[libcxx-commits] [libcxx] [libc++] Implement `views::join_with` (PR #65536)

[via libcxx-commits]

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+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___RANGES_JOIN_WITH_VIEW_H
+#define _LIBCPP___RANGES_JOIN_WITH_VIEW_H
+
+#include <__concepts/common_reference_with.h>
+#include <__concepts/common_with.h>
+#include <__concepts/constructible.h>
+#include <__concepts/convertible_to.h>
+#include <__concepts/derived_from.h>
+#include <__concepts/equality_comparable.h>
+#include <__config>
+#include <__functional/bind_back.h>
+#include <__iterator/concepts.h>
+#include <__iterator/incrementable_traits.h>
+#include <__iterator/iter_move.h>
+#include <__iterator/iter_swap.h>
+#include <__iterator/iterator_traits.h>
+#include <__memory/addressof.h>
+#include <__ranges/access.h>
+#include <__ranges/all.h>
+#include <__ranges/concepts.h>
+#include <__ranges/non_propagating_cache.h>
+#include <__ranges/range_adaptor.h>
+#include <__ranges/single_view.h>
+#include <__ranges/view_interface.h>
+#include <__type_traits/conditional.h>
+#include <__type_traits/decay.h>
+#include <__type_traits/is_reference.h>
+#include <__type_traits/maybe_const.h>
+#include <__utility/as_const.h>
+#include <__utility/as_lvalue.h>
+#include <__utility/empty.h>
+#include <__utility/forward.h>
+#include <__utility/move.h>
+#include <variant>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER >= 23
+
+namespace ranges {
+template <class _Range>
+concept __bidirectional_common = bidirectional_range<_Range> && common_range<_Range>;
+
+template <input_range _View, forward_range _Pattern>
+  requires view<_View> && input_range<range_reference_t<_View>> && view<_Pattern> &&
+           __concatable<range_reference_t<_View>, _Pattern>
+class join_with_view : public view_interface<join_with_view<_View, _Pattern>> {
+  using _InnerRng _LIBCPP_NODEBUG = range_reference_t<_View>;
+
+  _LIBCPP_NO_UNIQUE_ADDRESS _View __base_ = _View();
+
+  static constexpr bool _UseOuterItCache = !forward_range<_View>;
+  using _OuterItCache _LIBCPP_NODEBUG =
+      _If<_UseOuterItCache, __non_propagating_cache<iterator_t<_View>>, __empty_cache>;
+  _LIBCPP_NO_UNIQUE_ADDRESS _OuterItCache __outer_it_;
+
+  static constexpr bool _UseInnerCache = !is_reference_v<_InnerRng>;
+  using _InnerCache _LIBCPP_NODEBUG =
+      _If<_UseInnerCache, __non_propagating_cache<remove_cvref_t<_InnerRng>>, __empty_cache>;
+  _LIBCPP_NO_UNIQUE_ADDRESS _InnerCache __inner_;
+
+  _LIBCPP_NO_UNIQUE_ADDRESS _Pattern __pattern_ = _Pattern();
+
+  template <bool _Const>
+  struct __iterator;
+
+  template <bool _Const>
+  struct __sentinel;
+
+public:
+  _LIBCPP_HIDE_FROM_ABI join_with_view()
+    requires default_initializable<_View> && default_initializable<_Pattern>
+  = default;
+
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit join_with_view(_View __base, _Pattern __pattern)
+      : __base_(std::move(__base)), __pattern_(std::move(__pattern)) {}
+
+  template <input_range _Range>
+    requires constructible_from<_View, views::all_t<_Range>> &&
+                 constructible_from<_Pattern, single_view<range_value_t<_InnerRng>>>
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit join_with_view(_Range&& __r, range_value_t<_InnerRng> __e)
+      : __base_(views::all(std::forward<_Range>(__r))), __pattern_(views::single(std::move(__e))) {}
+
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI constexpr _View base() const&
+    requires copy_constructible<_View>
+  {
+    return __base_;
+  }
+
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI constexpr _View base() && { return std::move(__base_); }
+
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI constexpr auto begin() {
----------------
huixie90 wrote:

is there a reason why `[[__nodiscard__]]` is used rather than `[[nodiscard]]`?

also I notice that we don't add `[[nodiscard]]` in ranges library yet. not that I am against nodiscard, but I am not sure what we generally do in this case. For example, `begin` can be mutating and there might be a legitimate use case where `begin` is called explicitly to cache the results and discard it.

https://github.com/llvm/llvm-project/pull/65536