Add default constructor, update moved-from state

Author: patrickroberts

README.md

@@ -9,7 +9,7 @@ SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 ![Lint Check (pre-commit)](https://github.com/bemanproject/any_view/actions/workflows/pre-commit.yml/badge.svg)
 ![Standard Target](https://github.com/bemanproject/beman/blob/main/images/badges/cpp29.svg)
 
-**Implements**: `std::ranges::any_view` proposed in [any_view (P3411R2)](https://wg21.link/p3411r2).
+**Implements**: `std::ranges::any_view` proposed in [any_view (P3411)](https://wg21.link/p3411).
 
 **Status**: [Under development and not yet ready for production use.](https://github.com/bemanproject/beman/blob/main/docs/BEMAN_LIBRARY_MATURITY_MODEL.md#under-development-and-not-yet-ready-for-production-use)
 

include/beman/any_view/any_view.hpp

@@ -3,6 +3,7 @@
 #ifndef BEMAN_ANY_VIEW_ANY_VIEW_HPP
 #define BEMAN_ANY_VIEW_ANY_VIEW_HPP
 
+#include <beman/any_view/detail/default_view.hpp>
 #include <beman/any_view/detail/view_adaptor.hpp>
 
 namespace beman::any_view {
@@ -38,17 +39,24 @@ class any_view : public std::ranges::view_interface<any_view<ElementT, OptsV, Re
     constexpr any_view(RangeT&& range)
         : view_ptr(get_in_place_adaptor_type<RangeT>(), std::views::all(std::forward<RangeT>(range))) {}
 
+    constexpr any_view() noexcept : any_view(detail::default_view<ElementT, RefT, RValueRefT, DiffT>{}) {}
+
     constexpr any_view(const any_view&)
         requires(bool(OptsV& any_view_options::copyable))
     = default;
 
-    constexpr any_view(any_view&&) noexcept = default;
+    constexpr any_view(any_view&& other) noexcept : any_view() { swap(other); }
 
     constexpr any_view& operator=(const any_view&)
         requires(bool(OptsV& any_view_options::copyable))
     = default;
 
-    constexpr any_view& operator=(any_view&&) noexcept = default;
+    constexpr any_view& operator=(any_view&& other) noexcept {
+        view_ptr = {get_in_place_adaptor_type<detail::default_view<ElementT, RefT, RValueRefT, DiffT>>(),
+                    detail::default_view<ElementT, RefT, RValueRefT, DiffT>{}};
+        swap(other);
+        return *this;
+    }
 
     constexpr ~any_view() = default;
 

include/beman/any_view/detail/default_view.hpp

@@ -0,0 +1,21 @@
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#ifndef BEMAN_ANY_VIEW_DETAIL_DEFAULT_VIEW_HPP
+#define BEMAN_ANY_VIEW_DETAIL_DEFAULT_VIEW_HPP
+
+#include <beman/any_view/detail/unreachable_iterator.hpp>
+
+#include <ranges>
+
+namespace beman::any_view::detail {
+
+template <class ElementT, class RefT, class RValueRefT, class DiffT>
+class default_view : public std::ranges::subrange<unreachable_iterator<ElementT, RefT, RValueRefT, DiffT>> {
+  public:
+    using std::ranges::subrange<unreachable_iterator<ElementT, RefT, RValueRefT, DiffT>>::subrange;
+    using std::ranges::subrange<unreachable_iterator<ElementT, RefT, RValueRefT, DiffT>>::operator=;
+};
+
+} // namespace beman::any_view::detail
+
+#endif // BEMAN_ANY_VIEW_DETAIL_DEFAULT_VIEW_HPP

include/beman/any_view/detail/unreachable_iterator.hpp

@@ -0,0 +1,69 @@
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#ifndef BEMAN_ANY_VIEW_DETAIL_UNREACHABLE_ITERATOR_HPP
+#define BEMAN_ANY_VIEW_DETAIL_UNREACHABLE_ITERATOR_HPP
+
+#include <beman/any_view/detail/utility.hpp>
+
+#include <compare>
+#include <iterator>
+
+namespace beman::any_view::detail {
+
+template <class ElementT, class RefT, class RValueRefT, class DiffT>
+struct iterator_concept {
+    using type = std::random_access_iterator_tag;
+};
+
+template <class ElementT>
+struct iterator_concept<ElementT, ElementT&, ElementT&&, std::ptrdiff_t> {
+    using type = std::contiguous_iterator_tag;
+};
+
+template <class ElementT, class RefT, class RValueRefT, class DiffT>
+class unreachable_iterator {
+  public:
+    using iterator_concept = iterator_concept<ElementT, RefT, RValueRefT, DiffT>::type;
+    using element_type     = ElementT;
+    using difference_type  = DiffT;
+
+    constexpr auto operator*() const noexcept -> RefT { unreachable(); }
+
+    constexpr friend auto iter_move(unreachable_iterator) noexcept -> RValueRefT { unreachable(); }
+
+    constexpr auto operator++() noexcept -> unreachable_iterator& { return *this; }
+
+    constexpr auto operator++(int) noexcept -> unreachable_iterator { return *this; }
+
+    constexpr auto operator<=>(const unreachable_iterator&) const noexcept -> std::strong_ordering = default;
+
+    constexpr auto operator--() noexcept -> unreachable_iterator& { return *this; }
+
+    constexpr auto operator--(int) noexcept -> unreachable_iterator { return *this; }
+
+    constexpr auto operator-(unreachable_iterator) const noexcept -> difference_type { return 0; }
+
+    constexpr auto operator+=(difference_type) noexcept -> unreachable_iterator& { return *this; }
+
+    constexpr auto operator+(difference_type) const noexcept -> unreachable_iterator { return *this; }
+
+    constexpr friend auto operator+(difference_type, unreachable_iterator) noexcept -> unreachable_iterator {
+        return {};
+    }
+
+    constexpr auto operator-=(difference_type) noexcept -> unreachable_iterator& { return *this; }
+
+    constexpr auto operator-(difference_type) const noexcept -> unreachable_iterator { return *this; }
+
+    constexpr auto operator[](difference_type) const noexcept -> RefT { unreachable(); }
+
+    constexpr auto operator->() const noexcept -> element_type*
+        requires std::derived_from<iterator_concept, std::contiguous_iterator_tag>
+    {
+        return nullptr;
+    }
+};
+
+} // namespace beman::any_view::detail
+
+#endif // BEMAN_ANY_VIEW_DETAIL_UNREACHABLE_ITERATOR_HPP

tests/beman/any_view/constexpr.test.cpp

@@ -84,3 +84,23 @@ TEST(ConstexprTest, reference_lifetime) {
 #endif
     EXPECT_TRUE(set_front(std::vector{7}, 42));
 }
+
+constexpr auto is_empty(any_view<int, forward> view) { return view.empty(); }
+
+TEST(ConstexprTest, default_construct) {
+    static_assert(is_empty({}));
+
+    EXPECT_TRUE(is_empty({}));
+}
+
+TEST(ConstexprTest, moved_from) {
+    const auto make_moved_from = [] {
+        any_view<int, forward> view  = std::vector<int>{42};
+        auto                   other = std::move(view);
+        return view;
+    };
+
+    static_assert(is_empty(make_moved_from()));
+
+    EXPECT_TRUE(is_empty(make_moved_from()));
+}