about summary refs log tree commit diff
path: root/absl/container/fixed_array.h
blob: ec6fced16ababcc541f4620619a98ac97dde712a (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
// Copyright 2017 The Abseil Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//
// -----------------------------------------------------------------------------
// File: fixed_array.h
// -----------------------------------------------------------------------------
//
// A `FixedArray<T>` represents a non-resizable array of `T` where the length of
// the array can be determined at run-time. It is a good replacement for
// non-standard and deprecated uses of `alloca()` and variable length arrays
// within the GCC extension. (See
// https://gcc.gnu.org/onlinedocs/gcc/Variable-Length.html).
//
// `FixedArray` allocates small arrays inline, keeping performance fast by
// avoiding heap operations. It also helps reduce the chances of
// accidentally overflowing your stack if large input is passed to
// your function.

#ifndef ABSL_CONTAINER_FIXED_ARRAY_H_
#define ABSL_CONTAINER_FIXED_ARRAY_H_

#include <algorithm>
#include <array>
#include <cassert>
#include <cstddef>
#include <initializer_list>
#include <iterator>
#include <limits>
#include <memory>
#include <new>
#include <type_traits>

#include "absl/algorithm/algorithm.h"
#include "absl/base/dynamic_annotations.h"
#include "absl/base/internal/throw_delegate.h"
#include "absl/base/macros.h"
#include "absl/base/optimization.h"
#include "absl/base/port.h"
#include "absl/memory/memory.h"

namespace absl {

constexpr static auto kFixedArrayUseDefault = static_cast<size_t>(-1);

// -----------------------------------------------------------------------------
// FixedArray
// -----------------------------------------------------------------------------
//
// A `FixedArray` provides a run-time fixed-size array, allocating small arrays
// inline for efficiency and correctness.
//
// Most users should not specify an `inline_elements` argument and let
// `FixedArray<>` automatically determine the number of elements
// to store inline based on `sizeof(T)`. If `inline_elements` is specified, the
// `FixedArray<>` implementation will inline arrays of
// length <= `inline_elements`.
//
// Note that a `FixedArray` constructed with a `size_type` argument will
// default-initialize its values by leaving trivially constructible types
// uninitialized (e.g. int, int[4], double), and others default-constructed.
// This matches the behavior of c-style arrays and `std::array`, but not
// `std::vector`.
//
// Note that `FixedArray` does not provide a public allocator; if it requires a
// heap allocation, it will do so with global `::operator new[]()` and
// `::operator delete[]()`, even if T provides class-scope overrides for these
// operators.
template <typename T, size_t inlined = kFixedArrayUseDefault>
class FixedArray {
  static constexpr size_t kInlineBytesDefault = 256;

  // std::iterator_traits isn't guaranteed to be SFINAE-friendly until C++17,
  // but this seems to be mostly pedantic.
  template <typename Iter>
  using EnableIfForwardIterator = typename std::enable_if<
      std::is_convertible<
          typename std::iterator_traits<Iter>::iterator_category,
          std::forward_iterator_tag>::value,
      int>::type;

 public:
  // For playing nicely with stl:
  using value_type = T;
  using iterator = T*;
  using const_iterator = const T*;
  using reverse_iterator = std::reverse_iterator<iterator>;
  using const_reverse_iterator = std::reverse_iterator<const_iterator>;
  using reference = T&;
  using const_reference = const T&;
  using pointer = T*;
  using const_pointer = const T*;
  using difference_type = ptrdiff_t;
  using size_type = size_t;

  static constexpr size_type inline_elements =
      inlined == kFixedArrayUseDefault
          ? kInlineBytesDefault / sizeof(value_type)
          : inlined;

  FixedArray(const FixedArray& other) : rep_(other.begin(), other.end()) {}
  FixedArray(FixedArray&& other) noexcept(
  // clang-format off
      absl::allocator_is_nothrow<std::allocator<value_type>>::value &&
  // clang-format on
          std::is_nothrow_move_constructible<value_type>::value)
      : rep_(std::make_move_iterator(other.begin()),
             std::make_move_iterator(other.end())) {}

  // Creates an array object that can store `n` elements.
  // Note that trivially constructible elements will be uninitialized.
  explicit FixedArray(size_type n) : rep_(n) {}

  // Creates an array initialized with `n` copies of `val`.
  FixedArray(size_type n, const value_type& val) : rep_(n, val) {}

  // Creates an array initialized with the elements from the input
  // range. The array's size will always be `std::distance(first, last)`.
  // REQUIRES: Iter must be a forward_iterator or better.
  template <typename Iter, EnableIfForwardIterator<Iter> = 0>
  FixedArray(Iter first, Iter last) : rep_(first, last) {}

  // Creates the array from an initializer_list.
  FixedArray(std::initializer_list<T> init_list)
      : FixedArray(init_list.begin(), init_list.end()) {}

  ~FixedArray() {}

  // Assignments are deleted because they break the invariant that the size of a
  // `FixedArray` never changes.
  void operator=(FixedArray&&) = delete;
  void operator=(const FixedArray&) = delete;

  // FixedArray::size()
  //
  // Returns the length of the fixed array.
  size_type size() const { return rep_.size(); }

  // FixedArray::max_size()
  //
  // Returns the largest possible value of `std::distance(begin(), end())` for a
  // `FixedArray<T>`. This is equivalent to the most possible addressable bytes
  // over the number of bytes taken by T.
  constexpr size_type max_size() const {
    return std::numeric_limits<difference_type>::max() / sizeof(value_type);
  }

  // FixedArray::empty()
  //
  // Returns whether or not the fixed array is empty.
  bool empty() const { return size() == 0; }

  // FixedArray::memsize()
  //
  // Returns the memory size of the fixed array in bytes.
  size_t memsize() const { return size() * sizeof(value_type); }

  // FixedArray::data()
  //
  // Returns a const T* pointer to elements of the `FixedArray`. This pointer
  // can be used to access (but not modify) the contained elements.
  const_pointer data() const { return AsValue(rep_.begin()); }

  // Overload of FixedArray::data() to return a T* pointer to elements of the
  // fixed array. This pointer can be used to access and modify the contained
  // elements.
  pointer data() { return AsValue(rep_.begin()); }
  // FixedArray::operator[]
  //
  // Returns a reference the ith element of the fixed array.
  // REQUIRES: 0 <= i < size()
  reference operator[](size_type i) {
    assert(i < size());
    return data()[i];
  }

  // Overload of FixedArray::operator()[] to return a const reference to the
  // ith element of the fixed array.
  // REQUIRES: 0 <= i < size()
  const_reference operator[](size_type i) const {
    assert(i < size());
    return data()[i];
  }

  // FixedArray::at
  //
  // Bounds-checked access.  Returns a reference to the ith element of the
  // fiexed array, or throws std::out_of_range
  reference at(size_type i) {
    if (ABSL_PREDICT_FALSE(i >= size())) {
      base_internal::ThrowStdOutOfRange("FixedArray::at failed bounds check");
    }
    return data()[i];
  }

  // Overload of FixedArray::at() to return a const reference to the ith element
  // of the fixed array.
  const_reference at(size_type i) const {
    if (i >= size()) {
      base_internal::ThrowStdOutOfRange("FixedArray::at failed bounds check");
    }
    return data()[i];
  }

  // FixedArray::front()
  //
  // Returns a reference to the first element of the fixed array.
  reference front() { return *begin(); }

  // Overload of FixedArray::front() to return a reference to the first element
  // of a fixed array of const values.
  const_reference front() const { return *begin(); }

  // FixedArray::back()
  //
  // Returns a reference to the last element of the fixed array.
  reference back() { return *(end() - 1); }

  // Overload of FixedArray::back() to return a reference to the last element
  // of a fixed array of const values.
  const_reference back() const { return *(end() - 1); }

  // FixedArray::begin()
  //
  // Returns an iterator to the beginning of the fixed array.
  iterator begin() { return data(); }

  // Overload of FixedArray::begin() to return a const iterator to the
  // beginning of the fixed array.
  const_iterator begin() const { return data(); }

  // FixedArray::cbegin()
  //
  // Returns a const iterator to the beginning of the fixed array.
  const_iterator cbegin() const { return begin(); }

  // FixedArray::end()
  //
  // Returns an iterator to the end of the fixed array.
  iterator end() { return data() + size(); }

  // Overload of FixedArray::end() to return a const iterator to the end of the
  // fixed array.
  const_iterator end() const { return data() + size(); }

  // FixedArray::cend()
  //
  // Returns a const iterator to the end of the fixed array.
  const_iterator cend() const { return end(); }

  // FixedArray::rbegin()
  //
  // Returns a reverse iterator from the end of the fixed array.
  reverse_iterator rbegin() { return reverse_iterator(end()); }

  // Overload of FixedArray::rbegin() to return a const reverse iterator from
  // the end of the fixed array.
  const_reverse_iterator rbegin() const {
    return const_reverse_iterator(end());
  }

  // FixedArray::crbegin()
  //
  // Returns a const reverse iterator from the end of the fixed array.
  const_reverse_iterator crbegin() const { return rbegin(); }

  // FixedArray::rend()
  //
  // Returns a reverse iterator from the beginning of the fixed array.
  reverse_iterator rend() { return reverse_iterator(begin()); }

  // Overload of FixedArray::rend() for returning a const reverse iterator
  // from the beginning of the fixed array.
  const_reverse_iterator rend() const {
    return const_reverse_iterator(begin());
  }

  // FixedArray::crend()
  //
  // Returns a reverse iterator from the beginning of the fixed array.
  const_reverse_iterator crend() const { return rend(); }

  // FixedArray::fill()
  //
  // Assigns the given `value` to all elements in the fixed array.
  void fill(const T& value) { std::fill(begin(), end(), value); }

  // Relational operators. Equality operators are elementwise using
  // `operator==`, while order operators order FixedArrays lexicographically.
  friend bool operator==(const FixedArray& lhs, const FixedArray& rhs) {
    return absl::equal(lhs.begin(), lhs.end(), rhs.begin(), rhs.end());
  }

  friend bool operator!=(const FixedArray& lhs, const FixedArray& rhs) {
    return !(lhs == rhs);
  }

  friend bool operator<(const FixedArray& lhs, const FixedArray& rhs) {
    return std::lexicographical_compare(lhs.begin(), lhs.end(), rhs.begin(),
                                        rhs.end());
  }

  friend bool operator>(const FixedArray& lhs, const FixedArray& rhs) {
    return rhs < lhs;
  }

  friend bool operator<=(const FixedArray& lhs, const FixedArray& rhs) {
    return !(rhs < lhs);
  }

  friend bool operator>=(const FixedArray& lhs, const FixedArray& rhs) {
    return !(lhs < rhs);
  }

 private:
  // HolderTraits
  //
  // Wrapper to hold elements of type T for the case where T is an array type.
  // If 'T' is an array type, HolderTraits::type is a struct with a 'T v;'.
  // Otherwise, HolderTraits::type is simply 'T'.
  //
  // Maintainer's Note: The simpler solution would be to simply wrap T in a
  // struct whether it's an array or not: 'struct Holder { T v; };', but
  // that causes some paranoid diagnostics to misfire about uses of data(),
  // believing that 'data()' (aka '&rep_.begin().v') is a pointer to a single
  // element, rather than the packed array that it really is.
  // e.g.:
  //
  //     FixedArray<char> buf(1);
  //     sprintf(buf.data(), "foo");
  //
  //     error: call to int __builtin___sprintf_chk(etc...)
  //     will always overflow destination buffer [-Werror]
  //
  class HolderTraits {
    template <typename U>
    struct SelectImpl {
      using type = U;
      static pointer AsValue(type* p) { return p; }
    };

    // Partial specialization for elements of array type.
    template <typename U, size_t N>
    struct SelectImpl<U[N]> {
      struct Holder { U v[N]; };
      using type = Holder;
      static pointer AsValue(type* p) { return &p->v; }
    };
    using Impl = SelectImpl<value_type>;

   public:
    using type = typename Impl::type;

    static pointer AsValue(type *p) { return Impl::AsValue(p); }

    // TODO(billydonahue): fix the type aliasing violation
    // this assertion hints at.
    static_assert(sizeof(type) == sizeof(value_type),
                  "Holder must be same size as value_type");
  };

  using Holder = typename HolderTraits::type;
  static pointer AsValue(Holder *p) { return HolderTraits::AsValue(p); }

  // InlineSpace
  //
  // Allocate some space, not an array of elements of type T, so that we can
  // skip calling the T constructors and destructors for space we never use.
  // How many elements should we store inline?
  //   a. If not specified, use a default of kInlineBytesDefault bytes (This is
  //   currently 256 bytes, which seems small enough to not cause stack overflow
  //   or unnecessary stack pollution, while still allowing stack allocation for
  //   reasonably long character arrays).
  //   b. Never use 0 length arrays (not ISO C++)
  //
  template <size_type N, typename = void>
  class InlineSpace {
   public:
    Holder* data() { return reinterpret_cast<Holder*>(space_.data()); }
    void AnnotateConstruct(size_t n) const { Annotate(n, true); }
    void AnnotateDestruct(size_t n) const { Annotate(n, false); }

   private:
#ifndef ADDRESS_SANITIZER
    void Annotate(size_t, bool) const { }
#else
    void Annotate(size_t n, bool creating) const {
      if (!n) return;
      const void* bot = &left_redzone_;
      const void* beg = space_.data();
      const void* end = space_.data() + n;
      const void* top = &right_redzone_ + 1;
      // args: (beg, end, old_mid, new_mid)
      if (creating) {
        ANNOTATE_CONTIGUOUS_CONTAINER(beg, top, top, end);
        ANNOTATE_CONTIGUOUS_CONTAINER(bot, beg, beg, bot);
      } else {
        ANNOTATE_CONTIGUOUS_CONTAINER(beg, top, end, top);
        ANNOTATE_CONTIGUOUS_CONTAINER(bot, beg, bot, beg);
      }
    }
#endif  // ADDRESS_SANITIZER

    using Buffer =
        typename std::aligned_storage<sizeof(Holder), alignof(Holder)>::type;

    ADDRESS_SANITIZER_REDZONE(left_redzone_);
    std::array<Buffer, N> space_;
    ADDRESS_SANITIZER_REDZONE(right_redzone_);
  };

  // specialization when N = 0.
  template <typename U>
  class InlineSpace<0, U> {
   public:
    Holder* data() { return nullptr; }
    void AnnotateConstruct(size_t) const {}
    void AnnotateDestruct(size_t) const {}
  };

  // Rep
  //
  // A const Rep object holds FixedArray's size and data pointer.
  //
  class Rep : public InlineSpace<inline_elements> {
   public:
    Rep(size_type n, const value_type& val) : n_(n), p_(MakeHolder(n)) {
      std::uninitialized_fill_n(p_, n, val);
    }

    explicit Rep(size_type n) : n_(n), p_(MakeHolder(n)) {
      // Loop optimizes to nothing for trivially constructible T.
      for (Holder* p = p_; p != p_ + n; ++p)
        // Note: no parens: default init only.
        // Also note '::' to avoid Holder class placement new operator.
        ::new (static_cast<void*>(p)) Holder;
    }

    template <typename Iter>
    Rep(Iter first, Iter last)
        : n_(std::distance(first, last)), p_(MakeHolder(n_)) {
      std::uninitialized_copy(first, last, AsValue(p_));
    }

    ~Rep() {
      // Destruction must be in reverse order.
      // Loop optimizes to nothing for trivially destructible T.
      for (Holder* p = end(); p != begin();) (--p)->~Holder();
      if (IsAllocated(size())) {
        ::operator delete[](begin());
      } else {
        this->AnnotateDestruct(size());
      }
    }
    Holder* begin() const { return p_; }
    Holder* end() const { return p_ + n_; }
    size_type size() const { return n_; }

   private:
    Holder* MakeHolder(size_type n) {
      if (IsAllocated(n)) {
        return Allocate(n);
      } else {
        this->AnnotateConstruct(n);
        return this->data();
      }
    }

    Holder* Allocate(size_type n) {
      return static_cast<Holder*>(::operator new[](n * sizeof(Holder)));
    }

    bool IsAllocated(size_type n) const { return n > inline_elements; }

    const size_type n_;
    Holder* const p_;
  };


  // Data members
  Rep rep_;
};

template <typename T, size_t N>
constexpr size_t FixedArray<T, N>::inline_elements;

template <typename T, size_t N>
constexpr size_t FixedArray<T, N>::kInlineBytesDefault;

}  // namespace absl
#endif  // ABSL_CONTAINER_FIXED_ARRAY_H_