Спринт 7. Покоряем итераторы.

#include <cassert>
#include <cstddef>
#include <iterator>
#include <string>
#include <utility>

template <typename Type>
class SingleLinkedList {

    struct Node {
        Node() = default;
        Node(const Type& val, Node* next) : value(val), next_node(next) {}
        Type value;
        Node* next_node = nullptr;
    };

    template <typename ValueType>
    class BasicIterator {

        friend class SingleLinkedList;

        explicit BasicIterator(Node* node) {
            node_ = node;
        }

    public:
        using iterator_category = std::forward_iterator_tag;
        using value_type = Type;
        using difference_type = std::ptrdiff_t;
        using pointer = ValueType*;
        using reference = ValueType&;

        BasicIterator() = default;

        BasicIterator(const BasicIterator<Type>& other) noexcept {
            node_ = other.node_;
        }

        BasicIterator& operator=(const BasicIterator& rhs) = default;

        [[nodiscard]] bool operator==(const BasicIterator<const Type>& rhs) const noexcept {
            return node_ == rhs.node_;
        }

        [[nodiscard]] bool operator!=(const BasicIterator<const Type>& rhs) const noexcept {
            return node_ != rhs.node_;
        }

        [[nodiscard]] bool operator==(const BasicIterator<Type>& rhs) const noexcept {
            return node_ == rhs.node_;
        }

        [[nodiscard]] bool operator!=(const BasicIterator<Type>& rhs) const noexcept {
            return node_ != rhs.node_;
        }

        BasicIterator& operator++() noexcept {
            node_ = node_->next_node;
            return *this;
        }

        BasicIterator operator++(int) noexcept {
            auto old_value(*this);
            ++(*this);
            return old_value;
        }

        [[nodiscard]] reference operator*() const noexcept {
            return node_->value;
        }

        [[nodiscard]] pointer operator->() const noexcept {
            if(node_){
                return &node_->value;
            }else{
                return nullptr;
            }
        }

    private:
        Node* node_ = nullptr;
    };

public:
    SingleLinkedList() {}

    using value_type = Type;
    using reference = value_type&;
    using const_reference = const value_type&;

    using Iterator = BasicIterator<Type>;
    using ConstIterator = BasicIterator<const Type>;

    [[nodiscard]] Iterator begin() noexcept {
        return Iterator{head_.next_node};
    }

    [[nodiscard]] Iterator end() noexcept {
        return Iterator{nullptr};
    }

    [[nodiscard]] ConstIterator begin() const noexcept {
        return ConstIterator{head_.next_node};
    }

    [[nodiscard]] ConstIterator end() const noexcept {
        return ConstIterator{nullptr};
    }

    [[nodiscard]] ConstIterator cbegin() const noexcept {
        return ConstIterator{head_.next_node};
    }

    [[nodiscard]] ConstIterator cend() const noexcept {
        return ConstIterator{nullptr};
    }

    [[nodiscard]] size_t GetSize() const noexcept {
        return size_;
    }

    [[nodiscard]] bool IsEmpty() const noexcept {
        if (!size_) {
        return true;
        }
        return false;
    }

    void PushFront(const Type& value) {
        head_.next_node = new Node(value, head_.next_node);
        ++size_;
    }

    void Clear() noexcept {
        while (head_.next_node) {
            Node* new_head = head_.next_node->next_node;
            delete head_.next_node;
            head_.next_node = new_head;
        }
        size_ = 0;
    }

    ~SingleLinkedList() {
        Clear();
    }

private:
    Node head_;
    size_t size_ = 0;
};

void Test2() {
    {
        SingleLinkedList<int> list;

        const auto& const_list = list;

        assert(list.begin() == list.end());
        assert(const_list.begin() == const_list.end());
        assert(list.cbegin() == list.cend());
        assert(list.cbegin() == const_list.begin());
        assert(list.cend() == const_list.end());
    }

    {
        SingleLinkedList<int> list;
        const auto& const_list = list;

        list.PushFront(1);
        assert(list.GetSize() == 1u);
        assert(!list.IsEmpty());

        assert(const_list.begin() != const_list.end());
        assert(const_list.cbegin() != const_list.cend());
        assert(list.begin() != list.end());

        assert(const_list.begin() == const_list.cbegin());

        assert(*list.cbegin() == 1);
        *list.begin() = -1;
        assert(*list.cbegin() == -1);

        const auto old_begin = list.cbegin();
        list.PushFront(2);
        assert(list.GetSize() == 2);

        const auto new_begin = list.cbegin();
        assert(new_begin != old_begin);

        {
            auto new_begin_copy(new_begin);
            assert((++(new_begin_copy)) == old_begin);
        }

        {
            auto new_begin_copy(new_begin);
            assert(((new_begin_copy)++) == new_begin);
            assert(new_begin_copy == old_begin);
        }

        {
            auto old_begin_copy(old_begin);
            assert((++old_begin_copy) == list.end());
        }
    }

    {
        SingleLinkedList<int> list;
        list.PushFront(1);

        SingleLinkedList<int>::ConstIterator const_it(list.begin());
        assert(const_it == list.cbegin());
        assert(*const_it == *list.cbegin());

        SingleLinkedList<int>::ConstIterator const_it1;
        const_it1 = list.begin();
        assert(const_it1 == const_it);
    }

    {
        using namespace std;
        SingleLinkedList<std::string> string_list;

        string_list.PushFront("one"s);
        assert(string_list.cbegin()->length() == 3u);
        string_list.begin()->push_back('!');
        assert(*string_list.begin() == "one!"s);
    }
}

int main() {
    Test2();
}