This syntax regarding null checking baffles me

[12345swordy]

On Thursday, 7 January 2021 at 05:14:44 UTC, Basile B. wrote:
> On Thursday, 7 January 2021 at 04:57:55 UTC, 12345swordy wrote:
>> if (c !is null) Why?????
>>
>> Would it be simpler to type
>>
>> if (c is not null)
>>
>> on a related note. Why are not we allowed to do this?
>>
>> if (c != null)
>
> it is allowed
>
"For class objects, the == and != operators are intended to 
compare the contents of the objects, however an appropriate 
opEquals override must be defined for this to work. The default 
opEquals provided by the root Object class is equivalent to the 
is operator. Comparing against null is invalid, as null has no 
contents. Use the is and !is operators instead."

Not allowed for classes apparently.

-Alex