libcxx/include/__ranges/empty.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_H
  11 #define _LIBCPP___RANGES_EMPTY_H
  12
  13 #include <__concepts/class_or_enum.h>
  14 #include <__config>
  15 #include <__iterator/concepts.h>
  16 #include <__ranges/access.h>
  17 #include <__ranges/size.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 // [range.prim.empty]
  28
  29 namespace ranges {
  30 namespace __empty {
  31   template <class _Tp>
  32   concept __member_empty =
  33     __workaround_52970<_Tp> &&
  34     requires(_Tp&& __t) {
  35       bool(__t.empty());
  36     };
  37
  38   template<class _Tp>
  39   concept __can_invoke_size =
  40     !__member_empty<_Tp> &&
  41     requires(_Tp&& __t) { ranges::size(__t); };
  42
  43   template <class _Tp>
  44   concept __can_compare_begin_end =
  45     !__member_empty<_Tp> &&
  46     !__can_invoke_size<_Tp> &&
  47     requires(_Tp&& __t) {
  48       bool(ranges::begin(__t) == ranges::end(__t));
  49       { ranges::begin(__t) } -> forward_iterator;
  50     };
  51
  52   struct __fn {
  53     template <__member_empty _Tp>
  54     [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr bool operator()(_Tp&& __t) const
  55         noexcept(noexcept(bool(__t.empty()))) {
  56       return bool(__t.empty());
  57     }
  58
  59     template <__can_invoke_size _Tp>
  60     [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr bool operator()(_Tp&& __t) const
  61         noexcept(noexcept(ranges::size(__t))) {
  62       return ranges::size(__t) == 0;
  63     }
  64
  65     template<__can_compare_begin_end _Tp>
  66     [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr bool operator()(_Tp&& __t) const
  67         noexcept(noexcept(bool(ranges::begin(__t) == ranges::end(__t)))) {
  68       return ranges::begin(__t) == ranges::end(__t);
  69     }
  70   };
  71 } // namespace __empty
  72
  73 inline namespace __cpo {
  74   inline constexpr auto empty = __empty::__fn{};
  75 } // namespace __cpo
  76 } // namespace ranges
  77
  78 #endif // _LIBCPP_STD_VER >= 20
  79
  80 _LIBCPP_END_NAMESPACE_STD
  81
  82 #endif // _LIBCPP___RANGES_EMPTY_H