Detailed explanation of data and recursive types on the heap of Box in Rust

1. Basics of Box<T>

1.1 Division of labor between heap and stack

By default, Rust stores variables on the stack. However, the stack has limited space and is not applicable to data with unknown size or extremely large sizes.

useBox<T>, we can store data on the heap and keep only one pointer on the stack.

For example:

let b = Box::new(5);
println!("b = {}", b);

In this example, the variablebIt's oneBox<i32>, which points to the value stored on the heap5. whenbWhen leaving scope, Rust automatically cleans up pointers on the stack and data on the heap.

1.2 Performance Advantages

useBox<T>There are two main advantages:

• Memory efficiency: Although storing data on the heap may bring a small performance overhead, it is more efficient to use pointers to pass only copy fixed-size pointer data than copying large amounts of data directly on the stack.
• flexibility: When it is necessary to store data or large data blocks of unknown size,Box<T>The additional overhead caused by data replication can be avoided.

2. Use Box<T> to implement recursive types

2.1 Problems with recursive types

In some cases, we need to define recursive data structures, such as cons lists. In traditional recursive type definitions, each node may contain data for the next node.

If you directly nest this type, Rust cannot determine the size of the data structure at compile time, resulting in an "unlimited type size" error.

For example, the following enum definition will report an error:

enum List {
    Cons(i32, List),
    Nil,
}

becauseConsThe variant contains oneList, This results in infinite nesting, making it impossible to calculate the total size.

2.2 Breaking infinite nesting with Box<T>

To solve the above problems, we can useBox<T>Introduce an indirect level.

By lettingConsVariable storageBox<List>Instead of directly storingList, Rust will knowBox<T>The size of the data structure is calculated by the size of the entire data structure:

enum List {
    Cons(i32, Box<List>),
    Nil,
}

This way makes eachConsThe node contains ai32Value and a pointer to the next node.

Although the structure of the linked list is still recursive, since the pointer size is known, the compiler can successfully calculate the memory requirements of the entire data structure.

2.3 Cons List instance analysis

Cons list originated from the Lisp language and is used to build linked list data structures.

In Rust, we can use the above method to implement a simple cons list. For example, construct a list1, 2, 3Can be expressed as:

(1, (2, (3, Nil)))

In Rust, create this list by:

enum List {
    Cons(i32, Box<List>),
    Nil,
}

use List::{Cons, Nil};

fn main() {
    let list = Cons(1, Box::new(Cons(2, Box::new(Cons(3, Box::new(Nil))))));
    // You can add the operation on list here}

In this way, we not only successfully solved the problem of uncertain recursive type size, but also usedBox<T>The indirectness of the data structure maintains the flexibility of the data structure and memory efficiency.

3. More usage scenarios of Box<T>

In addition to being used for recursive types,Box<T>It is also very useful in several other scenarios:

• Size unknown type: When the size of the type is unknown at compile time,Box<T>It can help us put data on the heap, so that only pointers are saved on the stack.
• Efficient transfer of ownership: For the transfer of ownership of a large amount of data, it may take time to copy the entire data directly, while passing pointers is more efficient.
• Trait Object: When you only care about the implementation of a certain trait and not the specific type, useBox<dyn Trait>Can make your code more flexible. (See the content related to the trait object in Rust for details)

Summarize

In this article, we discussBox<T>Basic usage in Rust and its application in actual programming. By storing data on the heap,Box<T>It not only provides us with memory management convenience, but also solves problems such as recursive types with uncertain size. Whether it is to optimize the transfer of ownership of big data or to increase flexibility when using trait objects,Box<T>They are all very useful tools.

Once you master these concepts, you can use them confidently while writing more complex data structuresBox<T>, and have a deep understanding of Rust's memory management mechanism. I hope this article can help you better understand and apply itBox<T>。

The above is personal experience. I hope you can give you a reference and I hope you can support me more.