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

On Wednesday, 10 September 2025 at 14:01:29 UTC, Brother Bill wrote:

You have already asked this question here before. 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.

On Wednesday, 10 September 2025 at 14:01:29 UTC, Brother Bill wrote:

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

On Wednesday, 10 September 2025 at 14:25:05 UTC, IchorDev wrote:

Thank you so much. Got it.