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 = requires(_Tp&& __t) { bool(__t.empty()); };
  33
  34 template <class _Tp>
  35 concept __can_invoke_size = !__member_empty<_Tp> && requires(_Tp&& __t) { ranges::size(__t); };
  36
  37 template <class _Tp>
  38 concept __can_compare_begin_end = !__member_empty<_Tp> && !__can_invoke_size<_Tp> && requires(_Tp&& __t) {
  39   bool(ranges::begin(__t) == ranges::end(__t));
  40   { ranges::begin(__t) } -> forward_iterator;
  41 };
  42
  43 struct __fn {
  44   template <__member_empty _Tp>
  45   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr bool operator()(_Tp&& __t) const noexcept(noexcept(bool(__t.empty()))) {
  46     return bool(__t.empty());
  47   }
  48
  49   template <__can_invoke_size _Tp>
  50   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr bool operator()(_Tp&& __t) const noexcept(noexcept(ranges::size(__t))) {
  51     return ranges::size(__t) == 0;
  52   }
  53
  54   template <__can_compare_begin_end _Tp>
  55   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr bool operator()(_Tp&& __t) const
  56       noexcept(noexcept(bool(ranges::begin(__t) == ranges::end(__t)))) {
  57     return ranges::begin(__t) == ranges::end(__t);
  58   }
  59 };
  60 } // namespace __empty
  61
  62 inline namespace __cpo {
  63 inline constexpr auto empty = __empty::__fn{};
  64 } // namespace __cpo
  65 } // namespace ranges
  66
  67 #endif // _LIBCPP_STD_VER >= 20
  68
  69 _LIBCPP_END_NAMESPACE_STD
  70
  71 #endif // _LIBCPP___RANGES_EMPTY_H