libcxx/include/__ranges/empty_view.h

   1 // -*- C++ -*-
   2 //===----------------------------------------------------------------------===//
   3 //
   4 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   5 // See https://llvm.org/LICENSE.txt for license information.
   6 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   7 //
   8 //===----------------------------------------------------------------------===//
   9
  10 #ifndef _LIBCPP___RANGES_EMPTY_VIEW_H
  11 #define _LIBCPP___RANGES_EMPTY_VIEW_H
  12
  13 #include <__config>
  14 #include <__cstddef/size_t.h>
  15 #include <__ranges/enable_borrowed_range.h>
  16 #include <__ranges/view_interface.h>
  17 #include <__type_traits/is_object.h>
  18
  19 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
  20 #  pragma GCC system_header
  21 #endif
  22
  23 _LIBCPP_BEGIN_NAMESPACE_STD
  24
  25 #if _LIBCPP_STD_VER >= 20
  26
  27 namespace ranges {
  28 template <class _Tp>
  29   requires is_object_v<_Tp>
  30 class empty_view : public view_interface<empty_view<_Tp>> {
  31 public:
  32   _LIBCPP_HIDE_FROM_ABI static constexpr _Tp* begin() noexcept { return nullptr; }
  33   _LIBCPP_HIDE_FROM_ABI static constexpr _Tp* end() noexcept { return nullptr; }
  34   _LIBCPP_HIDE_FROM_ABI static constexpr _Tp* data() noexcept { return nullptr; }
  35   _LIBCPP_HIDE_FROM_ABI static constexpr size_t size() noexcept { return 0; }
  36   _LIBCPP_HIDE_FROM_ABI static constexpr bool empty() noexcept { return true; }
  37 };
  38
  39 template <class _Tp>
  40 inline constexpr bool enable_borrowed_range<empty_view<_Tp>> = true;
  41
  42 namespace views {
  43
  44 template <class _Tp>
  45 inline constexpr empty_view<_Tp> empty{};
  46
  47 } // namespace views
  48 } // namespace ranges
  49
  50 #endif // _LIBCPP_STD_VER >= 20
  51
  52 _LIBCPP_END_NAMESPACE_STD
  53
  54 #endif // _LIBCPP___RANGES_EMPTY_VIEW_H