Do structs inside Box::new need curly braces

jdevries3133 0
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Do empty structures inside Box::new() need curly braces? If not, is there a preferred style?

struct Empty;

// are these equivalent?

fn get_empty_box() -> Box<Empty> {
  Box::new(Empty)
}

fn get_empty_box_alt() -> Box<Empty> {
  Box::new(Empty {})
}
rust
1 Answers

Depends on how they are defined.

If they are defined as struct Foo {} then yes.

If they are defined as struct Foo(); then they need parentheses or braces, but it's uncommon to see them with braces.

If they're defined as struct Foo; then they don't need braces (although they can accept them) and usually instantiated without braces.

The technical reason for that is that struct Foo; defines, in addition to the struct itself, a constant with the same name that contains an instance of the struct. That is, const Foo: Foo = Foo {};. When you spell Foo without braces you just copy this constant.

In a similar fashion, tuple structs (struct Foo();) define, in addition to the struct itself, a function that instantiates it: fn Foo() -> Foo { Foo {} }.

Chayim Friedman0
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