不能将自定义迭代器转换为const_iterator

在自定义迭代器中定义const_iterator，并使其能够转换为const_iterator，例如：

class MyIterator {
public:
    using value_type = int;
    using difference_type = long;
    using pointer = int*;
    using reference = int&;
    using iterator_category = std::forward_iterator_tag;

    MyIterator(int* ptr) : ptr_(ptr) {}

    int& operator*() const {
        return *ptr_;
    }

    MyIterator& operator++() {
        ++ptr_;
        return *this;
    }

    MyIterator operator++(int) {
        MyIterator tmp(*this);
        ++(*this);
        return tmp;
    }

    bool operator==(const MyIterator& other) const {
        return ptr_ == other.ptr_;
    }

    bool operator!=(const MyIterator& other) const {
        return !(*this == other);
    }

    operator const MyIterator() const {
        return MyIterator(ptr_);
    }

    class const_iterator {
    public:
        using value_type = int;
        using difference_type = long;
        using pointer = const int*;
        using reference = const int&;
        using iterator_category = std::forward_iterator_tag;

        const_iterator(int* ptr) : ptr_(ptr) {}

        const_iterator& operator++() {
            ++ptr_;
            return *this;
        }

        const_iterator operator++(int) {
            const_iterator tmp(*this);
            ++(*this);
            return tmp;
        }

        const int& operator*() const {
            return *ptr_;
        }

        bool operator==(const const_iterator& other) const {
            return ptr_ == other.ptr_;
        }

        bool operator!=(const const_iterator& other) const {
            return !(*this == other);
        }

    private:
        int* ptr_;
    };

private:
    int* ptr_;
};

这个自定义迭代器包含一个const_iterator，可以通过转换将其转换为const_iterator。示例中，将MyIterator转换为const MyIterator，但是由于其const_iterator的定义，可以将const MyIterator转换为const_iterator，以便在需要const迭代器的地方使用。