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;
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"
69 ptr: Unique::new(heap::EMPTY as *mut T),
75 /// Creates a RawVec with exactly the capacity and alignment requirements
76 /// for a `[T; cap]`. This is equivalent to calling RawVec::new when `cap` is 0
77 /// or T is zero-sized. Note that if `T` is zero-sized this means you will *not*
78 /// get a RawVec with the requested capacity!
82 /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
83 /// * Panics on 32-bit platforms if the requested capacity exceeds
84 /// `isize::MAX` bytes.
89 pub fn with_capacity(cap: usize) -> Self {
91 let elem_size = mem::size_of::<T>();
93 let alloc_size = cap.checked_mul(elem_size).expect("capacity overflow");
94 alloc_guard(alloc_size);
96 // handles ZSTs and `cap = 0` alike
97 let ptr = if alloc_size == 0 {
98 heap::EMPTY as *mut u8
100 let align = mem::align_of::<T>();
101 let ptr = heap::allocate(alloc_size, align);
109 ptr: Unique::new(ptr as *mut _),
115 /// Reconstitutes a RawVec from a pointer and capacity.
117 /// # Undefined Behavior
119 /// The ptr must be allocated, and with the given capacity. The
120 /// capacity cannot exceed `isize::MAX` (only a concern on 32-bit systems).
121 /// If the ptr and capacity come from a RawVec, then this is guaranteed.
122 pub unsafe fn from_raw_parts(ptr: *mut T, cap: usize) -> Self {
124 ptr: Unique::new(ptr),
129 /// Converts a `Box<[T]>` into a `RawVec<T>`.
130 pub fn from_box(mut slice: Box<[T]>) -> Self {
132 let result = RawVec::from_raw_parts(slice.as_mut_ptr(), slice.len());
140 /// Gets a raw pointer to the start of the allocation. Note that this is
141 /// heap::EMPTY if `cap = 0` or T is zero-sized. In the former case, you must
143 pub fn ptr(&self) -> *mut T {
147 /// Gets the capacity of the allocation.
149 /// This will always be `usize::MAX` if `T` is zero-sized.
150 pub fn cap(&self) -> usize {
151 if mem::size_of::<T>() == 0 {
158 /// Doubles the size of the type's backing allocation. This is common enough
159 /// to want to do that it's easiest to just have a dedicated method. Slightly
160 /// more efficient logic can be provided for this than the general case.
162 /// This function is ideal for when pushing elements one-at-a-time because
163 /// you don't need to incur the costs of the more general computations
164 /// reserve needs to do to guard against overflow. You do however need to
165 /// manually check if your `len == cap`.
169 /// * Panics if T is zero-sized on the assumption that you managed to exhaust
170 /// all `usize::MAX` slots in your imaginary buffer.
171 /// * Panics on 32-bit platforms if the requested capacity exceeds
172 /// `isize::MAX` bytes.
181 /// struct MyVec<T> {
186 /// impl<T> MyVec<T> {
187 /// pub fn push(&mut self, elem: T) {
188 /// if self.len == self.buf.cap() { self.buf.double(); }
189 /// // double would have aborted or panicked if the len exceeded
190 /// // `isize::MAX` so this is safe to do unchecked now.
192 /// ptr::write(self.buf.ptr().offset(self.len as isize), elem);
200 pub fn double(&mut self) {
202 let elem_size = mem::size_of::<T>();
204 // since we set the capacity to usize::MAX when elem_size is
205 // 0, getting to here necessarily means the RawVec is overfull.
206 assert!(elem_size != 0, "capacity overflow");
208 let align = mem::align_of::<T>();
210 let (new_cap, ptr) = if self.cap == 0 {
211 // skip to 4 because tiny Vec's are dumb; but not if that would cause overflow
212 let new_cap = if elem_size > (!0) / 8 {
217 let ptr = heap::allocate(new_cap * elem_size, align);
220 // Since we guarantee that we never allocate more than isize::MAX bytes,
221 // `elem_size * self.cap <= isize::MAX` as a precondition, so this can't overflow
222 let new_cap = 2 * self.cap;
223 let new_alloc_size = new_cap * elem_size;
224 alloc_guard(new_alloc_size);
225 let ptr = heap::reallocate(self.ptr() as *mut _,
226 self.cap * elem_size,
232 // If allocate or reallocate fail, we'll get `null` back
237 self.ptr = Unique::new(ptr as *mut _);
242 /// Attempts to double the size of the type's backing allocation in place. This is common
243 /// enough to want to do that it's easiest to just have a dedicated method. Slightly
244 /// more efficient logic can be provided for this than the general case.
246 /// Returns true if the reallocation attempt has succeeded, or false otherwise.
250 /// * Panics if T is zero-sized on the assumption that you managed to exhaust
251 /// all `usize::MAX` slots in your imaginary buffer.
252 /// * Panics on 32-bit platforms if the requested capacity exceeds
253 /// `isize::MAX` bytes.
256 pub fn double_in_place(&mut self) -> bool {
258 let elem_size = mem::size_of::<T>();
259 let align = mem::align_of::<T>();
261 // since we set the capacity to usize::MAX when elem_size is
262 // 0, getting to here necessarily means the RawVec is overfull.
263 assert!(elem_size != 0, "capacity overflow");
265 // Since we guarantee that we never allocate more than isize::MAX bytes,
266 // `elem_size * self.cap <= isize::MAX` as a precondition, so this can't overflow
267 let new_cap = 2 * self.cap;
268 let new_alloc_size = new_cap * elem_size;
270 alloc_guard(new_alloc_size);
271 let size = heap::reallocate_inplace(self.ptr() as *mut _,
272 self.cap * elem_size,
275 if size >= new_alloc_size {
276 // We can't directly divide `size`.
279 size >= new_alloc_size
283 /// Ensures that the buffer contains at least enough space to hold
284 /// `used_cap + needed_extra_cap` elements. If it doesn't already,
285 /// will reallocate the minimum possible amount of memory necessary.
286 /// Generally this will be exactly the amount of memory necessary,
287 /// but in principle the allocator is free to give back more than
290 /// If `used_cap` exceeds `self.cap()`, this may fail to actually allocate
291 /// the requested space. This is not really unsafe, but the unsafe
292 /// code *you* write that relies on the behavior of this function may break.
296 /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
297 /// * Panics on 32-bit platforms if the requested capacity exceeds
298 /// `isize::MAX` bytes.
303 pub fn reserve_exact(&mut self, used_cap: usize, needed_extra_cap: usize) {
305 let elem_size = mem::size_of::<T>();
306 let align = mem::align_of::<T>();
308 // NOTE: we don't early branch on ZSTs here because we want this
309 // to actually catch "asking for more than usize::MAX" in that case.
310 // If we make it past the first branch then we are guaranteed to
313 // Don't actually need any more capacity.
314 // Wrapping in case they gave a bad `used_cap`.
315 if self.cap().wrapping_sub(used_cap) >= needed_extra_cap {
319 // Nothing we can really do about these checks :(
320 let new_cap = used_cap.checked_add(needed_extra_cap).expect("capacity overflow");
321 let new_alloc_size = new_cap.checked_mul(elem_size).expect("capacity overflow");
322 alloc_guard(new_alloc_size);
324 let ptr = if self.cap == 0 {
325 heap::allocate(new_alloc_size, align)
327 heap::reallocate(self.ptr() as *mut _,
328 self.cap * elem_size,
333 // If allocate or reallocate fail, we'll get `null` back
338 self.ptr = Unique::new(ptr as *mut _);
343 /// Calculates the buffer's new size given that it'll hold `used_cap +
344 /// needed_extra_cap` elements. This logic is used in amortized reserve methods.
345 /// Returns `(new_capacity, new_alloc_size)`.
346 fn amortized_new_size(&self, used_cap: usize, needed_extra_cap: usize) -> (usize, usize) {
347 let elem_size = mem::size_of::<T>();
348 // Nothing we can really do about these checks :(
349 let required_cap = used_cap.checked_add(needed_extra_cap)
350 .expect("capacity overflow");
351 // Cannot overflow, because `cap <= isize::MAX`, and type of `cap` is `usize`.
352 let double_cap = self.cap * 2;
353 // `double_cap` guarantees exponential growth.
354 let new_cap = cmp::max(double_cap, required_cap);
355 let new_alloc_size = new_cap.checked_mul(elem_size).expect("capacity overflow");
356 (new_cap, new_alloc_size)
359 /// Ensures that the buffer contains at least enough space to hold
360 /// `used_cap + needed_extra_cap` elements. If it doesn't already have
361 /// enough capacity, will reallocate enough space plus comfortable slack
362 /// space to get amortized `O(1)` behavior. Will limit this behavior
363 /// if it would needlessly cause itself to panic.
365 /// If `used_cap` exceeds `self.cap()`, this may fail to actually allocate
366 /// the requested space. This is not really unsafe, but the unsafe
367 /// code *you* write that relies on the behavior of this function may break.
369 /// This is ideal for implementing a bulk-push operation like `extend`.
373 /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
374 /// * Panics on 32-bit platforms if the requested capacity exceeds
375 /// `isize::MAX` bytes.
384 /// struct MyVec<T> {
389 /// impl<T> MyVec<T> {
390 /// pub fn push_all(&mut self, elems: &[T]) {
391 /// self.buf.reserve(self.len, elems.len());
392 /// // reserve would have aborted or panicked if the len exceeded
393 /// // `isize::MAX` so this is safe to do unchecked now.
396 /// ptr::write(self.buf.ptr().offset(self.len as isize), x.clone());
403 pub fn reserve(&mut self, used_cap: usize, needed_extra_cap: usize) {
405 let elem_size = mem::size_of::<T>();
406 let align = mem::align_of::<T>();
408 // NOTE: we don't early branch on ZSTs here because we want this
409 // to actually catch "asking for more than usize::MAX" in that case.
410 // If we make it past the first branch then we are guaranteed to
413 // Don't actually need any more capacity.
414 // Wrapping in case they give a bad `used_cap`
415 if self.cap().wrapping_sub(used_cap) >= needed_extra_cap {
419 let (new_cap, new_alloc_size) = self.amortized_new_size(used_cap, needed_extra_cap);
420 // FIXME: may crash and burn on over-reserve
421 alloc_guard(new_alloc_size);
423 let ptr = if self.cap == 0 {
424 heap::allocate(new_alloc_size, align)
426 heap::reallocate(self.ptr() as *mut _,
427 self.cap * elem_size,
432 // If allocate or reallocate fail, we'll get `null` back
437 self.ptr = Unique::new(ptr as *mut _);
442 /// Attempts to ensure that the buffer contains at least enough space to hold
443 /// `used_cap + needed_extra_cap` elements. If it doesn't already have
444 /// enough capacity, will reallocate in place enough space plus comfortable slack
445 /// space to get amortized `O(1)` behaviour. Will limit this behaviour
446 /// if it would needlessly cause itself to panic.
448 /// If `used_cap` exceeds `self.cap()`, this may fail to actually allocate
449 /// the requested space. This is not really unsafe, but the unsafe
450 /// code *you* write that relies on the behaviour of this function may break.
452 /// Returns true if the reallocation attempt has succeeded, or false otherwise.
456 /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
457 /// * Panics on 32-bit platforms if the requested capacity exceeds
458 /// `isize::MAX` bytes.
459 pub fn reserve_in_place(&mut self, used_cap: usize, needed_extra_cap: usize) -> bool {
461 let elem_size = mem::size_of::<T>();
462 let align = mem::align_of::<T>();
464 // NOTE: we don't early branch on ZSTs here because we want this
465 // to actually catch "asking for more than usize::MAX" in that case.
466 // If we make it past the first branch then we are guaranteed to
469 // Don't actually need any more capacity. If the current `cap` is 0, we can't
470 // reallocate in place.
471 // Wrapping in case they give a bad `used_cap`
472 if self.cap().wrapping_sub(used_cap) >= needed_extra_cap || self.cap == 0 {
476 let (_, new_alloc_size) = self.amortized_new_size(used_cap, needed_extra_cap);
477 // FIXME: may crash and burn on over-reserve
478 alloc_guard(new_alloc_size);
480 let size = heap::reallocate_inplace(self.ptr() as *mut _,
481 self.cap * elem_size,
484 if size >= new_alloc_size {
485 self.cap = new_alloc_size / elem_size;
487 size >= new_alloc_size
491 /// Shrinks the allocation down to the specified amount. If the given amount
492 /// is 0, actually completely deallocates.
496 /// Panics if the given amount is *larger* than the current capacity.
501 pub fn shrink_to_fit(&mut self, amount: usize) {
502 let elem_size = mem::size_of::<T>();
503 let align = mem::align_of::<T>();
505 // Set the `cap` because they might be about to promote to a `Box<[T]>`
511 // This check is my waterloo; it's the only thing Vec wouldn't have to do.
512 assert!(self.cap >= amount, "Tried to shrink to a larger capacity");
515 mem::replace(self, RawVec::new());
516 } else if self.cap != amount {
518 // Overflow check is unnecessary as the vector is already at
520 let ptr = heap::reallocate(self.ptr() as *mut _,
521 self.cap * elem_size,
527 self.ptr = Unique::new(ptr as *mut _);
533 /// Converts the entire buffer into `Box<[T]>`.
535 /// While it is not *strictly* Undefined Behavior to call
536 /// this procedure while some of the RawVec is unintialized,
537 /// it cetainly makes it trivial to trigger it.
539 /// Note that this will correctly reconstitute any `cap` changes
540 /// that may have been performed. (see description of type for details)
541 pub unsafe fn into_box(self) -> Box<[T]> {
542 // NOTE: not calling `cap()` here, actually using the real `cap` field!
543 let slice = slice::from_raw_parts_mut(self.ptr(), self.cap);
544 let output: Box<[T]> = Box::from_raw(slice);
549 /// This is a stupid name in the hopes that someone will find this in the
550 /// not too distant future and remove it with the rest of
551 /// #[unsafe_no_drop_flag]
552 pub fn unsafe_no_drop_flag_needs_drop(&self) -> bool {
553 self.cap != mem::POST_DROP_USIZE
557 impl<T> Drop for RawVec<T> {
558 #[unsafe_destructor_blind_to_params]
559 /// Frees the memory owned by the RawVec *without* trying to Drop its contents.
561 let elem_size = mem::size_of::<T>();
562 if elem_size != 0 && self.cap != 0 && self.unsafe_no_drop_flag_needs_drop() {
563 let align = mem::align_of::<T>();
565 let num_bytes = elem_size * self.cap;
567 heap::deallocate(*self.ptr as *mut _, num_bytes, align);
575 // We need to guarantee the following:
576 // * We don't ever allocate `> isize::MAX` byte-size objects
577 // * We don't overflow `usize::MAX` and actually allocate too little
579 // On 64-bit we just need to check for overflow since trying to allocate
580 // `> isize::MAX` bytes will surely fail. On 32-bit we need to add an extra
581 // guard for this in case we're running on a platform which can use all 4GB in
582 // user-space. e.g. PAE or x32
585 fn alloc_guard(alloc_size: usize) {
586 if mem::size_of::<usize>() < 8 {
587 assert!(alloc_size <= ::core::isize::MAX as usize,
588 "capacity overflow");
598 fn reserve_does_not_overallocate() {
600 let mut v: RawVec<u32> = RawVec::new();
601 // First `reserve` allocates like `reserve_exact`
603 assert_eq!(9, v.cap());
607 let mut v: RawVec<u32> = RawVec::new();
609 assert_eq!(7, v.cap());
610 // 97 if more than double of 7, so `reserve` should work
611 // like `reserve_exact`.
613 assert_eq!(97, v.cap());
617 let mut v: RawVec<u32> = RawVec::new();
619 assert_eq!(12, v.cap());
621 // 3 is less than half of 12, so `reserve` must grow
622 // exponentially. At the time of writing this test grow
623 // factor is 2, so new capacity is 24, however, grow factor
624 // of 1.5 is OK too. Hence `>= 18` in assert.
625 assert!(v.cap() >= 12 + 12 / 2);