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;
21 /// A low-level utility for more ergonomically allocating, reallocating, and deallocating a
22 /// a buffer of memory on the heap without having to worry about all the corner cases
23 /// involved. This type is excellent for building your own data structures like Vec and VecDeque.
26 /// * Produces heap::EMPTY on zero-sized types
27 /// * Produces heap::EMPTY on zero-length allocations
28 /// * Catches all overflows in capacity computations (promotes them to "capacity overflow" panics)
29 /// * Guards against 32-bit systems allocating more than isize::MAX bytes
30 /// * Guards against overflowing your length
32 /// * Avoids freeing heap::EMPTY
33 /// * Contains a ptr::Unique and thus endows the user with all related benefits
35 /// This type does not in anyway inspect the memory that it manages. When dropped it *will*
36 /// free its memory, but it *won't* try to Drop its contents. It is up to the user of RawVec
37 /// to handle the actual things *stored* inside of a RawVec.
39 /// Note that a RawVec always forces its capacity to be usize::MAX for zero-sized types.
40 /// This enables you to use capacity growing logic catch the overflows in your length
41 /// that might occur with zero-sized types.
43 /// However this means that you need to be careful when roundtripping this type
44 /// with a `Box<[T]>`: `cap()` won't yield the len. However `with_capacity`,
45 /// `shrink_to_fit`, and `from_box` will actually set RawVec's private capacity
46 /// field. This allows zero-sized types to not be special-cased by consumers of
48 #[unsafe_no_drop_flag]
49 pub struct RawVec<T> {
55 /// Creates the biggest possible RawVec without allocating. If T has positive
56 /// size, then this makes a RawVec with capacity 0. If T has 0 size, then it
57 /// it makes a RawVec with capacity `usize::MAX`. Useful for implementing
58 /// delayed allocation.
59 pub fn new() -> Self {
61 // !0 is usize::MAX. This branch should be stripped at compile time.
62 let cap = if mem::size_of::<T>() == 0 {
68 // heap::EMPTY doubles as "unallocated" and "zero-sized allocation"
70 ptr: Unique::new(heap::EMPTY as *mut T),
76 /// Creates a RawVec with exactly the capacity and alignment requirements
77 /// for a `[T; cap]`. This is equivalent to calling RawVec::new when `cap` is 0
78 /// or T is zero-sized. Note that if `T` is zero-sized this means you will *not*
79 /// get a RawVec with the requested capacity!
83 /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
84 /// * Panics on 32-bit platforms if the requested capacity exceeds
85 /// `isize::MAX` bytes.
90 pub fn with_capacity(cap: usize) -> Self {
92 let elem_size = mem::size_of::<T>();
94 let alloc_size = cap.checked_mul(elem_size).expect("capacity overflow");
95 alloc_guard(alloc_size);
97 // handles ZSTs and `cap = 0` alike
98 let ptr = if alloc_size == 0 {
99 heap::EMPTY as *mut u8
101 let align = mem::align_of::<T>();
102 let ptr = heap::allocate(alloc_size, align);
110 ptr: Unique::new(ptr as *mut _),
116 /// Reconstitutes a RawVec from a pointer and capacity.
118 /// # Undefined Behavior
120 /// The ptr must be allocated, and with the given capacity. The
121 /// capacity cannot exceed `isize::MAX` (only a concern on 32-bit systems).
122 /// If the ptr and capacity come from a RawVec, then this is guaranteed.
123 pub unsafe fn from_raw_parts(ptr: *mut T, cap: usize) -> Self {
125 ptr: Unique::new(ptr),
130 /// Converts a `Box<[T]>` into a `RawVec<T>`.
131 pub fn from_box(mut slice: Box<[T]>) -> Self {
133 let result = RawVec::from_raw_parts(slice.as_mut_ptr(), slice.len());
141 /// Gets a raw pointer to the start of the allocation. Note that this is
142 /// heap::EMPTY if `cap = 0` or T is zero-sized. In the former case, you must
144 pub fn ptr(&self) -> *mut T {
148 /// Gets the capacity of the allocation.
150 /// This will always be `usize::MAX` if `T` is zero-sized.
151 pub fn cap(&self) -> usize {
152 if mem::size_of::<T>() == 0 {
159 /// Doubles the size of the type's backing allocation. This is common enough
160 /// to want to do that it's easiest to just have a dedicated method. Slightly
161 /// more efficient logic can be provided for this than the general case.
163 /// This function is ideal for when pushing elements one-at-a-time because
164 /// you don't need to incur the costs of the more general computations
165 /// reserve needs to do to guard against overflow. You do however need to
166 /// manually check if your `len == cap`.
170 /// * Panics if T is zero-sized on the assumption that you managed to exhaust
171 /// all `usize::MAX` slots in your imaginary buffer.
172 /// * Panics on 32-bit platforms if the requested capacity exceeds
173 /// `isize::MAX` bytes.
182 /// struct MyVec<T> {
187 /// impl<T> MyVec<T> {
188 /// pub fn push(&mut self, elem: T) {
189 /// if self.len == self.buf.cap() { self.buf.double(); }
190 /// // double would have aborted or panicked if the len exceeded
191 /// // `isize::MAX` so this is safe to do unchecked now.
193 /// ptr::write(self.buf.ptr().offset(self.len as isize), elem);
201 pub fn double(&mut self) {
203 let elem_size = mem::size_of::<T>();
205 // since we set the capacity to usize::MAX when elem_size is
206 // 0, getting to here necessarily means the RawVec is overfull.
207 assert!(elem_size != 0, "capacity overflow");
209 let align = mem::align_of::<T>();
211 let (new_cap, ptr) = if self.cap == 0 {
212 // skip to 4 because tiny Vec's are dumb; but not if that would cause overflow
213 let new_cap = if elem_size > (!0) / 8 {
218 let ptr = heap::allocate(new_cap * elem_size, align);
221 // Since we guarantee that we never allocate more than isize::MAX bytes,
222 // `elem_size * self.cap <= isize::MAX` as a precondition, so this can't overflow
223 let new_cap = 2 * self.cap;
224 let new_alloc_size = new_cap * elem_size;
225 alloc_guard(new_alloc_size);
226 let ptr = heap::reallocate(self.ptr() as *mut _,
227 self.cap * elem_size,
233 // If allocate or reallocate fail, we'll get `null` back
238 self.ptr = Unique::new(ptr as *mut _);
243 /// Attempts to double the size of the type's backing allocation in place. This is common
244 /// enough to want to do that it's easiest to just have a dedicated method. Slightly
245 /// more efficient logic can be provided for this than the general case.
247 /// Returns true if the reallocation attempt has succeeded, or false otherwise.
251 /// * Panics if T is zero-sized on the assumption that you managed to exhaust
252 /// all `usize::MAX` slots in your imaginary buffer.
253 /// * Panics on 32-bit platforms if the requested capacity exceeds
254 /// `isize::MAX` bytes.
257 pub fn double_in_place(&mut self) -> bool {
259 let elem_size = mem::size_of::<T>();
260 let align = mem::align_of::<T>();
262 // since we set the capacity to usize::MAX when elem_size is
263 // 0, getting to here necessarily means the RawVec is overfull.
264 assert!(elem_size != 0, "capacity overflow");
266 // Since we guarantee that we never allocate more than isize::MAX bytes,
267 // `elem_size * self.cap <= isize::MAX` as a precondition, so this can't overflow
268 let new_cap = 2 * self.cap;
269 let new_alloc_size = new_cap * elem_size;
271 alloc_guard(new_alloc_size);
272 let size = heap::reallocate_inplace(self.ptr() as *mut _,
273 self.cap * elem_size,
276 if size >= new_alloc_size {
277 // We can't directly divide `size`.
280 size >= new_alloc_size
284 /// Ensures that the buffer contains at least enough space to hold
285 /// `used_cap + needed_extra_cap` elements. If it doesn't already,
286 /// will reallocate the minimum possible amount of memory necessary.
287 /// Generally this will be exactly the amount of memory necessary,
288 /// but in principle the allocator is free to give back more than
291 /// If `used_cap` exceeds `self.cap()`, this may fail to actually allocate
292 /// the requested space. This is not really unsafe, but the unsafe
293 /// code *you* write that relies on the behavior of this function may break.
297 /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
298 /// * Panics on 32-bit platforms if the requested capacity exceeds
299 /// `isize::MAX` bytes.
304 pub fn reserve_exact(&mut self, used_cap: usize, needed_extra_cap: usize) {
306 let elem_size = mem::size_of::<T>();
307 let align = mem::align_of::<T>();
309 // NOTE: we don't early branch on ZSTs here because we want this
310 // to actually catch "asking for more than usize::MAX" in that case.
311 // If we make it past the first branch then we are guaranteed to
314 // Don't actually need any more capacity.
315 // Wrapping in case they gave a bad `used_cap`.
316 if self.cap().wrapping_sub(used_cap) >= needed_extra_cap {
320 // Nothing we can really do about these checks :(
321 let new_cap = used_cap.checked_add(needed_extra_cap).expect("capacity overflow");
322 let new_alloc_size = new_cap.checked_mul(elem_size).expect("capacity overflow");
323 alloc_guard(new_alloc_size);
325 let ptr = if self.cap == 0 {
326 heap::allocate(new_alloc_size, align)
328 heap::reallocate(self.ptr() as *mut _,
329 self.cap * elem_size,
334 // If allocate or reallocate fail, we'll get `null` back
339 self.ptr = Unique::new(ptr as *mut _);
344 /// Calculates the buffer's new size given that it'll hold `used_cap +
345 /// needed_extra_cap` elements. This logic is used in amortized reserve methods.
346 /// Returns `(new_capacity, new_alloc_size)`.
347 fn amortized_new_size(&self, used_cap: usize, needed_extra_cap: usize) -> (usize, usize) {
348 let elem_size = mem::size_of::<T>();
349 // Nothing we can really do about these checks :(
350 let required_cap = used_cap.checked_add(needed_extra_cap)
351 .expect("capacity overflow");
352 // Cannot overflow, because `cap <= isize::MAX`, and type of `cap` is `usize`.
353 let double_cap = self.cap * 2;
354 // `double_cap` guarantees exponential growth.
355 let new_cap = cmp::max(double_cap, required_cap);
356 let new_alloc_size = new_cap.checked_mul(elem_size).expect("capacity overflow");
357 (new_cap, new_alloc_size)
360 /// Ensures that the buffer contains at least enough space to hold
361 /// `used_cap + needed_extra_cap` elements. If it doesn't already have
362 /// enough capacity, will reallocate enough space plus comfortable slack
363 /// space to get amortized `O(1)` behavior. Will limit this behavior
364 /// if it would needlessly cause itself to panic.
366 /// If `used_cap` exceeds `self.cap()`, this may fail to actually allocate
367 /// the requested space. This is not really unsafe, but the unsafe
368 /// code *you* write that relies on the behavior of this function may break.
370 /// This is ideal for implementing a bulk-push operation like `extend`.
374 /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
375 /// * Panics on 32-bit platforms if the requested capacity exceeds
376 /// `isize::MAX` bytes.
385 /// struct MyVec<T> {
390 /// impl<T> MyVec<T> {
391 /// pub fn push_all(&mut self, elems: &[T]) {
392 /// self.buf.reserve(self.len, elems.len());
393 /// // reserve would have aborted or panicked if the len exceeded
394 /// // `isize::MAX` so this is safe to do unchecked now.
397 /// ptr::write(self.buf.ptr().offset(self.len as isize), x.clone());
404 pub fn reserve(&mut self, used_cap: usize, needed_extra_cap: usize) {
406 let elem_size = mem::size_of::<T>();
407 let align = mem::align_of::<T>();
409 // NOTE: we don't early branch on ZSTs here because we want this
410 // to actually catch "asking for more than usize::MAX" in that case.
411 // If we make it past the first branch then we are guaranteed to
414 // Don't actually need any more capacity.
415 // Wrapping in case they give a bad `used_cap`
416 if self.cap().wrapping_sub(used_cap) >= needed_extra_cap {
420 let (new_cap, new_alloc_size) = self.amortized_new_size(used_cap, needed_extra_cap);
421 // FIXME: may crash and burn on over-reserve
422 alloc_guard(new_alloc_size);
424 let ptr = if self.cap == 0 {
425 heap::allocate(new_alloc_size, align)
427 heap::reallocate(self.ptr() as *mut _,
428 self.cap * elem_size,
433 // If allocate or reallocate fail, we'll get `null` back
438 self.ptr = Unique::new(ptr as *mut _);
443 /// Attempts to ensure that the buffer contains at least enough space to hold
444 /// `used_cap + needed_extra_cap` elements. If it doesn't already have
445 /// enough capacity, will reallocate in place enough space plus comfortable slack
446 /// space to get amortized `O(1)` behaviour. Will limit this behaviour
447 /// if it would needlessly cause itself to panic.
449 /// If `used_cap` exceeds `self.cap()`, this may fail to actually allocate
450 /// the requested space. This is not really unsafe, but the unsafe
451 /// code *you* write that relies on the behaviour of this function may break.
453 /// Returns true if the reallocation attempt has succeeded, or false otherwise.
457 /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
458 /// * Panics on 32-bit platforms if the requested capacity exceeds
459 /// `isize::MAX` bytes.
460 pub fn reserve_in_place(&mut self, used_cap: usize, needed_extra_cap: usize) -> bool {
462 let elem_size = mem::size_of::<T>();
463 let align = mem::align_of::<T>();
465 // NOTE: we don't early branch on ZSTs here because we want this
466 // to actually catch "asking for more than usize::MAX" in that case.
467 // If we make it past the first branch then we are guaranteed to
470 // Don't actually need any more capacity. If the current `cap` is 0, we can't
471 // reallocate in place.
472 // Wrapping in case they give a bad `used_cap`
473 if self.cap().wrapping_sub(used_cap) >= needed_extra_cap || self.cap == 0 {
477 let (_, new_alloc_size) = self.amortized_new_size(used_cap, needed_extra_cap);
478 // FIXME: may crash and burn on over-reserve
479 alloc_guard(new_alloc_size);
481 let size = heap::reallocate_inplace(self.ptr() as *mut _,
482 self.cap * elem_size,
485 if size >= new_alloc_size {
486 self.cap = new_alloc_size / elem_size;
488 size >= new_alloc_size
492 /// Shrinks the allocation down to the specified amount. If the given amount
493 /// is 0, actually completely deallocates.
497 /// Panics if the given amount is *larger* than the current capacity.
502 pub fn shrink_to_fit(&mut self, amount: usize) {
503 let elem_size = mem::size_of::<T>();
504 let align = mem::align_of::<T>();
506 // Set the `cap` because they might be about to promote to a `Box<[T]>`
512 // This check is my waterloo; it's the only thing Vec wouldn't have to do.
513 assert!(self.cap >= amount, "Tried to shrink to a larger capacity");
516 mem::replace(self, RawVec::new());
517 } else if self.cap != amount {
519 // Overflow check is unnecessary as the vector is already at
521 let ptr = heap::reallocate(self.ptr() as *mut _,
522 self.cap * elem_size,
528 self.ptr = Unique::new(ptr as *mut _);
534 /// Converts the entire buffer into `Box<[T]>`.
536 /// While it is not *strictly* Undefined Behavior to call
537 /// this procedure while some of the RawVec is unintialized,
538 /// it cetainly makes it trivial to trigger it.
540 /// Note that this will correctly reconstitute any `cap` changes
541 /// that may have been performed. (see description of type for details)
542 pub unsafe fn into_box(self) -> Box<[T]> {
543 // NOTE: not calling `cap()` here, actually using the real `cap` field!
544 let slice = slice::from_raw_parts_mut(self.ptr(), self.cap);
545 let output: Box<[T]> = Box::from_raw(slice);
550 /// This is a stupid name in the hopes that someone will find this in the
551 /// not too distant future and remove it with the rest of
552 /// #[unsafe_no_drop_flag]
553 pub fn unsafe_no_drop_flag_needs_drop(&self) -> bool {
554 self.cap != mem::POST_DROP_USIZE
558 impl<T> Drop for RawVec<T> {
559 #[unsafe_destructor_blind_to_params]
560 /// Frees the memory owned by the RawVec *without* trying to Drop its contents.
562 let elem_size = mem::size_of::<T>();
563 if elem_size != 0 && self.cap != 0 && self.unsafe_no_drop_flag_needs_drop() {
564 let align = mem::align_of::<T>();
566 let num_bytes = elem_size * self.cap;
568 heap::deallocate(*self.ptr as *mut _, num_bytes, align);
576 // We need to guarantee the following:
577 // * We don't ever allocate `> isize::MAX` byte-size objects
578 // * We don't overflow `usize::MAX` and actually allocate too little
580 // On 64-bit we just need to check for overflow since trying to allocate
581 // `> isize::MAX` bytes will surely fail. On 32-bit we need to add an extra
582 // guard for this in case we're running on a platform which can use all 4GB in
583 // user-space. e.g. PAE or x32
586 fn alloc_guard(alloc_size: usize) {
587 if core::usize::BITS < 64 {
588 assert!(alloc_size <= ::core::isize::MAX as usize,
589 "capacity overflow");
599 fn reserve_does_not_overallocate() {
601 let mut v: RawVec<u32> = RawVec::new();
602 // First `reserve` allocates like `reserve_exact`
604 assert_eq!(9, v.cap());
608 let mut v: RawVec<u32> = RawVec::new();
610 assert_eq!(7, v.cap());
611 // 97 if more than double of 7, so `reserve` should work
612 // like `reserve_exact`.
614 assert_eq!(97, v.cap());
618 let mut v: RawVec<u32> = RawVec::new();
620 assert_eq!(12, v.cap());
622 // 3 is less than half of 12, so `reserve` must grow
623 // exponentially. At the time of writing this test grow
624 // factor is 2, so new capacity is 24, however, grow factor
625 // of 1.5 is OK too. Hence `>= 18` in assert.
626 assert!(v.cap() >= 12 + 12 / 2);