core: iota_iterator (waiting for C++20)

TODO

@@ -9,6 +9,7 @@ DOC:
 * storage<void>
 * custom storage/view
 * examples (and credits) from @alanjfs :)
+* document iterator.hpp (see core.md)
 
 WIP:
 * handle: replace visit with an iterable object

src/entt/core/iterator.hpp

@@ -38,6 +38,90 @@ private:
     Type value;
 };
 
+/**
+ * @brief Plain iota iterator (waiting for C++20).
+ * @tparam Type Value type.
+ */
+template<typename Type>
+class iota_iterator final {
+    static_assert(std::is_integral_v<Type>, "Not an integral type");
+
+public:
+    /*! @brief Value type, likely an integral one. */
+    using value_type = Type;
+    /*! @brief Invalid pointer type. */
+    using pointer = void;
+    /*! @brief Non-reference type, same as value type. */
+    using reference = value_type;
+    /*! @brief Difference type. */
+    using difference_type = std::ptrdiff_t;
+    /*! @brief Iterator category. */
+    using iterator_category = std::input_iterator_tag;
+
+    /*! @brief Default constructor. */
+    constexpr iota_iterator() noexcept
+        : current{} {}
+
+    /**
+     * @brief Constructs an iota iterator from a given value.
+     * @param init The initial value assigned to the iota iterator.
+     */
+    constexpr iota_iterator(const value_type init) noexcept
+        : current{init} {}
+
+    /**
+     * @brief Pre-increment operator.
+     * @return This iota iterator.
+     */
+    constexpr iota_iterator &operator++() noexcept {
+        return ++current, *this;
+    }
+
+    /**
+     * @brief Post-increment operator.
+     * @return This iota iterator.
+     */
+    constexpr iota_iterator operator++(int) noexcept {
+        iota_iterator orig = *this;
+        return ++(*this), orig;
+    }
+
+    /**
+     * @brief Dereference operator.
+     * @return The underlying value.
+     */
+    [[nodiscard]] constexpr reference operator*() const noexcept {
+        return current;
+    }
+
+private:
+    value_type current;
+};
+
+/**
+ * @brief Comparison operator.
+ * @tparam Type Value type of the iota iterator.
+ * @param lhs A properly initialized iota iterator.
+ * @param rhs A properly initialized iota iterator.
+ * @return True if the two iterators are identical, false otherwise.
+ */
+template<typename Type>
+[[nodiscard]] constexpr bool operator==(const iota_iterator<Type> &lhs, const iota_iterator<Type> &rhs) noexcept {
+    return *lhs == *rhs;
+}
+
+/**
+ * @brief Comparison operator.
+ * @tparam Type Value type of the iota iterator.
+ * @param lhs A properly initialized iota iterator.
+ * @param rhs A properly initialized iota iterator.
+ * @return True if the two iterators differ, false otherwise.
+ */
+template<typename Type>
+[[nodiscard]] constexpr bool operator!=(const iota_iterator<Type> &lhs, const iota_iterator<Type> &rhs) noexcept {
+    return !(lhs == rhs);
+}
+
 /**
  * @brief Utility class to create an iterable object from a pair of iterators.
  * @tparam It Type of iterator.

test/entt/core/iterator.cpp

@@ -1,3 +1,4 @@
+#include <cstddef>
 #include <type_traits>
 #include <utility>
 #include <vector>
@@ -17,6 +18,20 @@ TEST(InputIteratorPointer, Functionalities) {
     ASSERT_EQ(ptr->value, 42);
 }
 
+TEST(IotaIterator, Functionalities) {
+    entt::iota_iterator<std::size_t> first{};
+    const entt::iota_iterator<std::size_t> last{2u};
+
+    ASSERT_NE(first, last);
+    ASSERT_FALSE(first == last);
+    ASSERT_TRUE(first != last);
+
+    ASSERT_EQ(*first++, 0u);
+    ASSERT_EQ(*first, 1u);
+    ASSERT_EQ(*++first, *last);
+    ASSERT_EQ(*first, 2u);
+}
+
 TEST(IterableAdaptor, Functionalities) {
     std::vector<int> vec{1, 2};
     entt::iterable_adaptor iterable{vec.begin(), vec.end()};