C++ Primer Plus习题及答案-第十二章
习题选自：C++ Primer Plus(第六版)
内容仅供参考，如有错误，欢迎指正 !
- c++使用new和delete运算符来动态控制内存。
- 对于静态成员，要在类声明之外使用单独语句进行初始化，因为静态类成员函数是单独存储的，而不是对象的组成部分，而且初始化是在方法文件中，而不是在头文件中，这是因为类声明位于头文件中，程序可能将头文件包括在其他几个文件中，如果在头文件中初始化，将出现多个初始化语句副本，从而引发错误。但如果静态成员是整形或枚举型const,则可以在类声明中初始化。
- 私有静态变量有且仅在初始化时，才可以在外部访问。
- 复制构造函数用于将一个对象复制到新创建的对象中，它用于初始化过程中（包括按值传递参数）。
复习题
1. 假设String 类有如下私有成员：
class String { private: char * str; //points to string allocated by new int len; //holds length of string //... };
a. 下述默认构造函数有什么问题？
String ::String() {}
b. 下述构造函数有什么问题？
String:: String (const char* s) { str = s: len = strlen(s); }
c. 下述构造西数有什么问题？
String: :String (const char* s) { strcpy(str, s); len = strlen(s); }
a. 语法是正确的，但该构造函数没有初始化str指针。该构造函数应该使用new[]来初始化它，或者将其设置为NULL。
b. 该构造函数没有创建新的字符串，只是复制了原有字符串的地址，它应当使用new[]和strcpy()。
c. 它虽然复制了字符串，但没有给他分配存储空间，应使用new char[len+1]来分配适当数量的内存。
2. 如果您定义了一个类，其指针成员是使用 new 初始化的，请指出可能出现的3个问题以及如何纠正这些问题。
可能出现的：
-
问题一：当对象过期的时候，对象的成员指针指向的数据仍保留在内存中，造成内存泄漏。
在析构函数中删除构造函数中new分配的内存，来解决该问题。
-
问题二：如果使用对象初1始化为另一个对象2时，采用默认初始化的方式，则在对象1过期后，在对象2过期过程中，其析构函数会对同一块内存释放两次，造成错误。
定义一个复制构造函数，使初始化复制指针指向的数据，而不是复制指针指向的地址。
-
问题三：将一个对象赋给另一个对象也将导致两个指针指向同一个数据。
重载赋值运算符，使之复制数据，而不是指针。
3. 如果没有显式提供类方法，编译器将自动生成哪些类方法？请描述这些隐式生成的函数的行为。
如果没有显示提供方法，c++将自动生成以下成员函数：
- 构造函数：默认构造函数不完成任何工作，但使得能够声明数组和未初始化对象。
- 复制构造函数：默认赋值构造函数使用成员赋值。
- 赋值运算符：默认赋值运算法使用成员赋值。
- 析构函数：默认析构函数不完成任何工作。
- 地址运算符：隐式地址运算符返回调用对象的地址（即this指针的值）。
4. 找出并改正下述类声明中的错误：
class nifty { // data char personality[]; int talents; // methods nifty(); nifty(char * s); ostream & operator<<(ostream & os, nifty & n); } nifty:nifty() { personality = NULL; talents = 0; } nifty:nifty(char * s) { personality = new char [strlen(s)]; personality = s; talents = 0; } ostream & nifty:operator<<(ostream & os, nifty & n) { os << n; }
应将personality成员声明为字符数组或car指针，或者将其声明为String对象。该声明没有将方法设置为公有的。修改后：
#include 5. 对于下面的类声明：
class Golfer { private: char * fullname; // points to string containing golfer's name int games; // holds number of golf games played int * scores; // points to first element of array of golf scores public: Golfer(); Golfer(const char * name, int g= 0); // creates empty dynamic array of g elements if g > 0 Golfer(const Golfer & g); ~Golfer(); };
a. 下列各条语句将调用哪些类方法？
Golfer nancy; // #1 Golfer lulu(“Little Lulu”); // #2 Golfer roy(“Roy Hobbs”, 12); // #3 Golfer * par = new Golfer; // #4 Golfer next = lulu; // #5 Golfer hazzard = “Weed Thwacker”; // #6 *par = nancy; // #7 nancy = “Nancy Putter”; // #8
b. 很明显，类需要有另外几个方法才能更有用，但是类需要哪些方法才能防止数据被损坏呢？
a.
Golfer nancy; // 默认构造函数
Golfer lulu("Little Lulu"); // Golfer(const char * name, int g)
Golfer roy("Roy Hobbs", 12); // Golfer(const char * name, int g)
Golfer* par = new Golfer; // 默认构造函数
Golfer next = lulu; // Golfer(const Golfer &g)
Golfer hazard = "Weed Thwacker"; // Golfer(const char * name, int g)
*par = nancy; // 默认赋值运算符
nancy = "Nancy Putter"; // 先调用Golfer(const char * name, int g), 再默认赋值运算符
b. 有以下两个方法：
- 防止拷贝，将赋值运算符（面向对象拷贝给对象的）／复制构造函数，放在私有部分；
- 类应定义一个复制数据（而不是地址）的赋值运算符。
编程练习
1. 对于下面的类声明：
class Cow { char name[20]; char * hobby; double weight; public: Cow(); Cow(const char * nm, const char * ho, double wt); Cow(const Cow c&); ~Cow(); Cow & operator=(const Cow & c); void ShowCow() const; // display all cow data };
给这个类提供实现，并编写一个使用所有成员函数的小程序。
cow.h:
#ifndef COW_H_
#define COW_H_
class Cow {
private:
char name[20];
char * hobby;
double weight;
public:
Cow();
Cow(const char * nm, const char * ho, double wt);
Cow(const Cow & c);
~Cow() { delete [] hobby; }
Cow & operator=(const Cow & c);
void show() const;
};
#endif
cow.cpp:
#include "cow.h"
#include main.cpp:
#include 2. 通过下面的工作来改进String类声明（即将String1.h升级为String2.h）。
a. 对+运算符进行重载，使之可将两个字符串合并成一个。
b. 提供一个Stringlow()成员函数，将字符串中所有的字母字符转换为小写（别忘了cctype系列字符函数）。
c. 提供String()成员函数，将字符串中所有字母字符转换成大写。
d. 提供一个这样的成员函数，它接受一个char参数，返回该字符在字符串中出现的次数。
使用下面的程序来测试您的工作：
// pe12_2.cpp #includeusing namespace std; #include "string2.h" int main() { String s1(" and I am a C++ student."); String s2 = "Please enter your name: "; String s3; cout << s2; // overloaded << operator cin >> s3; // overloaded >> operator s2 = "My name is " + s3; // overloaded =, + operators cout << s2 << ".\n"; s2 = s2 + s1; s2.stringup(); // converts string to uppercase cout << "The string\n" << s2 << "\ncontains " << s2.has('A') << " 'A' characters in it.\n"; s1 = "red"; // String(const char *), // then String & operator=(const String&) String rgb[3] = { String(s1), String("green"), String("blue")}; cout << "Enter the name of a primary color for mixing light: "; String ans; bool success = false; while (cin >> ans) { ans.stringlow(); // converts string to lowercase for (int i = 0; i < 3; i++) { if (ans == rgb[i]) // overloaded == operator { cout << "That's right!\n"; success = true; break; } } if (success) break; else cout << "Try again!\n"; } cout << "Bye\n"; return 0; }
输出应与下面相似：
Please enter your name: Fretta Farbo My name is Fretta Farbo. The string MY NAME IS FRETTA FARBO AND I AM A C++ STUDENT. contains 6 'A' characters in it. Enter the name of a primary color for mixing light: yellow Try again! BLUE That's right! Bye
string2.h:
#ifndef STRING2_H_
#define STRING2_H_
#include string.cpp:
#include "string2.h"
// #include 3. 新编写程序清单10.7和程序清单10.8描述的Stock类，使之使用动态分配的内存，而不是string类对象来存储股票名称。另外，使用重
载的operator<<()定义代替show()成员函数。再使用程序清单10.9测试新的定义程序。
stock20.h:
// stock20.h -- augmented version
#ifndef STOCK20_H_
#define STOCK20_H_
#include stock.cpp:
// stock20.h -- augmented version
#ifndef STOCK20_H_
#define STOCK20_H_
#include usestock20.cpp:
// stock20.cpp -- augmented version
#include "stock20.h"
#include 4.请看下面程序清单10.10定义的Stack类的变量：
// stack.h -- class declaration for the stack ADT typedef unsigned long Item; class Stack { private: enum {MAX = 10}; // constant specific to class Item * pitems; // holds stack items int size; // number of elements in stack int top; // index for top stack item public: Stack(int n = MAX); // creates stack with n elements Stack(const Stack & st); ~Stack(); bool isempty() const; bool isfull() const; // push() returns false if stack already is full, true otherwise bool push(const Item & item); // add item to stack // pop() returns false if stack already is empty, true otherwise bool pop(Item & item); // pop top into item Stack & operator=(const Stack & st); };
正如私有成员表明的，这个类使用动态分配的数组来保存栈项。请重新编写方法，以适应这种新的表示法，并编写一个程序来演示所有的方法，包括复制构造函数和赋值运算符。
stack.h:
// stack.h -- class declaration for the stack ADT
#ifndef STACK_H_
#define STACK_H_
#include stack.cpp:
// stack.cpp -- Stack member functions
#include "stack.h"
Stack::Stack(int n) // create an empty stack
{
size = MAX;
top = 0;
pitems = new Item[size];
for (int i = 0; i < size; ++i) pitems[i] = 0;
}
Stack::Stack(const Stack& st) {
delete[] pitems;
size = st.size;
top = st.top;
pitems = new Item[size];
for (int i = 0; i < size; ++i) pitems[i] = st.pitems[i];
}
Stack::~Stack() { delete[] pitems; }
bool Stack::isempty() const { return top == 0; }
bool Stack::isfull() const { return top == MAX; }
bool Stack::push(const Item& item) {
if (top < MAX) {
pitems[top++] = item;
return true;
} else
return false;
}
bool Stack::pop(Item& item) {
if (top > 0) {
item = pitems[--top];
return true;
} else
return false;
}
Stack& Stack::operator=(const Stack& st) {
if (this == &st) return *this;
delete[] pitems;
size = st.size;
top = st.top;
pitems = new Item[size];
for (int i = 0; i < size; ++i) pitems[i] = st.pitems[i];
return *this;
}
std::ostream& operator<<(std::ostream& os, const Stack& st) {
for (int i = 0; i < st.top; i++) {
os << st.pitems[i] << std::endl;
}
return os;
}
main.cpp:
// stacker.cpp -- testing the Stack class
#include 5. Heather银行进行的研究表明，ATM客户不希望排队时间不超过1分钟。使用程序清单12.10中的模拟，找出要使平均等候时间为1分钟，每小时到达的客户数应为多少（试验时间不短于100小时）？
根据实际情况可知，每小时到达的客户数目越多，则平均等候时间越长，若要求平均等候时间为1分钟（即平均等候时间不超过1分钟时的临界情况）对应的每小时到达客户数，则我们可以让程序从每小时客户数从1开始计算（不小于100小时）其对应的平均排队时间，直到平均等待时间刚好超过1分钟的时候停止，此时对应的客户数（或者对应客户数-1）即为答案。
queue.h(未修改):
// 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
queue.cpp(未修改):
// queue.cpp -- Queue and Customer methods
#include "queue.h"
#include main.cpp:
// bank.cpp -- using the Queue interface
// compile with queue.cpp
#include 实验过程中，队列最大数为10，模拟小时数为100，结果如下：
Case Study: Bank of Heather Automatic Teller
Enter maximum size of queue: 10
Enter the number of simulation hours: 100
customers accepted: 217
customers served: 217
turnaways: 0
average queue size: 0.00
average wait time: 0.06 minutes
customers accepted: 522
customers served: 522
turnaways: 0
average queue size: 0.01
average wait time: 0.07 minutes
customers accepted: 941
customers served: 941
turnaways: 0
average queue size: 0.01
average wait time: 0.09 minutes
customers accepted: 1465
customers served: 1465
turnaways: 0
average queue size: 0.02
average wait time: 0.10 minutes
customers accepted: 2078
customers served: 2078
turnaways: 0
average queue size: 0.04
average wait time: 0.12 minutes
customers accepted: 2821
customers served: 2821
turnaways: 0
average queue size: 0.07
average wait time: 0.15 minutes
customers accepted: 3640
customers served: 3640
turnaways: 0
average queue size: 0.10
average wait time: 0.17 minutes
customers accepted: 4525
customers served: 4525
turnaways: 0
average queue size: 0.14
average wait time: 0.19 minutes
customers accepted: 5534
customers served: 5534
turnaways: 0
average queue size: 0.20
average wait time: 0.21 minutes
customers accepted: 6617
customers served: 6617
turnaways: 0
average queue size: 0.26
average wait time: 0.24 minutes
customers accepted: 7780
customers served: 7780
turnaways: 0
average queue size: 0.35
average wait time: 0.27 minutes
customers accepted: 9094
customers served: 9094
turnaways: 0
average queue size: 0.47
average wait time: 0.31 minutes
customers accepted: 10427
customers served: 10427
turnaways: 0
average queue size: 0.60
average wait time: 0.34 minutes
customers accepted: 11989
customers served: 11989
turnaways: 0
average queue size: 0.82
average wait time: 0.41 minutes
customers accepted: 13588
customers served: 13588
turnaways: 0
average queue size: 1.02
average wait time: 0.45 minutes
customers accepted: 15277
customers served: 15277
turnaways: 0
average queue size: 1.25
average wait time: 0.49 minutes
customers accepted: 17118
customers served: 17116
turnaways: 0
average queue size: 1.55
average wait time: 0.54 minutes
customers accepted: 19000
customers served: 18998
turnaways: 0
average queue size: 1.90
average wait time: 0.60 minutes
customers accepted: 21012
customers served: 21009
turnaways: 0
average queue size: 2.39
average wait time: 0.68 minutes
customers accepted: 23139
customers served: 23136
turnaways: 0
average queue size: 2.94
average wait time: 0.76 minutes
customers accepted: 25374
customers served: 25368
turnaways: 0
average queue size: 3.59
average wait time: 0.85 minutes
customers accepted: 27661
customers served: 27653
turnaways: 0
average queue size: 4.36
average wait time: 0.95 minutes
customers accepted: 30018
customers served: 30010
turnaways: 1
average queue size: 5.32
average wait time: 1.06 minutes
When there comes 25.00 people per hour, the average wait time will be about 1 minute.
Done!
6．Heather银行想知道，如果再开设一台ATM，情况将如何。请对模拟进行修改，以包含两个队列。假设当第一台ATM前的排队人数少于第二台ATM时，客户将排在第一队，否则将排在第二队。然后再找出要使平均等候时间为1分钟，每小时到达的客户数应该为多少（注意，这是一个非线性问题，即将ATM数量加倍，并不能保证每小时处理的客户数量也翻倍，并确保客户等候的时间少于1分钟）？
queue.h(未修改):
// 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
queue.cpp(未修改):
// queue.cpp -- Queue and Customer methods
#include "queue.h"
#include main.cpp:
// bank.cpp -- using the Queue interface
// compile with queue.cpp
#include 实验过程中，队列最大数为10，模拟小时数为100，结果如下：
Case Study: Bank of Heather Automatic Teller
Enter maximum size of queue: 10
Enter the number of simulation hours: 100
customers accepted: 212
customers served: 212
turnaways: 0
average queue size: 0.00
average wait time: 0.03 minutes
customers accepted: 520
customers served: 520
turnaways: 0
average queue size: 0.00
average wait time: 0.06 minutes
customers accepted: 917
customers served: 917
turnaways: 0
average queue size: 0.01
average wait time: 0.06 minutes
customers accepted: 1372
customers served: 1372
turnaways: 0
average queue size: 0.02
average wait time: 0.07 minutes
customers accepted: 1987
customers served: 1987
turnaways: 0
average queue size: 0.03
average wait time: 0.10 minutes
customers accepted: 2715
customers served: 2715
turnaways: 0
average queue size: 0.05
average wait time: 0.11 minutes
customers accepted: 3471
customers served: 3471
turnaways: 0
average queue size: 0.07
average wait time: 0.12 minutes
customers accepted: 4367
customers served: 4367
turnaways: 0
average queue size: 0.09
average wait time: 0.13 minutes
customers accepted: 5368
customers served: 5368
turnaways: 0
average queue size: 0.13
average wait time: 0.14 minutes
customers accepted: 6454
customers served: 6454
turnaways: 0
average queue size: 0.17
average wait time: 0.16 minutes
customers accepted: 7639
customers served: 7639
turnaways: 0
average queue size: 0.22
average wait time: 0.17 minutes
customers accepted: 8909
customers served: 8909
turnaways: 0
average queue size: 0.27
average wait time: 0.18 minutes
customers accepted: 10292
customers served: 10292
turnaways: 0
average queue size: 0.33
average wait time: 0.19 minutes
customers accepted: 11830
customers served: 11830
turnaways: 0
average queue size: 0.41
average wait time: 0.21 minutes
customers accepted: 13347
customers served: 13347
turnaways: 0
average queue size: 0.48
average wait time: 0.21 minutes
customers accepted: 15033
customers served: 15033
turnaways: 0
average queue size: 0.57
average wait time: 0.23 minutes
customers accepted: 16837
customers served: 16836
turnaways: 0
average queue size: 0.67
average wait time: 0.24 minutes
customers accepted: 18717
customers served: 18716
turnaways: 0
average queue size: 0.79
average wait time: 0.25 minutes
customers accepted: 20775
customers served: 20774
turnaways: 0
average queue size: 0.91
average wait time: 0.26 minutes
customers accepted: 22874
customers served: 22873
turnaways: 0
average queue size: 1.05
average wait time: 0.28 minutes
customers accepted: 25038
customers served: 25037
turnaways: 0
average queue size: 1.20
average wait time: 0.29 minutes
customers accepted: 27395
customers served: 27394
turnaways: 0
average queue size: 1.37
average wait time: 0.30 minutes
customers accepted: 29826
customers served: 29825
turnaways: 0
average queue size: 1.54
average wait time: 0.31 minutes
customers accepted: 32313
customers served: 32312
turnaways: 0
average queue size: 1.73
average wait time: 0.32 minutes
customers accepted: 34782
customers served: 34781
turnaways: 0
average queue size: 1.90
average wait time: 0.33 minutes
customers accepted: 37513
customers served: 37511
turnaways: 0
average queue size: 2.12
average wait time: 0.34 minutes
customers accepted: 40250
customers served: 40248
turnaways: 0
average queue size: 2.33
average wait time: 0.35 minutes
customers accepted: 43151
customers served: 43149
turnaways: 0
average queue size: 2.56
average wait time: 0.36 minutes
customers accepted: 46116
customers served: 46114
turnaways: 0
average queue size: 2.80
average wait time: 0.36 minutes
customers accepted: 49234
customers served: 49232
turnaways: 0
average queue size: 3.07
average wait time: 0.37 minutes
customers accepted: 52400
customers served: 52398
turnaways: 0
average queue size: 3.35
average wait time: 0.38 minutes
customers accepted: 55651
customers served: 55648
turnaways: 0
average queue size: 3.65
average wait time: 0.39 minutes
customers accepted: 59012
customers served: 59008
turnaways: 0
average queue size: 3.97
average wait time: 0.40 minutes
customers accepted: 62468
customers served: 62464
turnaways: 0
average queue size: 4.29
average wait time: 0.41 minutes
customers accepted: 66015
customers served: 66010
turnaways: 0
average queue size: 4.62
average wait time: 0.42 minutes
customers accepted: 69696
customers served: 69691
turnaways: 0
average queue size: 4.97
average wait time: 0.43 minutes
customers accepted: 73509
customers served: 73503
turnaways: 0
average queue size: 5.38
average wait time: 0.44 minutes
customers accepted: 77483
customers served: 77476
turnaways: 0
average queue size: 5.81
average wait time: 0.45 minutes
customers accepted: 81552
customers served: 81544
turnaways: 0
average queue size: 6.24
average wait time: 0.46 minutes
customers accepted: 85676
customers served: 85668
turnaways: 0
average queue size: 6.70
average wait time: 0.47 minutes
customers accepted: 89893
customers served: 89885
turnaways: 0
average queue size: 7.18
average wait time: 0.48 minutes
customers accepted: 94185
customers served: 94177
turnaways: 0
average queue size: 7.68
average wait time: 0.49 minutes
customers accepted: 98572
customers served: 98564
turnaways: 0
average queue size: 8.22
average wait time: 0.50 minutes
customers accepted: 103067
customers served: 103058
turnaways: 0
average queue size: 8.78
average wait time: 0.51 minutes
customers accepted: 107697
customers served: 107688
turnaways: 0
average queue size: 9.38
average wait time: 0.52 minutes
customers accepted: 112391
customers served: 112382
turnaways: 0
average queue size: 9.98
average wait time: 0.53 minutes
customers accepted: 117190
customers served: 117181
turnaways: 0
average queue size: 10.67
average wait time: 0.55 minutes
customers accepted: 122085
customers served: 122075
turnaways: 0
average queue size: 11.43
average wait time: 0.56 minutes
customers accepted: 127075
customers served: 127065
turnaways: 0
average queue size: 12.17
average wait time: 0.57 minutes
customers accepted: 132223
customers served: 132212
turnaways: 0
average queue size: 13.06
average wait time: 0.59 minutes
customers accepted: 137410
customers served: 137398
turnaways: 0
average queue size: 14.04
average wait time: 0.61 minutes
customers accepted: 142710
customers served: 142697
turnaways: 0
average queue size: 15.11
average wait time: 0.63 minutes
customers accepted: 148126
customers served: 148112
turnaways: 0
average queue size: 16.21
average wait time: 0.66 minutes
customers accepted: 153631
customers served: 153617
turnaways: 0
average queue size: 17.51
average wait time: 0.68 minutes
customers accepted: 159249
customers served: 159234
turnaways: 0
average queue size: 19.50
average wait time: 0.73 minutes
customers accepted: 164952
customers served: 164936
turnaways: 0
average queue size: 22.05
average wait time: 0.80 minutes
customers accepted: 170746
customers served: 170720
turnaways: 0
average queue size: 24.75
average wait time: 0.87 minutes
customers accepted: 176635
customers served: 176603
turnaways: 11
average queue size: 29.79
average wait time: 1.01 minutes
When there comes 60.00 people per hour, the average wait time will be about 1 minute.
Done