is ==

[Neia Neutuladh]

On Friday, 18 May 2018 at 23:53:12 UTC, IntegratedDimensions 
wrote:
> Why does D complain when using == to compare with null? Is 
> there really any technical reason? if one just defines == null 
> to is null then there should be no problem. It seems like a 
> pedantic move by who ever implemented it and I'm hoping there 
> is actually a good technical reason for it.

tldr: this error is outdated.

In the days of yore, "obj == null" would call 
"obj.opEquals(null)". Attempting to call a virtual method on a 
null object is a quick path to a segmentation fault. So "obj == 
null" would either yield false or crash your program.

Except it's worse than that; your opEquals method had to 
explicitly check for null. So if your class had a custom equality 
function, "obj == null" was probably going to segfault no matter 
what.

Because of this common source of errors, in DMD 2.012 (2008), we 
got an error only for the case of comparing with a literal null. 
(The compiler isn't a mind-reader; it doesn't know whether that 
variable will be null when that line of code executes.)

This still sucked, so in 2015 we got a runtime function to handle 
object equality:
https://github.com/dlang/druntime/blob/dff824eda422b1fcdde5f2fe53120fcd71733aaa/src/object.d#L140

But we haven't removed the error message.

It *is* faster to call "foo is null" than "foo == null", but I 
don't think that's particularly worth a compiler error. The 
compiler could just convert it to "is null" automatically in that 
case.

One casualty of the current state of affairs is that no object 
may compare equal to null.