu]

On 2/1/14, 2:14 AM, Jonathan M Davis wrote:
> On Saturday, February 01, 2014 04:01:50 deadalnix wrote:
>> Dereferencing it is unsafe unless you put runtime check.
>
> How is it unsafe? It will segfault and kill your program, not corrupt memory.
> It can't even read any memory. It's a bug to dereference a null pointer or
> reference, but it's not unsafe, because it can't access _any_ memory, let
> alone memory that it's not supposed to be accessing, which is precisely what
> @safe is all about.

This has been discussed to death a number of times. A field access 
obj.field will use addressing with a constant offset. If that offset is 
larger than the lowest address allowed to the application, unsafety may 
occur.

The amount of low-address memory protected is OS-dependent. 4KB can 
virtually always be counted on. For fields placed beyond than that 
limit, a runtime test must be inserted. There are few enough 4KB objects 
out there to make this practically a non-issue. But the checks must be 
there.

>>   Which is stupid for something that can be verified at compile time.
>
> In the general case, you can only catch it at compile time if you disallow it
> completely, which is unnecessarily restrictive. Sure, some basic cases can be
> caught, but unless the code where the pointer/reference is defined is right
> next to the code where it's dereferenced, there's no way for the compiler to
> have any clue whether it's null or not. And yes, there's certainly code where
> it would make sense to use non-nullable references or pointers, because
> there's no need for them to be nullable, and having them be non-nullable
> avoids any risk of forgetting to initialize them, but that doesn't mean that
> nullable pointers and references aren't useful or that you can catch all
> instances of a null pointer or reference being dereferenced at compile time.

The Java community has a good experience with @Nullable: 
http://stackoverflow.com/questions/14076296/nullable-annotation-usage


Andrei