c++ primer plus 第15章友，异常和其他：15.2.2模板中的嵌套

c++ primer plus 第15章友，异常和其他：15.2.2模板中的嵌套

15.2.2模板中的嵌套

---

15.2.2模板中的嵌套

您知道，模板很适合用于实现诸如 Queue 等容器类。您可能会问，将Queue 类定义转换为模板时，是否会由于它包含嵌套类而带来问题?答案是不会。程序清单 15.5 演示了如何进行这种转换。和类模板一样,该头文件也包含类模板和方法函数模板。

程序清单15.5 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;       // save address of front item
        front = front->next;// reset pointer to next item
        delete temp;        // delete former front
    }
}

// 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;             // have rear point to new node
    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

---

程序清单 15.5中模板有趣的一点是，Node 是利用通用类型 Item 来定义的。所以，下面的声明将导致Node 被定义成用于存储 double 值:
QueueTpdg;
而下面的声明将导致 Node 被定义成用于存储 char 值:
QueueTpcq;
这两个 Node 类将在两个独立的 QueueTP 类中定义，因此不会发生名称冲突。即一个节点的类型为QueueTP::Node，另一个节点的类型为 QueueTP::Node。程序清单15.6是一个小程序，可用于测试这个新的类。

程序清单15.6 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";
    }
    // cin.get();
    return 0; 
}