C++模板中的嵌套

在下面的程序中，我們創建了一個模板類用於實現Queue容器的部分功能，並且在模板類中潛逃使用了一個Node類。
queuetp.h

// queuetp.h -- queue template with a nested class
#ifndef QUEUETP_H_
#define QUEUETP_H_

template <class Item>
class QueueTP
{
private:
    enum {Q_SIZE = 10};
    // Node is a nested class definition
    class Node
    {
    public:
        Item item;
        Node * next;
        Node(const Item & i) : item(i), next(0) {}
    };
    Node * front;       // pointer to front of Queue
    Node * rear;        // pointer to rear of Queue
    int items;          // current number of items in Queue
    const int qsize;    // maximum number of items in Queue
    QueueTP(const QueueTP & q) : qsize(0) {}
    QueueTP & operator=(const QueueTP & q) { return *this; }
public:
    QueueTP(int qs = Q_SIZE);
    ~QueueTP();
    bool isempty() const
    {
        return items == 0;
    }
    bool isfull() const
    {
        return items == qsize;
    }
    int queuecount() const
    {
        return items;
    }
    bool enqueue(const Item &item); // add item to end
    bool dequeue(Item &item);       // remove item from front
};
// QueueTP methods
template <class Item>
QueueTP<Item>::QueueTP(int qs) : qsize(qs)
{
    front = rear = 0;
    items = 0;
}

template <class Item>
QueueTP<Item>::~QueueTP()
{
    Node * temp;
    while (front != 0)      // while queue is not yet empty
    {
        temp = front;
        front = front->next;
        delete temp;
    }
}

// Add item to queue
template <class Item>
bool QueueTP<Item>::enqueue(const Item & item)
{
    if (isfull())
        return false;
    Node * add = new Node(item);    // create node
    // on failure, new throws std::bad_alloc exception
    items ++;
    if (front == 0)             // if queue is empty
        front = add;            // place item at front
    else
        rear->next = add;       // else place at rear
    rear = add;
    return true;
}

// Place front item into item variable and remove from queue
template <class Item>
bool QueueTP<Item>::dequeue(Item & item)
{
    if (front == 0)
        return false;
    item = front->item;         // set item to first item in queue
    items --;
    Node * temp = front;        // save location of first item
    front = front->next;        // reset front to next item
    delete temp;                // delete former first item
    if (items == 0)
        rear = 0;
    return true;
}

#endif // QUEUETP_H_

這里，Node是利用通用類型Item類定義的。所以，下面的聲明將導致Node被定義成用於存儲double值：
QueueTp<double> dq;
而下面的聲明將導致Node被定義沉用於存儲char值：
QueueTp<char> cq;
這兩個Node類將在兩個獨立的QueueTP類中定義，因此不會發生名稱沖突。即一個節點的類型為QueueTP<double>::Node，另一個節點的類型為QueueTP<char>::Node。
下面的程序用於測試這個新的類。
nested.cpp

//nested.cpp -- using a queue that has a nested class
#include <iostream>

#include <string>
#include "queuetp.h"

int main()
{
    using std::string;
    using std::cin;
    using std::cout;

    QueueTP<string> cs(5);
    string temp;

    while (!cs.isfull())
    {
        cout << "Please enter your name. You will be "
             << "served in the order of arrival.\n"
             << "name: ";
             getline(cin, temp);
             cs.enqueue(temp);
    }
    cout << "The queue is full. Processing begins!\n";

    while (!cs.isempty())
    {
        cs.dequeue(temp);
        cout << "Now processing " << temp << "...\n";
    }
    return 0;
}

效果：

Please enter your name. You will be served in the order of arrival.
name: moonlit
Please enter your name. You will be served in the order of arrival.
name: moon
Please enter your name. You will be served in the order of arrival.
name: light
Please enter your name. You will be served in the order of arrival.
name: moonlight poet
Please enter your name. You will be served in the order of arrival.
name: paul jackson
The queue is full. Processing begins!
Now processing moonlit...
Now processing moon...
Now processing light...
Now processing moonlight poet...
Now processing paul jackson...