不可变队列

    实现一个高效的不可变队列，队列有enqueue和dequeue，分别从队列push、pop一个元素，并返回新队列。原有队列保持不变。

template<typename T>
class const_queue
{
public:
    const_queue()
        : m_nFrontIdx(0),
        m_nEndIdx(0),
        m_pdqData(new deque<T>())
    {

    }

    const_queue<T> enqueue(T t) const
    {
        // Share data.
        if (m_pdqData->size() == m_nEndIdx)
        {
            const_queue q(*this);

            q.m_pdqData->push_back(t);
            q.m_nFrontIdx = m_nFrontIdx;
            q.m_nEndIdx = m_nEndIdx + 1;

            return q;
        }
        // Copy data, share failed.
        else
        {
            const_queue q;
            deque<T> dq(
                m_pdqData->begin() + m_nFrontIdx,
                m_pdqData->begin() + m_nEndIdx);

            q.m_pdqData->swap(dq);
            q.m_pdqData->push_back(t);
            q.m_nFrontIdx = 0;
            q.m_nEndIdx = m_nEndIdx - m_nFrontIdx + 1;

            return q;
        }
    }

    const_queue<T> dequeue() const
    {
        // Share data.
        const_queue q(*this);

        q.m_nFrontIdx = m_nFrontIdx + 1;
        q.m_nEndIdx = m_nEndIdx;

        return q;
    }

    T peek() const
    {
        if (!isEmpty())
        {
            return m_pdqData->at(m_nFrontIdx);
        }

        throw "const_queue is empty";
    }

    size_t size() const
    {
        if (!isEmpty())
        {
            return m_nEndIdx - m_nFrontIdx;
        }

        return 0;
    }

    bool isEmpty() const
    {
        return m_nEndIdx <= m_nFrontIdx;
    }

private:
    size_t m_nFrontIdx;
    size_t m_nEndIdx;
    shared_ptr<deque<T> > m_pdqData;
};

void test()
{
    const_queue<int> q;

    const_queue<int> q1 = q.enqueue(1);
    cout << q1.size() << " " << q1.peek() << endl;
    const_queue<int> q2 = q.enqueue(2);
    cout << q2.size() << " " << q2.peek() << endl;
    const_queue<int> q3 = q1.enqueue(3);
    cout << q3.size() << " " << q3.peek() << endl;
    const_queue<int> q4 = q3.dequeue();
    cout << q4.size() << " " << q4.peek() << endl;
    const_queue<int> q5 = q2.enqueue(4);
    cout << q5.size() << " " << q5.peek() << endl;

    cout << q1.size() << " " << q1.peek() << endl;
    cout << q2.size() << " " << q2.peek() << endl;
    cout << q3.size() << " " << q3.peek() << endl;
    cout << q4.size() << " " << q4.peek() << endl;
    cout << q5.size() << " " << q5.peek() << endl;
}