libstdc++
span
Go to the documentation of this file.
1 // Components for manipulating non-owning sequences of objects -*- C++ -*-
2 
3 // Copyright (C) 2019-2024 Free Software Foundation, Inc.
4 //
5 // This file is part of the GNU ISO C++ Library. This library is free
6 // software; you can redistribute it and/or modify it under the
7 // terms of the GNU General Public License as published by the
8 // Free Software Foundation; either version 3, or (at your option)
9 // any later version.
10 
11 // This library is distributed in the hope that it will be useful,
12 // but WITHOUT ANY WARRANTY; without even the implied warranty of
13 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
14 // GNU General Public License for more details.
15 
16 // Under Section 7 of GPL version 3, you are granted additional
17 // permissions described in the GCC Runtime Library Exception, version
18 // 3.1, as published by the Free Software Foundation.
19 
20 // You should have received a copy of the GNU General Public License and
21 // a copy of the GCC Runtime Library Exception along with this program;
22 // see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
23 // <http://www.gnu.org/licenses/>.
24 
25 /** @file span
26  * This is a Standard C++ Library header.
27  */
28 
29 //
30 // P0122 span library
31 // Contributed by ThePhD
32 //
33 
34 #ifndef _GLIBCXX_SPAN
35 #define _GLIBCXX_SPAN 1
36 
37 #pragma GCC system_header
38 
39 #define __glibcxx_want_span
40 #include <bits/version.h>
41 
42 #ifdef __cpp_lib_span // C++ >= 20 && concepts
43 #include <array>
44 #include <cstddef>
45 #include <bits/stl_iterator.h>
46 #include <bits/ranges_base.h>
47 namespace std _GLIBCXX_VISIBILITY(default)
48 {
49 _GLIBCXX_BEGIN_NAMESPACE_VERSION
50 
51  inline constexpr size_t dynamic_extent = static_cast<size_t>(-1);
52 
53  template<typename _Type, size_t _Extent>
54  class span;
55 
56  namespace __detail
57  {
58  template<typename _Tp>
59  inline constexpr bool __is_span = false;
60 
61  template<typename _Tp, size_t _Num>
62  inline constexpr bool __is_span<span<_Tp, _Num>> = true;
63 
64  template<typename _Tp>
65  inline constexpr bool __is_std_array = false;
66 
67  template<typename _Tp, size_t _Num>
68  inline constexpr bool __is_std_array<std::array<_Tp, _Num>> = true;
69 
70  template<size_t _Extent>
71  class __extent_storage
72  {
73  public:
74  constexpr
75  __extent_storage(size_t) noexcept
76  { }
77 
78  static constexpr size_t
79  _M_extent() noexcept
80  { return _Extent; }
81  };
82 
83  template<>
84  class __extent_storage<dynamic_extent>
85  {
86  public:
87  constexpr
88  __extent_storage(size_t __extent) noexcept
89  : _M_extent_value(__extent)
90  { }
91 
92  constexpr size_t
93  _M_extent() const noexcept
94  { return this->_M_extent_value; }
95 
96  private:
97  size_t _M_extent_value;
98  };
99  } // namespace __detail
100 
101  template<typename _Type, size_t _Extent = dynamic_extent>
102  class span
103  {
104  template<size_t _Offset, size_t _Count>
105  static constexpr size_t
106  _S_subspan_extent()
107  {
108  if constexpr (_Count != dynamic_extent)
109  return _Count;
110  else if constexpr (extent != dynamic_extent)
111  return _Extent - _Offset;
112  else
113  return dynamic_extent;
114  }
115 
116  // _GLIBCXX_RESOLVE_LIB_DEFECTS
117  // 3255. span's array constructor is too strict
118  template<typename _Tp, size_t _ArrayExtent>
119  requires (_Extent == dynamic_extent || _ArrayExtent == _Extent)
120  using __is_compatible_array = __is_array_convertible<_Type, _Tp>;
121 
122  template<typename _Ref>
123  using __is_compatible_ref
124  = __is_array_convertible<_Type, remove_reference_t<_Ref>>;
125 
126  public:
127  // member types
128  using element_type = _Type;
129  using value_type = remove_cv_t<_Type>;
130  using size_type = size_t;
131  using difference_type = ptrdiff_t;
132  using pointer = _Type*;
133  using const_pointer = const _Type*;
134  using reference = element_type&;
135  using const_reference = const element_type&;
136  using iterator = __gnu_cxx::__normal_iterator<pointer, span>;
137  using reverse_iterator = std::reverse_iterator<iterator>;
138 #if __cplusplus > 202002L
139  using const_iterator = std::const_iterator<iterator>;
140  using const_reverse_iterator = std::const_iterator<reverse_iterator>;
141 #endif
142 
143  // member constants
144  static constexpr size_t extent = _Extent;
145 
146  // constructors, copy and assignment
147 
148  constexpr
149  span() noexcept
150  requires (_Extent == dynamic_extent || _Extent == 0)
151  : _M_ptr(nullptr), _M_extent(0)
152  { }
153 
154  template<contiguous_iterator _It>
155  requires __is_compatible_ref<iter_reference_t<_It>>::value
156  constexpr explicit(extent != dynamic_extent)
157  span(_It __first, size_type __count)
158  noexcept
159  : _M_ptr(std::to_address(__first)), _M_extent(__count)
160  {
161  if constexpr (_Extent != dynamic_extent)
162  {
163  __glibcxx_assert(__count == _Extent);
164  }
165  __glibcxx_requires_valid_range(__first, __first + __count);
166  }
167 
168  template<contiguous_iterator _It, sized_sentinel_for<_It> _End>
169  requires __is_compatible_ref<iter_reference_t<_It>>::value
170  && (!is_convertible_v<_End, size_type>)
171  constexpr explicit(extent != dynamic_extent)
172  span(_It __first, _End __last)
173  noexcept(noexcept(__last - __first))
174  : _M_ptr(std::to_address(__first)),
175  _M_extent(static_cast<size_type>(__last - __first))
176  {
177  if constexpr (_Extent != dynamic_extent)
178  {
179  __glibcxx_assert((__last - __first) == _Extent);
180  }
181  __glibcxx_requires_valid_range(__first, __last);
182  }
183 
184  template<size_t _ArrayExtent>
185  requires (_Extent == dynamic_extent || _ArrayExtent == _Extent)
186  constexpr
187  span(type_identity_t<element_type> (&__arr)[_ArrayExtent]) noexcept
188  : _M_ptr(__arr), _M_extent(_ArrayExtent)
189  { }
190 
191  template<typename _Tp, size_t _ArrayExtent>
192  requires __is_compatible_array<_Tp, _ArrayExtent>::value
193  constexpr
194  span(array<_Tp, _ArrayExtent>& __arr) noexcept
195  : _M_ptr(__arr.data()), _M_extent(_ArrayExtent)
196  { }
197 
198  template<typename _Tp, size_t _ArrayExtent>
199  requires __is_compatible_array<const _Tp, _ArrayExtent>::value
200  constexpr
201  span(const array<_Tp, _ArrayExtent>& __arr) noexcept
202  : _M_ptr(__arr.data()), _M_extent(_ArrayExtent)
203  { }
204 
205  template<typename _Range>
206  requires (!__detail::__is_span<remove_cvref_t<_Range>>)
207  && (!__detail::__is_std_array<remove_cvref_t<_Range>>)
208  && (!is_array_v<remove_cvref_t<_Range>>)
209  && ranges::contiguous_range<_Range> && ranges::sized_range<_Range>
210  && (ranges::borrowed_range<_Range> || is_const_v<element_type>)
211  && __is_compatible_ref<ranges::range_reference_t<_Range>>::value
212  constexpr explicit(extent != dynamic_extent)
213  span(_Range&& __range)
214  noexcept(noexcept(ranges::data(__range))
215  && noexcept(ranges::size(__range)))
216  : _M_ptr(ranges::data(__range)), _M_extent(ranges::size(__range))
217  {
218  if constexpr (extent != dynamic_extent)
219  {
220  __glibcxx_assert(ranges::size(__range) == extent);
221  }
222  }
223 
224  constexpr
225  span(const span&) noexcept = default;
226 
227  template<typename _OType, size_t _OExtent>
228  requires (_Extent == dynamic_extent || _OExtent == dynamic_extent
229  || _Extent == _OExtent)
230  && (__is_array_convertible<_Type, _OType>::value)
231  constexpr
232  explicit(extent != dynamic_extent && _OExtent == dynamic_extent)
233  span(const span<_OType, _OExtent>& __s) noexcept
234  : _M_ptr(__s.data()), _M_extent(__s.size())
235  {
236  if constexpr (extent != dynamic_extent)
237  {
238  __glibcxx_assert(__s.size() == extent);
239  }
240  }
241 
242  ~span() noexcept = default;
243 
244  constexpr span&
245  operator=(const span&) noexcept = default;
246 
247  // observers
248 
249  [[nodiscard]]
250  constexpr size_type
251  size() const noexcept
252  { return this->_M_extent._M_extent(); }
253 
254  [[nodiscard]]
255  constexpr size_type
256  size_bytes() const noexcept
257  { return this->_M_extent._M_extent() * sizeof(element_type); }
258 
259  [[nodiscard]]
260  constexpr bool
261  empty() const noexcept
262  { return size() == 0; }
263 
264  // element access
265 
266  [[nodiscard]]
267  constexpr reference
268  front() const noexcept
269  {
270  __glibcxx_assert(!empty());
271  return *this->_M_ptr;
272  }
273 
274  [[nodiscard]]
275  constexpr reference
276  back() const noexcept
277  {
278  __glibcxx_assert(!empty());
279  return *(this->_M_ptr + (size() - 1));
280  }
281 
282  [[nodiscard]]
283  constexpr reference
284  operator[](size_type __idx) const noexcept
285  {
286  __glibcxx_assert(__idx < size());
287  return *(this->_M_ptr + __idx);
288  }
289 
290 #if __cpp_lib_span >= 202311L // >= C++26
291  [[nodiscard]]
292  constexpr reference
293  at(size_type __idx) const
294  {
295  if (__idx >= size())
296  __throw_out_of_range_fmt(__N("span::at(%zu) out-of-range for span "
297  "of size %zu"), __idx, this->size());
298  return *(this->_M_ptr + __idx);
299  }
300 #endif
301 
302  [[nodiscard]]
303  constexpr pointer
304  data() const noexcept
305  { return this->_M_ptr; }
306 
307  // iterator support
308 
309  [[nodiscard]]
310  constexpr iterator
311  begin() const noexcept
312  { return iterator(this->_M_ptr); }
313 
314  [[nodiscard]]
315  constexpr iterator
316  end() const noexcept
317  { return iterator(this->_M_ptr + this->size()); }
318 
319  [[nodiscard]]
320  constexpr reverse_iterator
321  rbegin() const noexcept
322  { return reverse_iterator(this->end()); }
323 
324  [[nodiscard]]
325  constexpr reverse_iterator
326  rend() const noexcept
327  { return reverse_iterator(this->begin()); }
328 
329 #if __cplusplus > 202002L
330  [[nodiscard]]
331  constexpr const_iterator
332  cbegin() const noexcept
333  { return begin(); }
334 
335  [[nodiscard]]
336  constexpr const_iterator
337  cend() const noexcept
338  { return end(); }
339 
340  [[nodiscard]]
341  constexpr const_reverse_iterator
342  crbegin() const noexcept
343  { return rbegin(); }
344 
345  [[nodiscard]]
346  constexpr const_reverse_iterator
347  crend() const noexcept
348  { return rend(); }
349 #endif
350 
351  // subviews
352 
353  template<size_t _Count>
354  [[nodiscard]]
355  constexpr span<element_type, _Count>
356  first() const noexcept
357  {
358  if constexpr (_Extent == dynamic_extent)
359  __glibcxx_assert(_Count <= size());
360  else
361  static_assert(_Count <= extent);
362  using _Sp = span<element_type, _Count>;
363  return _Sp{ this->data(), _Count };
364  }
365 
366  [[nodiscard]]
367  constexpr span<element_type, dynamic_extent>
368  first(size_type __count) const noexcept
369  {
370  __glibcxx_assert(__count <= size());
371  return { this->data(), __count };
372  }
373 
374  template<size_t _Count>
375  [[nodiscard]]
376  constexpr span<element_type, _Count>
377  last() const noexcept
378  {
379  if constexpr (_Extent == dynamic_extent)
380  __glibcxx_assert(_Count <= size());
381  else
382  static_assert(_Count <= extent);
383  using _Sp = span<element_type, _Count>;
384  return _Sp{ this->data() + (this->size() - _Count), _Count };
385  }
386 
387  [[nodiscard]]
388  constexpr span<element_type, dynamic_extent>
389  last(size_type __count) const noexcept
390  {
391  __glibcxx_assert(__count <= size());
392  return { this->data() + (this->size() - __count), __count };
393  }
394 
395  template<size_t _Offset, size_t _Count = dynamic_extent>
396  [[nodiscard]]
397  constexpr auto
398  subspan() const noexcept
399  -> span<element_type, _S_subspan_extent<_Offset, _Count>()>
400  {
401  if constexpr (_Extent == dynamic_extent)
402  {
403  __glibcxx_assert(_Offset <= size());
404  }
405  else
406  static_assert(_Offset <= extent);
407 
408  using _Sp = span<element_type, _S_subspan_extent<_Offset, _Count>()>;
409 
410  if constexpr (_Count == dynamic_extent)
411  return _Sp{ this->data() + _Offset, this->size() - _Offset };
412  else
413  {
414  if constexpr (_Extent == dynamic_extent)
415  {
416  __glibcxx_assert(_Count <= size());
417  __glibcxx_assert(_Count <= (size() - _Offset));
418  }
419  else
420  {
421  static_assert(_Count <= extent);
422  static_assert(_Count <= (extent - _Offset));
423  }
424  return _Sp{ this->data() + _Offset, _Count };
425  }
426  }
427 
428  [[nodiscard]]
429  constexpr span<element_type, dynamic_extent>
430  subspan(size_type __offset, size_type __count = dynamic_extent) const
431  noexcept
432  {
433  __glibcxx_assert(__offset <= size());
434  if (__count == dynamic_extent)
435  __count = this->size() - __offset;
436  else
437  {
438  __glibcxx_assert(__count <= size());
439  __glibcxx_assert(__offset + __count <= size());
440  }
441  return {this->data() + __offset, __count};
442  }
443 
444  private:
445  pointer _M_ptr;
446  [[no_unique_address]] __detail::__extent_storage<extent> _M_extent;
447  };
448 
449  // deduction guides
450 
451  template<typename _Type, size_t _ArrayExtent>
452  span(_Type(&)[_ArrayExtent]) -> span<_Type, _ArrayExtent>;
453 
454  template<typename _Type, size_t _ArrayExtent>
455  span(array<_Type, _ArrayExtent>&) -> span<_Type, _ArrayExtent>;
456 
457  template<typename _Type, size_t _ArrayExtent>
458  span(const array<_Type, _ArrayExtent>&)
459  -> span<const _Type, _ArrayExtent>;
460 
461  template<contiguous_iterator _Iter, typename _End>
462  span(_Iter, _End)
463  -> span<remove_reference_t<iter_reference_t<_Iter>>>;
464 
465  template<ranges::contiguous_range _Range>
466  span(_Range &&)
467  -> span<remove_reference_t<ranges::range_reference_t<_Range&>>>;
468 
469  template<typename _Type, size_t _Extent>
470  [[nodiscard]]
471  inline
472  span<const byte, _Extent == dynamic_extent
473  ? dynamic_extent : _Extent * sizeof(_Type)>
474  as_bytes(span<_Type, _Extent> __sp) noexcept
475  {
476  auto data = reinterpret_cast<const byte*>(__sp.data());
477  auto size = __sp.size_bytes();
478  constexpr auto extent = _Extent == dynamic_extent
479  ? dynamic_extent : _Extent * sizeof(_Type);
480  return span<const byte, extent>{data, size};
481  }
482 
483  template<typename _Type, size_t _Extent>
484  requires (!is_const_v<_Type>)
485  inline
486  span<byte, _Extent == dynamic_extent
487  ? dynamic_extent : _Extent * sizeof(_Type)>
488  as_writable_bytes [[nodiscard]] (span<_Type, _Extent> __sp) noexcept
489  {
490  auto data = reinterpret_cast<byte*>(__sp.data());
491  auto size = __sp.size_bytes();
492  constexpr auto extent = _Extent == dynamic_extent
493  ? dynamic_extent : _Extent * sizeof(_Type);
494  return span<byte, extent>{data, size};
495  }
496 
497  namespace ranges
498  {
499  // Opt-in to borrowed_range concept
500  template<typename _ElementType, size_t _Extent>
501  inline constexpr bool
502  enable_borrowed_range<span<_ElementType, _Extent>> = true;
503 
504  // Opt-in to view concept
505  template<typename _ElementType, size_t _Extent>
506  inline constexpr bool
507  enable_view<span<_ElementType, _Extent>> = true;
508  }
509 _GLIBCXX_END_NAMESPACE_VERSION
510 } // namespace std
511 #endif // __cpp_lib_span
512 #endif // _GLIBCXX_SPAN