Express "Class argument may not be null" ?

[Steven Schveighoffer via Digitalmars-d-learn]

On 8/9/17 1:05 PM, Johan wrote:
> On Wednesday, 9 August 2017 at 12:47:49 UTC, Steven Schveighoffer wrote:
>> On 8/8/17 3:59 PM, Johan Engelen wrote:
>>>
>>> In C++, it is clear that the _caller_ is doing the dereferencing, and 
>>> the dereference is also explicit.
>>
>> In fact it's not doing any dereferencing. It's just under the hood 
>> passing a pointer.
> 
> Come on Steven, I thought you knew I am a compiler dev.

Of course I do! I'm not a compiler dev though :)

> The caller _is_ doing dereferencing. That's what the '*' means 
> semantically and it matters. [1]

As a layman user, I consider dereferencing to be when you actually use 
the data pointed at by the pointer (i.e. you've followed the reference).

What you are doing when passing a dereferenced pointer through a 
reference is the same as just passing a pointer, it doesn't seem to me 
like there should be a difference.

>>>> However, the in contract does actually enforce the requirement.
>>>
>>> And adds null pointer checks even when clearly not needed.
>>
>> Clearly not needed? I thought the point was to ensure the reference is 
>> not null?
> 
> I meant code like this: `k->something(); foo(*k);` or `auto k = new 
> Klass(); foo(*k);`.
> 
> Anyway, the point was to express the intent nicely.
> I wasn't expecting having to explain the merits of passing by reference.
> 

Sorry, not a C++ developer. I just think in terms of how the code 
translates to the instructions to the computer. To me, pointers and 
references seem very similar.

> [1] Going off-topic.
> The alternative is dereferencing inside the callee (which is apparently 
> the only option D semantically provides for classes). I showed the 
> impact of this semantic difference in my DConf 2017 talk; here are links 
> to  examples:
> https://godbolt.org/g/rgvHTC  (C++)
> https://godbolt.org/g/NCTeXo  (D)
> It's a pity we are not able to pass classes "by reference" (per C++ 
> parlance), for reasoning and readability, and performance. I still have 
> to come up with a rationale for not having an explicit "*" for classes 
> for when I am going to teach D.

I didn't think this comes from the dereferencing, I thought it came from 
the ability to rebind.

That is, inside the loop, the pointer potentially could have been set to 
something different, so it has to reload the data.

What you really want perhaps is a class parameter that cannot be 
rebound? That would more accurately duplicate a ref parameter.

-Steve