1 // Copyright 2015 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
11 use core::ptr::Unique;
13 use core::slice::{self, SliceExt};
16 use super::boxed::Box;
20 /// A low-level utility for more ergonomically allocating, reallocating, and deallocating a
21 /// a buffer of memory on the heap without having to worry about all the corner cases
22 /// involved. This type is excellent for building your own data structures like Vec and VecDeque.
25 /// * Produces heap::EMPTY on zero-sized types
26 /// * Produces heap::EMPTY on zero-length allocations
27 /// * Catches all overflows in capacity computations (promotes them to "capacity overflow" panics)
28 /// * Guards against 32-bit systems allocating more than isize::MAX bytes
29 /// * Guards against overflowing your length
31 /// * Avoids freeing heap::EMPTY
32 /// * Contains a ptr::Unique and thus endows the user with all related benefits
34 /// This type does not in anyway inspect the memory that it manages. When dropped it *will*
35 /// free its memory, but it *won't* try to Drop its contents. It is up to the user of RawVec
36 /// to handle the actual things *stored* inside of a RawVec.
38 /// Note that a RawVec always forces its capacity to be usize::MAX for zero-sized types.
39 /// This enables you to use capacity growing logic catch the overflows in your length
40 /// that might occur with zero-sized types.
42 /// However this means that you need to be careful when roundtripping this type
43 /// with a `Box<[T]>`: `cap()` won't yield the len. However `with_capacity`,
44 /// `shrink_to_fit`, and `from_box` will actually set RawVec's private capacity
45 /// field. This allows zero-sized types to not be special-cased by consumers of
47 #[unsafe_no_drop_flag]
48 pub struct RawVec<T> {
54 /// Creates the biggest possible RawVec without allocating. If T has positive
55 /// size, then this makes a RawVec with capacity 0. If T has 0 size, then it
56 /// it makes a RawVec with capacity `usize::MAX`. Useful for implementing
57 /// delayed allocation.
58 pub fn new() -> Self {
60 // !0 is usize::MAX. This branch should be stripped at compile time.
61 let cap = if mem::size_of::<T>() == 0 {
67 // heap::EMPTY doubles as "unallocated" and "zero-sized allocation"
68 RawVec { ptr: Unique::new(heap::EMPTY as *mut T), cap: cap }
72 /// Creates a RawVec with exactly the capacity and alignment requirements
73 /// for a `[T; cap]`. This is equivalent to calling RawVec::new when `cap` is 0
74 /// or T is zero-sized. Note that if `T` is zero-sized this means you will *not*
75 /// get a RawVec with the requested capacity!
79 /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
80 /// * Panics on 32-bit platforms if the requested capacity exceeds
81 /// `isize::MAX` bytes.
86 pub fn with_capacity(cap: usize) -> Self {
88 let elem_size = mem::size_of::<T>();
90 let alloc_size = cap.checked_mul(elem_size).expect("capacity overflow");
91 alloc_guard(alloc_size);
93 // handles ZSTs and `cap = 0` alike
94 let ptr = if alloc_size == 0 {
95 heap::EMPTY as *mut u8
97 let align = mem::align_of::<T>();
98 let ptr = heap::allocate(alloc_size, align);
105 RawVec { ptr: Unique::new(ptr as *mut _), cap: cap }
109 /// Reconstitutes a RawVec from a pointer and capacity.
111 /// # Undefined Behaviour
113 /// The ptr must be allocated, and with the given capacity. The
114 /// capacity cannot exceed `isize::MAX` (only a concern on 32-bit systems).
115 /// If the ptr and capacity come from a RawVec, then this is guaranteed.
116 pub unsafe fn from_raw_parts(ptr: *mut T, cap: usize) -> Self {
117 RawVec { ptr: Unique::new(ptr), cap: cap }
120 /// Converts a `Box<[T]>` into a `RawVec<T>`.
121 pub fn from_box(mut slice: Box<[T]>) -> Self {
123 let result = RawVec::from_raw_parts(slice.as_mut_ptr(), slice.len());
131 /// Gets a raw pointer to the start of the allocation. Note that this is
132 /// heap::EMPTY if `cap = 0` or T is zero-sized. In the former case, you must
134 pub fn ptr(&self) -> *mut T {
138 /// Gets the capacity of the allocation.
140 /// This will always be `usize::MAX` if `T` is zero-sized.
141 pub fn cap(&self) -> usize {
142 if mem::size_of::<T>() == 0 {
149 /// Doubles the size of the type's backing allocation. This is common enough
150 /// to want to do that it's easiest to just have a dedicated method. Slightly
151 /// more efficient logic can be provided for this than the general case.
153 /// This function is ideal for when pushing elements one-at-a-time because
154 /// you don't need to incur the costs of the more general computations
155 /// reserve needs to do to guard against overflow. You do however need to
156 /// manually check if your `len == cap`.
160 /// * Panics if T is zero-sized on the assumption that you managed to exhaust
161 /// all `usize::MAX` slots in your imaginary buffer.
162 /// * Panics on 32-bit platforms if the requested capacity exceeds
163 /// `isize::MAX` bytes.
172 /// struct MyVec<T> {
177 /// impl<T> MyVec<T> {
178 /// pub fn push(&mut self, elem: T) {
179 /// if self.len == self.buf.cap() { self.buf.double(); }
180 /// // double would have aborted or panicked if the len exceeded
181 /// // `isize::MAX` so this is safe to do unchecked now.
183 /// ptr::write(self.buf.ptr().offset(self.len as isize), elem);
191 pub fn double(&mut self) {
193 let elem_size = mem::size_of::<T>();
195 // since we set the capacity to usize::MAX when elem_size is
196 // 0, getting to here necessarily means the RawVec is overfull.
197 assert!(elem_size != 0, "capacity overflow");
199 let align = mem::align_of::<T>();
201 let (new_cap, ptr) = if self.cap == 0 {
202 // skip to 4 because tiny Vec's are dumb; but not if that would cause overflow
203 let new_cap = if elem_size > (!0) / 8 {
208 let ptr = heap::allocate(new_cap * elem_size, align);
211 // Since we guarantee that we never allocate more than isize::MAX bytes,
212 // `elem_size * self.cap <= isize::MAX` as a precondition, so this can't overflow
213 let new_cap = 2 * self.cap;
214 let new_alloc_size = new_cap * elem_size;
215 alloc_guard(new_alloc_size);
216 let ptr = heap::reallocate(self.ptr() as *mut _,
217 self.cap * elem_size,
223 // If allocate or reallocate fail, we'll get `null` back
228 self.ptr = Unique::new(ptr as *mut _);
233 /// Ensures that the buffer contains at least enough space to hold
234 /// `used_cap + needed_extra_cap` elements. If it doesn't already,
235 /// will reallocate the minimum possible amount of memory necessary.
236 /// Generally this will be exactly the amount of memory necessary,
237 /// but in principle the allocator is free to give back more than
240 /// If `used_cap` exceeds `self.cap()`, this may fail to actually allocate
241 /// the requested space. This is not really unsafe, but the unsafe
242 /// code *you* write that relies on the behaviour of this function may break.
246 /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
247 /// * Panics on 32-bit platforms if the requested capacity exceeds
248 /// `isize::MAX` bytes.
253 pub fn reserve_exact(&mut self, used_cap: usize, needed_extra_cap: usize) {
255 let elem_size = mem::size_of::<T>();
256 let align = mem::align_of::<T>();
258 // NOTE: we don't early branch on ZSTs here because we want this
259 // to actually catch "asking for more than usize::MAX" in that case.
260 // If we make it past the first branch then we are guaranteed to
263 // Don't actually need any more capacity.
264 // Wrapping in case they gave a bad `used_cap`.
265 if self.cap().wrapping_sub(used_cap) >= needed_extra_cap {
269 // Nothing we can really do about these checks :(
270 let new_cap = used_cap.checked_add(needed_extra_cap).expect("capacity overflow");
271 let new_alloc_size = new_cap.checked_mul(elem_size).expect("capacity overflow");
272 alloc_guard(new_alloc_size);
274 let ptr = if self.cap == 0 {
275 heap::allocate(new_alloc_size, align)
277 heap::reallocate(self.ptr() as *mut _,
278 self.cap * elem_size,
283 // If allocate or reallocate fail, we'll get `null` back
288 self.ptr = Unique::new(ptr as *mut _);
293 /// Ensures that the buffer contains at least enough space to hold
294 /// `used_cap + needed_extra_cap` elements. If it doesn't already have
295 /// enough capacity, will reallocate enough space plus comfortable slack
296 /// space to get amortized `O(1)` behaviour. Will limit this behaviour
297 /// if it would needlessly cause itself to panic.
299 /// If `used_cap` exceeds `self.cap()`, this may fail to actually allocate
300 /// the requested space. This is not really unsafe, but the unsafe
301 /// code *you* write that relies on the behaviour of this function may break.
303 /// This is ideal for implementing a bulk-push operation like `extend`.
307 /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
308 /// * Panics on 32-bit platforms if the requested capacity exceeds
309 /// `isize::MAX` bytes.
318 /// struct MyVec<T> {
323 /// impl<T> MyVec<T> {
324 /// pub fn push_all(&mut self, elems: &[T]) {
325 /// self.buf.reserve(self.len, elems.len());
326 /// // reserve would have aborted or panicked if the len exceeded
327 /// // `isize::MAX` so this is safe to do unchecked now.
330 /// ptr::write(self.buf.ptr().offset(self.len as isize), x.clone());
337 pub fn reserve(&mut self, used_cap: usize, needed_extra_cap: usize) {
339 let elem_size = mem::size_of::<T>();
340 let align = mem::align_of::<T>();
342 // NOTE: we don't early branch on ZSTs here because we want this
343 // to actually catch "asking for more than usize::MAX" in that case.
344 // If we make it past the first branch then we are guaranteed to
347 // Don't actually need any more capacity.
348 // Wrapping in case they give a bas `used_cap`
349 if self.cap().wrapping_sub(used_cap) >= needed_extra_cap {
353 // Nothing we can really do about these checks :(
354 let new_cap = used_cap.checked_add(needed_extra_cap)
355 .and_then(|cap| cap.checked_mul(2))
356 .expect("capacity overflow");
357 let new_alloc_size = new_cap.checked_mul(elem_size).expect("capacity overflow");
358 // FIXME: may crash and burn on over-reserve
359 alloc_guard(new_alloc_size);
361 let ptr = if self.cap == 0 {
362 heap::allocate(new_alloc_size, align)
364 heap::reallocate(self.ptr() as *mut _,
365 self.cap * elem_size,
370 // If allocate or reallocate fail, we'll get `null` back
375 self.ptr = Unique::new(ptr as *mut _);
380 /// Shrinks the allocation down to the specified amount. If the given amount
381 /// is 0, actually completely deallocates.
385 /// Panics if the given amount is *larger* than the current capacity.
390 pub fn shrink_to_fit(&mut self, amount: usize) {
391 let elem_size = mem::size_of::<T>();
392 let align = mem::align_of::<T>();
394 // Set the `cap` because they might be about to promote to a `Box<[T]>`
400 // This check is my waterloo; it's the only thing Vec wouldn't have to do.
401 assert!(self.cap >= amount,
402 "Tried to shrink to a larger capacity");
405 mem::replace(self, RawVec::new());
406 } else if self.cap != amount {
408 // Overflow check is unnecessary as the vector is already at
410 let ptr = heap::reallocate(self.ptr() as *mut _,
411 self.cap * elem_size,
417 self.ptr = Unique::new(ptr as *mut _);
423 /// Converts the entire buffer into `Box<[T]>`.
425 /// While it is not *strictly* Undefined Behaviour to call
426 /// this procedure while some of the RawVec is unintialized,
427 /// it cetainly makes it trivial to trigger it.
429 /// Note that this will correctly reconstitute any `cap` changes
430 /// that may have been performed. (see description of type for details)
431 pub unsafe fn into_box(self) -> Box<[T]> {
432 // NOTE: not calling `cap()` here, actually using the real `cap` field!
433 let slice = slice::from_raw_parts_mut(self.ptr(), self.cap);
434 let output: Box<[T]> = Box::from_raw(slice);
439 /// This is a stupid name in the hopes that someone will find this in the
440 /// not too distant future and remove it with the rest of
441 /// #[unsafe_no_drop_flag]
442 pub fn unsafe_no_drop_flag_needs_drop(&self) -> bool {
443 self.cap != mem::POST_DROP_USIZE
447 impl<T> Drop for RawVec<T> {
448 /// Frees the memory owned by the RawVec *without* trying to Drop its contents.
450 let elem_size = mem::size_of::<T>();
451 if elem_size != 0 && self.cap != 0 && self.unsafe_no_drop_flag_needs_drop() {
452 let align = mem::align_of::<T>();
454 let num_bytes = elem_size * self.cap;
456 heap::deallocate(*self.ptr as *mut _, num_bytes, align);
464 // We need to guarantee the following:
465 // * We don't ever allocate `> isize::MAX` byte-size objects
466 // * We don't overflow `usize::MAX` and actually allocate too little
468 // On 64-bit we just need to check for overflow since trying to allocate
469 // `> isize::MAX` bytes will surely fail. On 32-bit we need to add an extra
470 // guard for this in case we're running on a platform which can use all 4GB in
471 // user-space. e.g. PAE or x32
474 fn alloc_guard(alloc_size: usize) {
475 if core::usize::BITS < 64 {
476 assert!(alloc_size <= ::core::isize::MAX as usize,
477 "capacity overflow");