Rust: Vec类型的into_boxed_slice()方法

songroom2023-07-27 14:37

比如，我们经常看到Vec类型，但取转其裸指针，经常会看到into_boxed_slice()方法，这是为何？

use std::{fmt, slice};

#[derive(Clone, Copy)]
struct RawBuffer {
    ptr: *mut u8,
    len: usize,
}

impl From<Vec<u8>> for RawBuffer {
    fn from(vec: Vec<u8>) -> Self {
        let slice = vec.into_boxed_slice();
        Self {
            len: slice.len(),
            // into_raw 之后，Box 就不管这块内存的释放了，RawBuffer 需要处理释放
            ptr: Box::into_raw(slice) as *mut u8,
        }
    }
}

其实，你看标准文档，就很清楚，

pub fn into_boxed_slice(self) -> Box<[T], A>

//Converts the vector into Box<[T]>.

//If the vector has excess capacity, its items will be moved into a newly-allocated buffer with exactly the right capacity.

也就是说，转成了Box<[T]>后，指针所指向的类型，更简短了。

let v = vec![1, 2, 3];
let slice = v.into_boxed_slice();
let mut vec = Vec::with_capacity(10);
vec.extend([1, 2, 3]);

assert!(vec.capacity() >= 10);
let slice = vec.into_boxed_slice();
assert_eq!(slice.into_vec().capacity(), 3);

这样，从操作上来讲，更加节省内存空间。