31-Customer类

12.7.2　Customer类

接下来需要设计客户类。通常，ATM客户有很多属性，例如姓名、账户和账户结余。然而，这里的模拟需要使用的唯一一个属性是客户何时进入队列以及客户交易所需的时间。当模拟生成新客户时，程序将创建一个新的客户对象，并在其中存储客户的到达时间以及一个随机生成的交易时间。当客户到达队首时，程序将记录此时的时间，并将其与进入队列的时间相减，得到客户的等候时间。下面的代码演示了如何定义和实现Customer类：

class Customer
{
private:
    long arrive;     // arrival time for customer
    int processtime; // processing time for customer
public:
    Customer() { arrive = processtime = 0; }
    void set(long when);
    long when() const { return arrive; }
    int ptime() const { return processtime; }
};
void Customer::set(long when)
{
    processtime = std::rand() % 3 + 1;
    arrive = when;
}

默认构造函数创建一个空客户。set()成员函数将到达时间设置为参数，并将处理时间设置为1～3中的一个随机值。

程序清单12.10将Queue和Customer类声明放到一起，而程序清单12.11列出了方法。

程序清单12.10　queue.h

// queue.h -- interface for a queue
#ifndef QUEUE_H_
#define QUEUE_H_
// This queue will contain Customer items
class Customer
{
private:
    long arrive;     // arrival time for customer
    int processtime; // processing time for customer
public:
    Customer() { arrive = processtime = 0; }
    void set(long when);
    long when() const { return arrive; }
    int ptime() const { return processtime; }
};
typedef Customer Item;
class Queue
{
private:
// class scope definitions
    // Node is a nested structure definition local to this class
    struct Node { Item item; struct Node * next;};
    enum {Q_SIZE = 10};
// private class members
    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
    // preemptive definitions to prevent public copying
    Queue(const Queue & q) : qsize(0) { }
    Queue & operator=(const Queue & q) { return *this;}
public:
    Queue(int qs = Q_SIZE); // create queue with a qs limit
    ~Queue();
    bool isempty() const;
    bool isfull() const;
    int queuecount() const;
    bool enqueue(const Item &item); // add item to end
    bool dequeue(Item &item); // remove item from front
};
#endif

程序清单12.11　queue.cpp

// queue.cpp -- Queue and Customer methods
#include "queue.h"
#include <cstdlib>           // (or stdlib.h) for rand()
// Queue methods
Queue::Queue(int qs) : qsize(qs)
{
    front = rear = NULL;     // or nullptr
    items = 0;
}
Queue::~Queue()
{
    Node * temp;
    while (front != NULL)    // 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
    }
}
bool Queue::isempty() const
{
    return items == 0;
}
bool Queue::isfull() const
{
    return items == qsize;
}
int Queue::queuecount() const
{
    return items;
}
// Add item to queue
bool Queue::enqueue(const Item & item)
{
    if (isfull())
        return false;
    Node * add = new Node; // create node
// on failure, new throws std::bad_alloc exception
    add->item = item;      // set node pointers
    add->next = NULL;      // or nullptr;
    items++;
    if (front == NULL)     // 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
bool Queue::dequeue(Item & item)
{
    if (front == NULL)
        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 = NULL;
    return true;
}
// customer method
// when is the time at which the customer arrives
// the arrival time is set to when and the processing
// time set to a random value in the range 1 - 3
void Customer::set(long when)
{
    processtime = std::rand() % 3 + 1;
    arrive = when;
}