1 // Copyright 2018 The Abseil Authors.
2 //
3 // Licensed under the Apache License, Version 2.0 (the "License");
4 // you may not use this file except in compliance with the License.
5 // You may obtain a copy of the License at
6 //
7 // http://www.apache.org/licenses/LICENSE-2.0
8 //
9 // Unless required by applicable law or agreed to in writing, software
10 // distributed under the License is distributed on an "AS IS" BASIS,
11 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
12 // See the License for the specific language governing permissions and
13 // limitations under the License.
14 //
15 // -----------------------------------------------------------------------------
16 // File: fixed_array.h
17 // -----------------------------------------------------------------------------
18 //
19 // A `FixedArray<T>` represents a non-resizable array of `T` where the length of
20 // the array can be determined at run-time. It is a good replacement for
21 // non-standard and deprecated uses of `alloca()` and variable length arrays
22 // within the GCC extension. (See
23 // https://gcc.gnu.org/onlinedocs/gcc/Variable-Length.html).
24 //
25 // `FixedArray` allocates small arrays inline, keeping performance fast by
26 // avoiding heap operations. It also helps reduce the chances of
27 // accidentally overflowing your stack if large input is passed to
28 // your function.
29
30 #ifndef ABSL_CONTAINER_FIXED_ARRAY_H_
31 #define ABSL_CONTAINER_FIXED_ARRAY_H_
32
33 #include <algorithm>
34 #include <array>
35 #include <cassert>
36 #include <cstddef>
37 #include <initializer_list>
38 #include <iterator>
39 #include <limits>
40 #include <memory>
41 #include <new>
42 #include <type_traits>
43
44 #include "absl/algorithm/algorithm.h"
45 #include "absl/base/dynamic_annotations.h"
46 #include "absl/base/internal/throw_delegate.h"
47 #include "absl/base/macros.h"
48 #include "absl/base/optimization.h"
49 #include "absl/base/port.h"
50 #include "absl/container/internal/compressed_tuple.h"
51 #include "absl/memory/memory.h"
52
53 namespace absl {
54
55 constexpr static auto kFixedArrayUseDefault = static_cast<size_t>(-1);
56
57 // -----------------------------------------------------------------------------
58 // FixedArray
59 // -----------------------------------------------------------------------------
60 //
61 // A `FixedArray` provides a run-time fixed-size array, allocating a small array
62 // inline for efficiency.
63 //
64 // Most users should not specify an `inline_elements` argument and let
65 // `FixedArray` automatically determine the number of elements
66 // to store inline based on `sizeof(T)`. If `inline_elements` is specified, the
67 // `FixedArray` implementation will use inline storage for arrays with a
68 // length <= `inline_elements`.
69 //
70 // Note that a `FixedArray` constructed with a `size_type` argument will
71 // default-initialize its values by leaving trivially constructible types
72 // uninitialized (e.g. int, int[4], double), and others default-constructed.
73 // This matches the behavior of c-style arrays and `std::array`, but not
74 // `std::vector`.
75 //
76 // Note that `FixedArray` does not provide a public allocator; if it requires a
77 // heap allocation, it will do so with global `::operator new[]()` and
78 // `::operator delete[]()`, even if T provides class-scope overrides for these
79 // operators.
80 template <typename T, size_t N = kFixedArrayUseDefault,
81 typename A = std::allocator<T>>
82 class FixedArray {
83 static_assert(!std::is_array<T>::value || std::extent<T>::value > 0,
84 "Arrays with unknown bounds cannot be used with FixedArray.");
85
86 static constexpr size_t kInlineBytesDefault = 256;
87
88 using AllocatorTraits = std::allocator_traits<A>;
89 // std::iterator_traits isn't guaranteed to be SFINAE-friendly until C++17,
90 // but this seems to be mostly pedantic.
91 template <typename Iterator>
92 using EnableIfForwardIterator = absl::enable_if_t<std::is_convertible<
93 typename std::iterator_traits<Iterator>::iterator_category,
94 std::forward_iterator_tag>::value>;
NoexceptCopyable()95 static constexpr bool NoexceptCopyable() {
96 return std::is_nothrow_copy_constructible<StorageElement>::value &&
97 absl::allocator_is_nothrow<allocator_type>::value;
98 }
NoexceptMovable()99 static constexpr bool NoexceptMovable() {
100 return std::is_nothrow_move_constructible<StorageElement>::value &&
101 absl::allocator_is_nothrow<allocator_type>::value;
102 }
DefaultConstructorIsNonTrivial()103 static constexpr bool DefaultConstructorIsNonTrivial() {
104 return !absl::is_trivially_default_constructible<StorageElement>::value;
105 }
106
107 public:
108 using allocator_type = typename AllocatorTraits::allocator_type;
109 using value_type = typename allocator_type::value_type;
110 using pointer = typename allocator_type::pointer;
111 using const_pointer = typename allocator_type::const_pointer;
112 using reference = typename allocator_type::reference;
113 using const_reference = typename allocator_type::const_reference;
114 using size_type = typename allocator_type::size_type;
115 using difference_type = typename allocator_type::difference_type;
116 using iterator = pointer;
117 using const_iterator = const_pointer;
118 using reverse_iterator = std::reverse_iterator<iterator>;
119 using const_reverse_iterator = std::reverse_iterator<const_iterator>;
120
121 static constexpr size_type inline_elements =
122 (N == kFixedArrayUseDefault ? kInlineBytesDefault / sizeof(value_type)
123 : static_cast<size_type>(N));
124
125 FixedArray(
126 const FixedArray& other,
noexcept(NoexceptCopyable ())127 const allocator_type& a = allocator_type()) noexcept(NoexceptCopyable())
128 : FixedArray(other.begin(), other.end(), a) {}
129
130 FixedArray(
131 FixedArray&& other,
noexcept(NoexceptMovable ())132 const allocator_type& a = allocator_type()) noexcept(NoexceptMovable())
133 : FixedArray(std::make_move_iterator(other.begin()),
134 std::make_move_iterator(other.end()), a) {}
135
136 // Creates an array object that can store `n` elements.
137 // Note that trivially constructible elements will be uninitialized.
138 explicit FixedArray(size_type n, const allocator_type& a = allocator_type())
storage_(n,a)139 : storage_(n, a) {
140 if (DefaultConstructorIsNonTrivial()) {
141 memory_internal::ConstructStorage(storage_.alloc(), storage_.begin(),
142 storage_.end());
143 }
144 }
145
146 // Creates an array initialized with `n` copies of `val`.
147 FixedArray(size_type n, const value_type& val,
148 const allocator_type& a = allocator_type())
storage_(n,a)149 : storage_(n, a) {
150 memory_internal::ConstructStorage(storage_.alloc(), storage_.begin(),
151 storage_.end(), val);
152 }
153
154 // Creates an array initialized with the size and contents of `init_list`.
155 FixedArray(std::initializer_list<value_type> init_list,
156 const allocator_type& a = allocator_type())
157 : FixedArray(init_list.begin(), init_list.end(), a) {}
158
159 // Creates an array initialized with the elements from the input
160 // range. The array's size will always be `std::distance(first, last)`.
161 // REQUIRES: Iterator must be a forward_iterator or better.
162 template <typename Iterator, EnableIfForwardIterator<Iterator>* = nullptr>
163 FixedArray(Iterator first, Iterator last,
164 const allocator_type& a = allocator_type())
storage_(std::distance (first,last),a)165 : storage_(std::distance(first, last), a) {
166 memory_internal::CopyToStorageFromRange(storage_.alloc(), storage_.begin(),
167 first, last);
168 }
169
~FixedArray()170 ~FixedArray() noexcept {
171 for (auto* cur = storage_.begin(); cur != storage_.end(); ++cur) {
172 AllocatorTraits::destroy(*storage_.alloc(), cur);
173 }
174 }
175
176 // Assignments are deleted because they break the invariant that the size of a
177 // `FixedArray` never changes.
178 void operator=(FixedArray&&) = delete;
179 void operator=(const FixedArray&) = delete;
180
181 // FixedArray::size()
182 //
183 // Returns the length of the fixed array.
size()184 size_type size() const { return storage_.size(); }
185
186 // FixedArray::max_size()
187 //
188 // Returns the largest possible value of `std::distance(begin(), end())` for a
189 // `FixedArray<T>`. This is equivalent to the most possible addressable bytes
190 // over the number of bytes taken by T.
max_size()191 constexpr size_type max_size() const {
192 return std::numeric_limits<difference_type>::max() / sizeof(value_type);
193 }
194
195 // FixedArray::empty()
196 //
197 // Returns whether or not the fixed array is empty.
empty()198 bool empty() const { return size() == 0; }
199
200 // FixedArray::memsize()
201 //
202 // Returns the memory size of the fixed array in bytes.
memsize()203 size_t memsize() const { return size() * sizeof(value_type); }
204
205 // FixedArray::data()
206 //
207 // Returns a const T* pointer to elements of the `FixedArray`. This pointer
208 // can be used to access (but not modify) the contained elements.
data()209 const_pointer data() const { return AsValueType(storage_.begin()); }
210
211 // Overload of FixedArray::data() to return a T* pointer to elements of the
212 // fixed array. This pointer can be used to access and modify the contained
213 // elements.
data()214 pointer data() { return AsValueType(storage_.begin()); }
215
216 // FixedArray::operator[]
217 //
218 // Returns a reference the ith element of the fixed array.
219 // REQUIRES: 0 <= i < size()
220 reference operator[](size_type i) {
221 assert(i < size());
222 return data()[i];
223 }
224
225 // Overload of FixedArray::operator()[] to return a const reference to the
226 // ith element of the fixed array.
227 // REQUIRES: 0 <= i < size()
228 const_reference operator[](size_type i) const {
229 assert(i < size());
230 return data()[i];
231 }
232
233 // FixedArray::at
234 //
235 // Bounds-checked access. Returns a reference to the ith element of the
236 // fiexed array, or throws std::out_of_range
at(size_type i)237 reference at(size_type i) {
238 if (ABSL_PREDICT_FALSE(i >= size())) {
239 base_internal::ThrowStdOutOfRange("FixedArray::at failed bounds check");
240 }
241 return data()[i];
242 }
243
244 // Overload of FixedArray::at() to return a const reference to the ith element
245 // of the fixed array.
at(size_type i)246 const_reference at(size_type i) const {
247 if (ABSL_PREDICT_FALSE(i >= size())) {
248 base_internal::ThrowStdOutOfRange("FixedArray::at failed bounds check");
249 }
250 return data()[i];
251 }
252
253 // FixedArray::front()
254 //
255 // Returns a reference to the first element of the fixed array.
front()256 reference front() { return *begin(); }
257
258 // Overload of FixedArray::front() to return a reference to the first element
259 // of a fixed array of const values.
front()260 const_reference front() const { return *begin(); }
261
262 // FixedArray::back()
263 //
264 // Returns a reference to the last element of the fixed array.
back()265 reference back() { return *(end() - 1); }
266
267 // Overload of FixedArray::back() to return a reference to the last element
268 // of a fixed array of const values.
back()269 const_reference back() const { return *(end() - 1); }
270
271 // FixedArray::begin()
272 //
273 // Returns an iterator to the beginning of the fixed array.
begin()274 iterator begin() { return data(); }
275
276 // Overload of FixedArray::begin() to return a const iterator to the
277 // beginning of the fixed array.
begin()278 const_iterator begin() const { return data(); }
279
280 // FixedArray::cbegin()
281 //
282 // Returns a const iterator to the beginning of the fixed array.
cbegin()283 const_iterator cbegin() const { return begin(); }
284
285 // FixedArray::end()
286 //
287 // Returns an iterator to the end of the fixed array.
end()288 iterator end() { return data() + size(); }
289
290 // Overload of FixedArray::end() to return a const iterator to the end of the
291 // fixed array.
end()292 const_iterator end() const { return data() + size(); }
293
294 // FixedArray::cend()
295 //
296 // Returns a const iterator to the end of the fixed array.
cend()297 const_iterator cend() const { return end(); }
298
299 // FixedArray::rbegin()
300 //
301 // Returns a reverse iterator from the end of the fixed array.
rbegin()302 reverse_iterator rbegin() { return reverse_iterator(end()); }
303
304 // Overload of FixedArray::rbegin() to return a const reverse iterator from
305 // the end of the fixed array.
rbegin()306 const_reverse_iterator rbegin() const {
307 return const_reverse_iterator(end());
308 }
309
310 // FixedArray::crbegin()
311 //
312 // Returns a const reverse iterator from the end of the fixed array.
crbegin()313 const_reverse_iterator crbegin() const { return rbegin(); }
314
315 // FixedArray::rend()
316 //
317 // Returns a reverse iterator from the beginning of the fixed array.
rend()318 reverse_iterator rend() { return reverse_iterator(begin()); }
319
320 // Overload of FixedArray::rend() for returning a const reverse iterator
321 // from the beginning of the fixed array.
rend()322 const_reverse_iterator rend() const {
323 return const_reverse_iterator(begin());
324 }
325
326 // FixedArray::crend()
327 //
328 // Returns a reverse iterator from the beginning of the fixed array.
crend()329 const_reverse_iterator crend() const { return rend(); }
330
331 // FixedArray::fill()
332 //
333 // Assigns the given `value` to all elements in the fixed array.
fill(const value_type & val)334 void fill(const value_type& val) { std::fill(begin(), end(), val); }
335
336 // Relational operators. Equality operators are elementwise using
337 // `operator==`, while order operators order FixedArrays lexicographically.
338 friend bool operator==(const FixedArray& lhs, const FixedArray& rhs) {
339 return absl::equal(lhs.begin(), lhs.end(), rhs.begin(), rhs.end());
340 }
341
342 friend bool operator!=(const FixedArray& lhs, const FixedArray& rhs) {
343 return !(lhs == rhs);
344 }
345
346 friend bool operator<(const FixedArray& lhs, const FixedArray& rhs) {
347 return std::lexicographical_compare(lhs.begin(), lhs.end(), rhs.begin(),
348 rhs.end());
349 }
350
351 friend bool operator>(const FixedArray& lhs, const FixedArray& rhs) {
352 return rhs < lhs;
353 }
354
355 friend bool operator<=(const FixedArray& lhs, const FixedArray& rhs) {
356 return !(rhs < lhs);
357 }
358
359 friend bool operator>=(const FixedArray& lhs, const FixedArray& rhs) {
360 return !(lhs < rhs);
361 }
362 private:
363 // StorageElement
364 //
365 // For FixedArrays with a C-style-array value_type, StorageElement is a POD
366 // wrapper struct called StorageElementWrapper that holds the value_type
367 // instance inside. This is needed for construction and destruction of the
368 // entire array regardless of how many dimensions it has. For all other cases,
369 // StorageElement is just an alias of value_type.
370 //
371 // Maintainer's Note: The simpler solution would be to simply wrap value_type
372 // in a struct whether it's an array or not. That causes some paranoid
373 // diagnostics to misfire, believing that 'data()' returns a pointer to a
374 // single element, rather than the packed array that it really is.
375 // e.g.:
376 //
377 // FixedArray<char> buf(1);
378 // sprintf(buf.data(), "foo");
379 //
380 // error: call to int __builtin___sprintf_chk(etc...)
381 // will always overflow destination buffer [-Werror]
382 //
383 template <typename OuterT = value_type,
384 typename InnerT = absl::remove_extent_t<OuterT>,
385 size_t InnerN = std::extent<OuterT>::value>
386 struct StorageElementWrapper {
387 InnerT array[InnerN];
388 };
389
390 using StorageElement =
391 absl::conditional_t<std::is_array<value_type>::value,
392 StorageElementWrapper<value_type>, value_type>;
393 using StorageElementBuffer =
394 absl::aligned_storage_t<sizeof(StorageElement), alignof(StorageElement)>;
395
AsValueType(pointer ptr)396 static pointer AsValueType(pointer ptr) { return ptr; }
AsValueType(StorageElementWrapper<value_type> * ptr)397 static pointer AsValueType(StorageElementWrapper<value_type>* ptr) {
398 return std::addressof(ptr->array);
399 }
400
401 static_assert(sizeof(StorageElement) == sizeof(value_type), "");
402 static_assert(alignof(StorageElement) == alignof(value_type), "");
403
404 struct NonEmptyInlinedStorage {
dataNonEmptyInlinedStorage405 StorageElement* data() {
406 return reinterpret_cast<StorageElement*>(inlined_storage_.data());
407 }
408
409 #ifdef ADDRESS_SANITIZER
RedzoneBeginNonEmptyInlinedStorage410 void* RedzoneBegin() { return &redzone_begin_; }
RedzoneEndNonEmptyInlinedStorage411 void* RedzoneEnd() { return &redzone_end_ + 1; }
412 #endif // ADDRESS_SANITIZER
413
414 void AnnotateConstruct(size_type);
415 void AnnotateDestruct(size_type);
416
417 ADDRESS_SANITIZER_REDZONE(redzone_begin_);
418 std::array<StorageElementBuffer, inline_elements> inlined_storage_;
419 ADDRESS_SANITIZER_REDZONE(redzone_end_);
420 };
421
422 struct EmptyInlinedStorage {
dataEmptyInlinedStorage423 StorageElement* data() { return nullptr; }
AnnotateConstructEmptyInlinedStorage424 void AnnotateConstruct(size_type) {}
AnnotateDestructEmptyInlinedStorage425 void AnnotateDestruct(size_type) {}
426 };
427
428 using InlinedStorage =
429 absl::conditional_t<inline_elements == 0, EmptyInlinedStorage,
430 NonEmptyInlinedStorage>;
431
432 // Storage
433 //
434 // An instance of Storage manages the inline and out-of-line memory for
435 // instances of FixedArray. This guarantees that even when construction of
436 // individual elements fails in the FixedArray constructor body, the
437 // destructor for Storage will still be called and out-of-line memory will be
438 // properly deallocated.
439 //
440 class Storage : public InlinedStorage {
441 public:
Storage(size_type n,const allocator_type & a)442 Storage(size_type n, const allocator_type& a)
443 : size_alloc_(n, a), data_(InitializeData()) {}
444
~Storage()445 ~Storage() noexcept {
446 if (UsingInlinedStorage(size())) {
447 InlinedStorage::AnnotateDestruct(size());
448 } else {
449 AllocatorTraits::deallocate(*alloc(), AsValueType(begin()), size());
450 }
451 }
452
size()453 size_type size() const { return size_alloc_.template get<0>(); }
begin()454 StorageElement* begin() const { return data_; }
end()455 StorageElement* end() const { return begin() + size(); }
alloc()456 allocator_type* alloc() {
457 return std::addressof(size_alloc_.template get<1>());
458 }
459
460 private:
UsingInlinedStorage(size_type n)461 static bool UsingInlinedStorage(size_type n) {
462 return n <= inline_elements;
463 }
464
InitializeData()465 StorageElement* InitializeData() {
466 if (UsingInlinedStorage(size())) {
467 InlinedStorage::AnnotateConstruct(size());
468 return InlinedStorage::data();
469 } else {
470 return reinterpret_cast<StorageElement*>(
471 AllocatorTraits::allocate(*alloc(), size()));
472 }
473 }
474
475 // `CompressedTuple` takes advantage of EBCO for stateless `allocator_type`s
476 container_internal::CompressedTuple<size_type, allocator_type> size_alloc_;
477 StorageElement* data_;
478 };
479
480 Storage storage_;
481 };
482
483 template <typename T, size_t N, typename A>
484 constexpr size_t FixedArray<T, N, A>::kInlineBytesDefault;
485
486 template <typename T, size_t N, typename A>
487 constexpr typename FixedArray<T, N, A>::size_type
488 FixedArray<T, N, A>::inline_elements;
489
490 template <typename T, size_t N, typename A>
AnnotateConstruct(typename FixedArray<T,N,A>::size_type n)491 void FixedArray<T, N, A>::NonEmptyInlinedStorage::AnnotateConstruct(
492 typename FixedArray<T, N, A>::size_type n) {
493 #ifdef ADDRESS_SANITIZER
494 if (!n) return;
495 ANNOTATE_CONTIGUOUS_CONTAINER(data(), RedzoneEnd(), RedzoneEnd(), data() + n);
496 ANNOTATE_CONTIGUOUS_CONTAINER(RedzoneBegin(), data(), data(), RedzoneBegin());
497 #endif // ADDRESS_SANITIZER
498 static_cast<void>(n); // Mark used when not in asan mode
499 }
500
501 template <typename T, size_t N, typename A>
AnnotateDestruct(typename FixedArray<T,N,A>::size_type n)502 void FixedArray<T, N, A>::NonEmptyInlinedStorage::AnnotateDestruct(
503 typename FixedArray<T, N, A>::size_type n) {
504 #ifdef ADDRESS_SANITIZER
505 if (!n) return;
506 ANNOTATE_CONTIGUOUS_CONTAINER(data(), RedzoneEnd(), data() + n, RedzoneEnd());
507 ANNOTATE_CONTIGUOUS_CONTAINER(RedzoneBegin(), data(), RedzoneBegin(), data());
508 #endif // ADDRESS_SANITIZER
509 static_cast<void>(n); // Mark used when not in asan mode
510 }
511 } // namespace absl
512 #endif // ABSL_CONTAINER_FIXED_ARRAY_H_
513