• Brother Bill (30/30) Sep 10 Page 119 of Programming in D
• IchorDev (9/10) Sep 10 You have already asked this question [here
• Brother Bill (2/12) Sep 10 Thank you so much. Got it.
• Nick Treleaven (10/14) Sep 10 Feature. An AA has reference semantics. Removing all elements

Brother Bill <brotherbill mail.com> writes:

Page 119 of Programming in D

It seems odd that == null and is null have different values?
Is this a bug or feature?
If a feature, what is the meanings of == null vs. is null?

source/app.d
```
import std.stdio;

void main()
{
     // value[key]
     int[string] dayNumbers =
          //   key     : value
         [
             "Monday": 0, "Tuesday": 1, "Wednesday": 2, 
"Thursday": 3, "Friday": 4,
             "Saturday": 5, "Sunday": 6
         ];

     dayNumbers.remove("Tuesday");
     // writeln(dayNumbers["Tuesday"]);		// ← run-time ERROR

     dayNumbers.clear;
     writeln("dayNumbers == null? ", dayNumbers == null);

     // Better, use 'is'
     writeln("dayNumbers is null? ", dayNumbers is null);
}

```

Console output:
```
dayNumbers == null? true
dayNumbers is null? false
```

IchorDev <zxinsworld gmail.com> writes:

On Wednesday, 10 September 2025 at 14:01:29 UTC, Brother Bill 
wrote:
 what is the meanings of == null vs. is null?

You have already asked this question [here 
before](https://forum.dlang.org/thread/nvaiwzvcrahnwziorazc forum.dlang.org).
The spec outlines the meaning of equality and identity expressions here:
- https://dlang.org/spec/expression.html#equality_expressions
- https://dlang.org/spec/expression.html#identity_expressions

You should try to reference the spec when you don't understand 
something. It's essentially the best resource for learning about 
D's finer points.

Brother Bill <brotherbill mail.com> writes:

On Wednesday, 10 September 2025 at 14:25:05 UTC, IchorDev wrote:
 On Wednesday, 10 September 2025 at 14:01:29 UTC, Brother Bill 
 wrote:
 what is the meanings of == null vs. is null?

 You have already asked this question [here 
 before](https://forum.dlang.org/thread/nvaiwzvcrahnwziorazc forum.dlang.org).
The spec outlines the meaning of equality and identity expressions here:
 - https://dlang.org/spec/expression.html#equality_expressions
 - https://dlang.org/spec/expression.html#identity_expressions

 You should try to reference the spec when you don't understand 
 something. It's essentially the best resource for learning 
 about D's finer points.

Thank you so much. Got it.

Nick Treleaven <nick geany.org> writes:

On Wednesday, 10 September 2025 at 14:01:29 UTC, Brother Bill 
wrote:
 Page 119 of Programming in D

 It seems odd that == null and is null have different values?
 Is this a bug or feature?
 If a feature, what is the meanings of == null vs. is null?

Feature. An AA has reference semantics. Removing all elements 
does not set it to `null` (the `.init` value), because it is 
already allocated, which may be useful for future operations.

Comparing for equality against null is true because both AA 
references have no elements (null is a valid AA reference).

Comparing for identity is false because `null` has no AA 
allocation whereas `dayNumbers` does - it isn't null.

https://dlang.org/spec/hash-map.html#construction_and_ref_semantic