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86 lines
2 KiB
Markdown
86 lines
2 KiB
Markdown
---
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date: 2020-09-29
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id: d20e0dd7-ac1d-4dbb-b4e7-a6780b77bd69
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title: Box\<T\>
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---
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# Introduction
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Boxes allow you to store data on the heap rather than the stack. What
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remains on the stack is the pointer to the heap data.
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Boxes don’t have performance overhead, other than storing their data on
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the heap instead of on the stack. But they don’t have many extra
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capabilities either. There are three typical user cases for Boxes:
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- When you have a type whose size can’t be known at compile time and
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you want to use a value of that type in a context that requires an
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exact size
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- When you have a large amount of data and you want to transfer
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ownership but ensure the data won’t be copied when you do so
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- When you want to own a value and you care only that it’s a type that
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implements a particular trait rather than being of a specific type
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``` rust
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fn main() {
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let b = Box::new(5);
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println!("b = {}", b);
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}
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```
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# Reason to choose Box\<T\>
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`Box<T>` allows immutable or mutable borrows checked at compile time;
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[Rc\<T\>](20200929145534-rc_t) allows only immutable borrows checked at
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compile time; [RefCell\<T\>](20200929152628-refcell_t) allows immutable
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or mutable borrows checked at runtime.
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# Usercases
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## Type whose size can't be known at compile time
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When you have a type whose size can’t be known at compile time and you
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want to use a value of that type in a context that requires an exact
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size.
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The following won't compile as the `List` type doesn't have a known
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size:
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``` rust
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enum List {
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Cons(i32, List),
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Nil,
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}
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fn main() {}
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```
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This won't fly either:
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``` rust
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enum List {
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Cons(i32, List),
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Nil,
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}
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use crate::List::{Cons, Nil};
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fn main() {
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let list = Cons(1, Cons(2, Cons(3, Nil)));
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}
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```
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With pointers all things are possible, huzzah:
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``` rust
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enum List {
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Cons(i32, Box<List>),
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Nil,
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}
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use crate::List::{Cons, Nil};
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fn main() {
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let _list = Cons(1, Box::new(Cons(2, Box::new(Cons(3, Box::new(Nil))))));
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}
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```
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