libstdc++
span
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1 // Components for manipulating non-owning sequences of objects -*- C++ -*-
2 
3 // Copyright (C) 2019-2025 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 #ifdef _GLIBCXX_SYSHDR
38 #pragma GCC system_header
39 #endif
40 
41 #define __glibcxx_want_span
42 #define __glibcxx_want_span_initializer_list
43 #include <bits/version.h>
44 
45 #ifdef __cpp_lib_span // C++ >= 20 && concepts
46 #include <array>
47 #include <cstddef>
48 #include <bits/stl_iterator.h>
49 #include <bits/ranges_base.h>
50 #ifdef __cpp_lib_span_initializer_list
51 # include <initializer_list>
52 #endif
53 namespace std _GLIBCXX_VISIBILITY(default)
54 {
55 _GLIBCXX_BEGIN_NAMESPACE_VERSION
56 
57  inline constexpr size_t dynamic_extent = static_cast<size_t>(-1);
58 
59  template<typename _Type, size_t _Extent>
60  class span;
61 
62  namespace __detail
63  {
64  template<typename _Tp>
65  inline constexpr bool __is_span = false;
66 
67  template<typename _Tp, size_t _Num>
68  inline constexpr bool __is_span<span<_Tp, _Num>> = true;
69 
70  template<typename _Tp>
71  inline constexpr bool __is_std_array = false;
72 
73  template<typename _Tp, size_t _Num>
74  inline constexpr bool __is_std_array<std::array<_Tp, _Num>> = true;
75 
76  template<size_t _Extent>
77  class __extent_storage
78  {
79  public:
80  // Used for runtime sizes that must satisfy the precondition.
81  constexpr
82  __extent_storage([[maybe_unused]] size_t __n) noexcept
83  { __glibcxx_assert(__n == _Extent); }
84 
85  // Used for constant sizes that are already known to be correct.
86  consteval
87  __extent_storage(integral_constant<size_t, _Extent>) noexcept
88  { }
89 
90  // "I've made a huge mistake" - George Oscar Bluth II
91  template<size_t _Gob>
92  __extent_storage(integral_constant<size_t, _Gob>) = delete;
93 
94  [[__gnu__::__always_inline__]]
95  static constexpr size_t
96  _M_extent() noexcept
97  { return _Extent; }
98  };
99 
100  template<>
101  class __extent_storage<dynamic_extent>
102  {
103  public:
104  [[__gnu__::__always_inline__]]
105  constexpr
106  __extent_storage(size_t __extent) noexcept
107  : _M_extent_value(__extent)
108  { }
109 
110  [[__gnu__::__always_inline__]]
111  constexpr size_t
112  _M_extent() const noexcept
113  { return this->_M_extent_value; }
114 
115  private:
116  size_t _M_extent_value;
117  };
118 
119  template<typename _Type> struct __span_ptr { _Type* const _M_ptr; };
120 
121  } // namespace __detail
122 
123  template<typename _Type, size_t _Extent = dynamic_extent>
124  class span
125  {
126  template<size_t _Offset, size_t _Count>
127  static constexpr size_t
128  _S_subspan_extent()
129  {
130  if constexpr (_Count != dynamic_extent)
131  return _Count;
132  else if constexpr (extent != dynamic_extent)
133  return _Extent - _Offset;
134  else
135  return dynamic_extent;
136  }
137 
138  // _GLIBCXX_RESOLVE_LIB_DEFECTS
139  // 3255. span's array constructor is too strict
140  template<typename _Tp, size_t _ArrayExtent>
141  requires (_Extent == dynamic_extent || _ArrayExtent == _Extent)
142  using __is_compatible_array = __is_array_convertible<_Type, _Tp>;
143 
144  template<typename _Ref>
145  using __is_compatible_ref
146  = __is_array_convertible<_Type, remove_reference_t<_Ref>>;
147 
148  // Nested type so that _Type is not an associated class of iterator.
149  struct __iter_tag;
150 
151  template<size_t _Nm>
152  static inline constexpr integral_constant<size_t, _Nm> __v{};
153 
154  public:
155  // member types
156  using element_type = _Type;
157  using value_type = remove_cv_t<_Type>;
158  using size_type = size_t;
159  using difference_type = ptrdiff_t;
160  using pointer = _Type*;
161  using const_pointer = const _Type*;
162  using reference = element_type&;
163  using const_reference = const element_type&;
164  using iterator = __gnu_cxx::__normal_iterator<pointer, __iter_tag>;
165  using reverse_iterator = std::reverse_iterator<iterator>;
166 #if __cplusplus > 202002L
167  using const_iterator = std::const_iterator<iterator>;
168  using const_reverse_iterator = std::const_iterator<reverse_iterator>;
169 #endif
170 
171  // member constants
172  static constexpr size_t extent = _Extent;
173 
174  // constructors, copy and assignment
175 
176  constexpr
177  span() noexcept
178  requires (_Extent == dynamic_extent || _Extent == 0)
179  : _M_ptr(nullptr), _M_extent(__v<0>)
180  { }
181 
182  template<contiguous_iterator _It>
183  requires __is_compatible_ref<iter_reference_t<_It>>::value
184  constexpr explicit(extent != dynamic_extent)
185  span(_It __first, size_type __count)
186  noexcept
187  : _M_ptr(std::to_address(__first)), _M_extent(__count)
188  { __glibcxx_requires_valid_range(__first, __first + __count); }
189 
190  template<contiguous_iterator _It, sized_sentinel_for<_It> _End>
191  requires __is_compatible_ref<iter_reference_t<_It>>::value
192  && (!is_convertible_v<_End, size_type>)
193  constexpr explicit(extent != dynamic_extent)
194  span(_It __first, _End __last)
195  noexcept(noexcept(__last - __first))
196  : _M_ptr(std::to_address(__first)),
197  _M_extent(static_cast<size_type>(__last - __first))
198  { __glibcxx_requires_valid_range(__first, __last); }
199 
200  template<size_t _ArrayExtent>
201  requires (_Extent == dynamic_extent || _ArrayExtent == _Extent)
202  constexpr
203  span(type_identity_t<element_type> (&__arr)[_ArrayExtent]) noexcept
204  : _M_ptr(__arr), _M_extent(__v<_ArrayExtent>)
205  { }
206 
207  template<typename _Tp, size_t _ArrayExtent>
208  requires __is_compatible_array<_Tp, _ArrayExtent>::value
209  constexpr
210  span(array<_Tp, _ArrayExtent>& __arr) noexcept
211  : _M_ptr(__arr.data()), _M_extent(__v<_ArrayExtent>)
212  { }
213 
214  template<typename _Tp, size_t _ArrayExtent>
215  requires __is_compatible_array<const _Tp, _ArrayExtent>::value
216  constexpr
217  span(const array<_Tp, _ArrayExtent>& __arr) noexcept
218  : _M_ptr(__arr.data()), _M_extent(__v<_ArrayExtent>)
219  { }
220 
221  template<typename _Range>
222  requires (!__detail::__is_span<remove_cvref_t<_Range>>)
223  && (!__detail::__is_std_array<remove_cvref_t<_Range>>)
224  && (!is_array_v<remove_cvref_t<_Range>>)
225  && ranges::contiguous_range<_Range> && ranges::sized_range<_Range>
226  && (ranges::borrowed_range<_Range> || is_const_v<element_type>)
227  && __is_compatible_ref<ranges::range_reference_t<_Range>>::value
228  constexpr explicit(extent != dynamic_extent)
229  span(_Range&& __range)
230  noexcept(noexcept(ranges::data(__range))
231  && noexcept(ranges::size(__range)))
232  : _M_ptr(ranges::data(__range)), _M_extent(ranges::size(__range))
233  { }
234 
235 #if __cpp_lib_span_initializer_list >= 202311L // >= C++26
236 #pragma GCC diagnostic push
237 #pragma GCC diagnostic ignored "-Winit-list-lifetime"
238  constexpr
239  explicit(extent != dynamic_extent)
240  span(initializer_list<value_type> __il)
241  requires (is_const_v<_Type>)
242  : _M_ptr(__il.begin()), _M_extent(__il.size())
243  { }
244 #pragma GCC diagnostic pop
245 #endif
246 
247  constexpr
248  span(const span&) noexcept = default;
249 
250  template<typename _OType, size_t _OExtent>
251  requires (_Extent == dynamic_extent || _OExtent == dynamic_extent
252  || _Extent == _OExtent)
253  && (__is_array_convertible<_Type, _OType>::value)
254  constexpr
255  explicit(extent != dynamic_extent && _OExtent == dynamic_extent)
256  span(const span<_OType, _OExtent>& __s) noexcept
257  : _M_ptr(__s.data()), _M_extent(__s.size())
258  { }
259 
260  constexpr span&
261  operator=(const span&) noexcept = default;
262 
263  // observers
264 
265  [[nodiscard]]
266  constexpr size_type
267  size() const noexcept
268  { return this->_M_extent._M_extent(); }
269 
270  [[nodiscard]]
271  constexpr size_type
272  size_bytes() const noexcept
273  { return this->_M_extent._M_extent() * sizeof(element_type); }
274 
275  [[nodiscard]]
276  constexpr bool
277  empty() const noexcept
278  { return size() == 0; }
279 
280  // element access
281 
282  [[nodiscard]]
283  constexpr reference
284  front() const noexcept
285  {
286  __glibcxx_assert(!empty());
287  return *this->_M_ptr;
288  }
289 
290  [[nodiscard]]
291  constexpr reference
292  back() const noexcept
293  {
294  __glibcxx_assert(!empty());
295  return *(this->_M_ptr + (size() - 1));
296  }
297 
298  [[nodiscard]]
299  constexpr reference
300  operator[](size_type __idx) const noexcept
301  {
302  __glibcxx_assert(__idx < size());
303  return *(this->_M_ptr + __idx);
304  }
305 
306 #if __cpp_lib_span >= 202311L // >= C++26
307  [[nodiscard]]
308  constexpr reference
309  at(size_type __idx) const
310  {
311  if (__idx >= size())
312  __throw_out_of_range_fmt(__N("span::at(%zu) out-of-range for span "
313  "of size %zu"), __idx, this->size());
314  return *(this->_M_ptr + __idx);
315  }
316 #endif
317 
318  [[nodiscard]]
319  constexpr pointer
320  data() const noexcept
321  { return this->_M_ptr; }
322 
323  // iterator support
324 
325  [[nodiscard]]
326  constexpr iterator
327  begin() const noexcept
328  { return iterator(this->_M_ptr); }
329 
330  [[nodiscard]]
331  constexpr iterator
332  end() const noexcept
333  { return iterator(this->_M_ptr + this->size()); }
334 
335  [[nodiscard]]
336  constexpr reverse_iterator
337  rbegin() const noexcept
338  { return reverse_iterator(this->end()); }
339 
340  [[nodiscard]]
341  constexpr reverse_iterator
342  rend() const noexcept
343  { return reverse_iterator(this->begin()); }
344 
345 #if __cplusplus > 202002L
346  [[nodiscard]]
347  constexpr const_iterator
348  cbegin() const noexcept
349  { return begin(); }
350 
351  [[nodiscard]]
352  constexpr const_iterator
353  cend() const noexcept
354  { return end(); }
355 
356  [[nodiscard]]
357  constexpr const_reverse_iterator
358  crbegin() const noexcept
359  { return rbegin(); }
360 
361  [[nodiscard]]
362  constexpr const_reverse_iterator
363  crend() const noexcept
364  { return rend(); }
365 #endif
366 
367  // subviews
368 
369  template<size_t _Count>
370  [[nodiscard]]
371  constexpr span<element_type, _Count>
372  first() const noexcept
373  {
374  if constexpr (_Extent == dynamic_extent)
375  __glibcxx_assert(_Count <= size());
376  else
377  static_assert(_Count <= extent);
378  using _Sp = span<element_type, _Count>;
379  return _Sp{ _SizedPtr{this->data()} };
380  }
381 
382  [[nodiscard]]
383  constexpr span<element_type, dynamic_extent>
384  first(size_type __count) const noexcept
385  {
386  __glibcxx_assert(__count <= size());
387  return { this->data(), __count };
388  }
389 
390  template<size_t _Count>
391  [[nodiscard]]
392  constexpr span<element_type, _Count>
393  last() const noexcept
394  {
395  if constexpr (_Extent == dynamic_extent)
396  __glibcxx_assert(_Count <= size());
397  else
398  static_assert(_Count <= extent);
399  using _Sp = span<element_type, _Count>;
400  return _Sp{ _SizedPtr{this->data() + (this->size() - _Count)} };
401  }
402 
403  [[nodiscard]]
404  constexpr span<element_type, dynamic_extent>
405  last(size_type __count) const noexcept
406  {
407  __glibcxx_assert(__count <= size());
408  return { this->data() + (this->size() - __count), __count };
409  }
410 
411  template<size_t _Offset, size_t _Count = dynamic_extent>
412  [[nodiscard]]
413  constexpr auto
414  subspan() const noexcept
415  -> span<element_type, _S_subspan_extent<_Offset, _Count>()>
416  {
417  if constexpr (_Extent == dynamic_extent)
418  {
419  __glibcxx_assert(_Offset <= size());
420  }
421  else
422  static_assert(_Offset <= extent);
423 
424  using _Sp = span<element_type, _S_subspan_extent<_Offset, _Count>()>;
425 
426  if constexpr (_Count == dynamic_extent)
427  return _Sp{ this->data() + _Offset, this->size() - _Offset };
428  else
429  {
430  if constexpr (_Extent == dynamic_extent)
431  {
432  __glibcxx_assert(_Count <= size());
433  __glibcxx_assert(_Count <= (size() - _Offset));
434  }
435  else
436  {
437  static_assert(_Count <= extent);
438  static_assert(_Count <= (extent - _Offset));
439  }
440  return _Sp{ _SizedPtr{this->data() + _Offset} };
441  }
442  }
443 
444  [[nodiscard]]
445  constexpr span<element_type, dynamic_extent>
446  subspan(size_type __offset, size_type __count = dynamic_extent) const
447  noexcept
448  {
449  __glibcxx_assert(__offset <= size());
450  if (__count == dynamic_extent)
451  __count = this->size() - __offset;
452  else
453  {
454  __glibcxx_assert(__count <= size());
455  __glibcxx_assert(__offset + __count <= size());
456  }
457  return {this->data() + __offset, __count};
458  }
459 
460  private:
461  template<typename, size_t> friend class span;
462 
463  // Tag type for pointer that has known extent.
464  using _SizedPtr = __detail::__span_ptr<_Type>;
465 
466  // Private constructor with an implied extent.
467  [[__gnu__::__always_inline__]]
468  constexpr explicit
469  span(_SizedPtr __ptr) noexcept
470  requires (extent != dynamic_extent)
471  : _M_ptr(__ptr._M_ptr), _M_extent(__v<extent>)
472  { }
473 
474  pointer _M_ptr;
475  [[no_unique_address]] __detail::__extent_storage<extent> _M_extent;
476  };
477 
478  // deduction guides
479 
480  template<typename _Type, size_t _ArrayExtent>
481  span(_Type(&)[_ArrayExtent]) -> span<_Type, _ArrayExtent>;
482 
483  template<typename _Type, size_t _ArrayExtent>
484  span(array<_Type, _ArrayExtent>&) -> span<_Type, _ArrayExtent>;
485 
486  template<typename _Type, size_t _ArrayExtent>
487  span(const array<_Type, _ArrayExtent>&)
488  -> span<const _Type, _ArrayExtent>;
489 
490  template<contiguous_iterator _Iter, typename _End>
491  span(_Iter, _End)
492  -> span<remove_reference_t<iter_reference_t<_Iter>>>;
493 
494  template<ranges::contiguous_range _Range>
495  span(_Range &&)
496  -> span<remove_reference_t<ranges::range_reference_t<_Range&>>>;
497 
498  template<typename _Type, size_t _Extent>
499  [[nodiscard]]
500  inline
501  span<const byte, _Extent == dynamic_extent
502  ? dynamic_extent : _Extent * sizeof(_Type)>
503  as_bytes(span<_Type, _Extent> __sp) noexcept
504  {
505  auto data = reinterpret_cast<const byte*>(__sp.data());
506  auto size = __sp.size_bytes();
507  constexpr auto extent = _Extent == dynamic_extent
508  ? dynamic_extent : _Extent * sizeof(_Type);
509  return span<const byte, extent>{data, size};
510  }
511 
512  template<typename _Type, size_t _Extent>
513  requires (!is_const_v<_Type>)
514  inline
515  span<byte, _Extent == dynamic_extent
516  ? dynamic_extent : _Extent * sizeof(_Type)>
517  as_writable_bytes [[nodiscard]] (span<_Type, _Extent> __sp) noexcept
518  {
519  auto data = reinterpret_cast<byte*>(__sp.data());
520  auto size = __sp.size_bytes();
521  constexpr auto extent = _Extent == dynamic_extent
522  ? dynamic_extent : _Extent * sizeof(_Type);
523  return span<byte, extent>{data, size};
524  }
525 
526  namespace ranges
527  {
528  // Opt-in to borrowed_range concept
529  template<typename _ElementType, size_t _Extent>
530  inline constexpr bool
531  enable_borrowed_range<span<_ElementType, _Extent>> = true;
532 
533  // Opt-in to view concept
534  template<typename _ElementType, size_t _Extent>
535  inline constexpr bool
536  enable_view<span<_ElementType, _Extent>> = true;
537  }
538 _GLIBCXX_END_NAMESPACE_VERSION
539 } // namespace std
540 #endif // __cpp_lib_span
541 #endif // _GLIBCXX_SPAN