Does the magic have a name <Trait>::<fn>() ex Default::default()

help
1

Hi,

I just came across as I'm learning the use of Default::default(), as used like this assert_eq!(SomeEnum::Empty, Default::default()); and in the docs like this let x: SomeStruct = Default::default();. This is throwing me as by looking I can't reason about the behavior. I understand the magic or automatic behavior, but nothing signals it other than obvious name, Default, in the syntax.

Does this have a name?
Is it used anywhere else other than Default::default()?

I've seen trait bounds and trait impls and so on. Had not seen a traits impl fn called in this way until now. Thanks for the help.

2

Default is a trait that you implement to define some good default values.

3

You can view it as an abbreviated form of fully-qualified syntax.

let _ = <String as Default>::default();
let _: String = <_ as Default>::default();
let _: String = <_>::default();
let _: String = Default::default();

It's not Default specific (but more common for trait functions that don't have self receivers for obvious reasons).

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4

Thanks for replying, I do understand that. Not really my question though, does this "pattern" have a name, example ::<> has a name turbo fish and it has a behavior it defines. However the compiler magic is hard to reason about just from the syntax.

5

Thank you. Wow, that now clears it up. Especially the note about self receivers. Aha! Thank you

6

There is no magic. Default is a trait and Default::default() is a trait method. Since its return type is Self, annotating the return type with a concrete type makes type inference infer that Self == ThatConcreteType, and looks up its Default impl.

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7

Thanks, I am not a person in the know, but your reply seems to fit; However, for me it is not as clear as the fully qualified syntax especially the doc page wording. Especially the note about sugar. Coming from OOP the sugar gave me a different impression vs what the compiler is actually allowing and doing.

8

The docs don't seem to have caught up, unless I missed it. as has an additional use in fully qualified syntax or Disambiguating Function Calls as I have learned.

as
as type cast reference

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