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comprehensive-rust/src/std/box-niche.md
Martin Geisler f16b41f012
Further simplify Box diagrams (#409) 
* Further simplify `Box` diagrams

I think we can improve the drawing by simplifying them and making them
more symbolic. Followup to #374.

* Apply suggestions from code review
2023-02-15 18:10:50 +00:00

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Niche Optimization

#[derive(Debug)]
enum List<T> {
    Cons(T, Box<List<T>>),
    Nil,
}

fn main() {
    let list: List<i32> = List::Cons(1, Box::new(List::Cons(2, Box::new(List::Nil))));
    println!("{list:?}");
}

A Box cannot be empty, so the pointer is always valid and non-null. This allows the compiler to optimize the memory layout:

 Stack                           Heap
.- - - - - - - - - - - - -.     .- - - - - - - - - - - - - - - - - - - - - - -.
:                         :     :                                             :
:    list                 :     :                                             :
:   +----+----+           :     :    +----+----+    +----+------+             :
:   | 1  | o--+-----------+-----+--->| 2  | o--+--->| // | null |             :
:   +----+----+           :     :    +----+----+    +----+------+             :
:                         :     :                                             :
:                         :     :                                             :
`- - - - - - - - - - - - -'     '- - - - - - - - - - - - - - - - - - - - - - -'