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comprehensive-rust/src/std/box-recursive.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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Box with Recursive Data Structures

Recursive data types or data types with dynamic sizes need to use a Box:

#[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:?}");
}

 Stack                           Heap
.- - - - - - - - - - - - -.     .- - - - - - - - - - - - - - - - - - - - - - - -.
:                         :     :                                               :
:    list                 :     :                                               :
:   +------+----+----+    :     :    +------+----+----+    +------+----+----+   :
:   | Cons | 1  | o--+----+-----+--->| Cons | 2  | o--+--->| Nil  | // | // |   :
:   +------+----+----+    :     :    +------+----+----+    +------+----+----+   :
:                         :     :                                               :
:                         :     :                                               :
'- - - - - - - - - - - - -'     '- - - - - - - - - - - - - - - - - - - - - - - -'

  • If the Box was not used here and we attempted to embed a List directly into the List, the compiler would not compute a fixed size of the struct in memory, it would look infinite.

  • Box solves this problem as it has the same size as a regular pointer and just points at the next element of the List in the heap.

  • Remove the Box in the List definition and show the compiler error. "Recursive with indirection" is a hint you might want to use a Box or reference of some kind, instead of storing a value directly.