我试图通过使用像 C 的灵活数组成员这样的东西来避免多个堆分配。为此,我需要分配一个未确定大小的结构,但我没有找到任何通过智能指针来实现的方法。我对 Rc
特别感兴趣,但这也是 Box
的情况,这就是我将在示例中使用的内容。
这是我迄今为止最接近的:
use std::alloc::{self, Layout};
struct Inner {/* Sized fields */}
#[repr(C)] // Ensure the array is always last
// Both `inner` and `arr` need to be allocated, but preferably not separately
struct Unsized {
inner: Inner,
arr: [usize],
}
pub struct Exposed(Box<Unsized>);
impl Exposed {
pub fn new(capacity: usize) -> Self {
// Create a layout of an `Inner` followed by the array
let (layout, arr_base) = Layout::array::<usize>(capacity)
.and_then(|arr_layout| Layout::new::<Inner>().extend(arr_layout))
.unwrap();
let ptr = unsafe { alloc::alloc(layout) };
// At this point, `ptr` is `*mut u8` and the compiler doesn't know the size of the allocation
if ptr.is_null() {
panic!("Internal allocation error");
}
unsafe {
ptr.cast::<Inner>()
.write(Inner {/* Initialize sized fields */});
let tmp_ptr = ptr.add(arr_base).cast::<usize>();
// Initialize the array elements, in this case to 0
(0..capacity).for_each(|i| tmp_ptr.add(i).write(0));
// At this point everything is initialized and can safely be converted to `Box`
Self(Box::from_raw(ptr as *mut _))
}
}
}
这不编译:error[E0607]: cannot cast thin pointer `*mut u8` to fat pointer `*mut Unsized`
--> src/lib.rs:32:28
|
32 | Self(Box::from_raw(ptr as *mut _))
| ^^^^^^^^^^^^^
我可以直接使用 *mut u8
,但这似乎非常容易出错,并且需要手动删除。有没有办法从
ptr
创建一个胖指针? ,因为我实际上知道分配大小,还是从复合未大小类型创建智能指针?
最佳答案
问题是指针*mut Unsized
是一个宽指针,所以不仅仅是一个地址,而是一个地址和切片中元素的数量。指针*mut u8
另一方面,不包含有关切片长度的信息。标准库提供了
std::ptr::slice_from_raw_parts
和,std::ptr::slice_from_raw_parts_mut
对于这种情况。所以你首先创建一个假的(和错误的)
*mut usize
ptr as *mut usize
然后允许slice_from_raw_parts_mut(ptr as *mut usize, capacity)
创建一个假的(仍然是错误的)*mut [usize]
在宽指针中使用正确的长度字段,然后我们毫不客气地将其强制转换slice_from_raw_parts_mut(ptr as *mut usize, capacity) as *mut Unsized
它只改变类型(值不变),所以我们得到了正确的指针,我们现在可以最终输入 Box::from_raw
展示这篇文章的完整示例:use std::alloc::{self, Layout};
struct Inner {/* Sized fields */}
#[repr(C)] // Ensure the array is always last
// Both `inner` and `arr` need to be allocated, but preferably not separately
struct Unsized {
inner: Inner,
arr: [usize],
}
pub struct Exposed(Box<Unsized>);
impl Exposed {
pub fn new(capacity: usize) -> Self {
// Create a layout of an `Inner` followed by the array
let (layout, arr_base) = Layout::array::<usize>(capacity)
.and_then(|arr_layout| Layout::new::<Inner>().extend(arr_layout))
.unwrap();
let ptr = unsafe { alloc::alloc(layout) };
// At this point, `ptr` is `*mut u8` and the compiler doesn't know the size of the allocation
if ptr.is_null() {
panic!("Internal allocation error");
}
unsafe {
ptr.cast::<Inner>()
.write(Inner {/* Initialize sized fields */});
let tmp_ptr = ptr.add(arr_base).cast::<usize>();
// Initialize the array elements, in this case to 0
(0..capacity).for_each(|i| tmp_ptr.add(i).write(0));
}
// At this point everything is initialized and can safely be converted to `Box`
unsafe {
Self(Box::from_raw(
std::ptr::slice_from_raw_parts_mut(ptr as *mut usize, capacity) as *mut Unsized,
))
}
}
}
Playground旁注:您不需要
#[repr(C)]
确保未调整大小的切片字段在最后,这是有保证的。您需要它来了解字段的偏移量。
关于rust - 如何使用嵌入切片创建指向未调整大小类型的智能指针?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/64120001/