queues

    // Generic FIFO queue ----------------------------------------------------------

    template<typename T>
    class queue {
     public:
      virtual ~queue() {}
      virtual void enqueue(const T& t) = 0;
      virtual void enqueue(T&& t) = 0;
      virtual T dequeue() = 0;
    };

    // Bounded queues --------------------------------------------------------------

    class upper_bounded_queue : public virtual queue {
    public:
      virtual bool full() = 0;
    };

    class lower_bounded_queue : public virtual queue {
    public:
      virtual bool empty() = 0;
    };

    class bounded_queue : public virtual upper_bounded_queue,
                      public virtual lower_bounded_queue {}

    // Unbounded queues ------------------------------------------------------------

    class upper_unbounded_queue : public virtual upper_bounded_queue {
    public:
      virtual bool full() { return false; } final;
    };

    class lower_unbounded_queue : public virtual lower_bounded_queue {
    public:
      virtual bool empty() { return false; } final;
    };

    class unbounded_queue : public virtual bounded_queue,
                        public virtual upper_bounded_queue,
                        public virtual lower_bounded_queue {}