[libcxx-commits] [libcxx] WIP [libc++][ranges] P3138R3: `views::cache_latest` (PR #151450)

[Hristo Hristov via libcxx-commits]

================
@@ -0,0 +1,81 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: std-at-least-c++26
+
+// <ranges>
+
+// class cache_latest_view
+
+// constexpr auto reserve_hint()
+//     requires approximately_sized_range<V>;
+// constexpr auto reserve_hint() const
+//     requires approximately_sized_range<const V>;
+
+#include <cassert>
+#include <ranges>
+#include <utility>
+
+#include "test_iterators.h"
+
+template <class T>
+concept HasMemberReserveHint = requires(T t) { t.reserve_hint(); };
+
+static constexpr int globalBuff[8] = {};
+
+struct NonApproximatelySizedView : std::ranges::view_base {
+  using iterator = forward_iterator<const int*>;
+  iterator begin() const;
+  iterator end() const;
+};
+
+static_assert(!std::ranges::approximately_sized_range<NonApproximatelySizedView>);
+static_assert(!std::ranges::approximately_sized_range<const NonApproximatelySizedView>);
+
+static_assert(!HasMemberReserveHint<std::ranges::cache_latest_view<NonApproximatelySizedView>>);
+static_assert(!HasMemberReserveHint<const std::ranges::cache_latest_view<NonApproximatelySizedView>>);
+
+struct ApproximatelySizedView : std::ranges::view_base {
+  unsigned int size_;
+  constexpr explicit ApproximatelySizedView(unsigned int hint) : size_(hint) {}
+  constexpr auto begin() const { return forward_iterator<const int*>(globalBuff); }
+  constexpr auto end() const { return forward_iterator<const int*>(globalBuff + 8); }
+  constexpr unsigned int reserve_hint() const { return size_; }
+};
+
+static_assert(HasMemberReserveHint<std::ranges::cache_latest_view<ApproximatelySizedView>>);
+static_assert(HasMemberReserveHint<const std::ranges::cache_latest_view<ApproximatelySizedView>>);
+
+struct ApproximatelySizedNotConstView : std::ranges::view_base {
+  unsigned int size_;
+  constexpr explicit ApproximatelySizedNotConstView(unsigned int hint) : size_(hint) {}
+  constexpr auto begin() const { return forward_iterator<const int*>(globalBuff); }
+  constexpr auto end() const { return forward_iterator<const int*>(globalBuff + 8); }
+  constexpr unsigned int reserve_hint() { return size_; }
+};
+
+static_assert(HasMemberReserveHint<std::ranges::cache_latest_view<ApproximatelySizedNotConstView>>);
+static_assert(!HasMemberReserveHint<const std::ranges::cache_latest_view<ApproximatelySizedNotConstView>>);
+
+constexpr bool test() {
+  // Non-const and const are reserve_hint-able
+  {
+    auto view = std::views::cache_latest(ApproximatelySizedView{82});
+    assert(view.reserve_hint() == 82);
+    assert(std::as_const(view).reserve_hint() == 82);
+  }
+
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+
+  return 0;
+}
----------------
Zingam wrote:

```suggestion
}

```

https://github.com/llvm/llvm-project/pull/151450