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* 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
42 lines
1.6 KiB
Markdown
42 lines
1.6 KiB
Markdown
# Box with Recursive Data Structures
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Recursive data types or data types with dynamic sizes need to use a `Box`:
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```rust,editable
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#[derive(Debug)]
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enum List<T> {
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Cons(T, Box<List<T>>),
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Nil,
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}
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fn main() {
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let list: List<i32> = List::Cons(1, Box::new(List::Cons(2, Box::new(List::Nil))));
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println!("{list:?}");
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}
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```
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```bob
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Stack Heap
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.- - - - - - - - - - - - -. .- - - - - - - - - - - - - - - - - - - - - - - -.
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: : : :
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: list : : :
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: +------+----+----+ : : +------+----+----+ +------+----+----+ :
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: | Cons | 1 | o--+----+-----+--->| Cons | 2 | o--+--->| Nil | // | // | :
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: +------+----+----+ : : +------+----+----+ +------+----+----+ :
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: : : :
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: : : :
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'- - - - - - - - - - - - -' '- - - - - - - - - - - - - - - - - - - - - - - -'
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```
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<details>
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* If the `Box` was not used here and we attempted to embed a `List` directly into the `List`,
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the compiler would not compute a fixed size of the struct in memory, it would look infinite.
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* `Box` solves this problem as it has the same size as a regular pointer and just points at the next
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element of the `List` in the heap.
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* 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.
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</details>
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