#include <cassert>

struct Reader {
    virtual int read() const = 0;
    virtual ~Reader() = default;
};
struct Writer {
    virtual void write(int value) = 0;
    virtual ~Writer() = default;
};
class Cell final : public Reader, public Writer {
    int value_ = 3;
public:
    int read() const override { return value_; }
    void write(int value) override { value_ = value; }
};

int main() {
    Cell cell;
    Reader* reader = &cell;
    Writer* writer = &cell;
    assert(reader->read() == 3);
    writer->write(8);
    assert(reader->read() == 8);
    assert(dynamic_cast<Writer*>(reader) == writer);
    assert(dynamic_cast<void*>(reader) == static_cast<void*>(&cell));
}