习题选自：C Primer Plus(第六版) 内容仅供参考，如有错误，欢迎指正 ! 友元类和嵌套类 RTTI和类型转换运算符
复习题
1. 下面建立友元的尝试有什么错误？
a.
代码语言：javascript复制class snap {
friend clasp;
...
};
class clasp { ... };
b.
代码语言：javascript复制class cuff {
public:
void snip(muff &) { ... }
...
};
class muff {
friend void cuff::snip(muff &);
...
};
c.
代码语言：javascript复制class muff {
friend void cuff::snip(muff &);
...
};
class cuff {
public:
void snip(muff &) { ... }
...
};
a. friend clasp;缺少class声明，应该改为friend class clasp;。
b. 缺少muff的前项声明，应该在第一行添加class muff。
c. cuff类声明应在muff类之前，以便编译器可以理解cuff::snip( )。同时编译器需要muff的一个前向声明，以便可以理解snip(muff &)。修改后应该为：
class muff; // forward declaration
class cuff {
public:
void snip(muff &) { ... }
...
};
class muff {
friend void cuff::snip(muff &);
...
};
2. 您知道了如何建立相互类友元的方法。能够创建一种更为严格的友情关系，即类B只有部分成员是类A的友元，而类A只有部分成员是类B的友元吗？请解释原因。
不能，为了使类B部分成员是类A的友元，需要将类B的声明位于类A的前面，并且要在类A指出类B中要作为类A友元的成员。同样的使类A部分成员成为类A的友元，需要同样的要求，而这两个要求是互斥的，因此无法创建该友情关系。
3. 下面的嵌套类声明中可能存在什么问题？
代码语言：javascript复制class Ribs
{
private:
class Sauce
{
int soy;
int sugar;
public:
Sauce(int s1, int s2) : soy(s1), sugar(s2) { }
};
...
};
嵌套类Sauce中的soy和sugar都是私有的，Ribs只能通过Sauce的构造函数创建它们。
4. throw和return之间的区别何在？
假设函数f1( )调用函数f2( )。f2( )中的返回语句导致程序执行在函数f1( )中调用函数f2( )后面的一条语句。throw语句导致程序沿函数调用的当前序列回溯，直到找到直接或间接包含对f2( )的调用的try语句块为止。它可能在f1( )中、调用f1( )的函数中或其他函数中。找到这样的try语句块后，将执行下一个匹配的catch语句块，而不是函数调用后的语句。
5. 假设有一个从异常基类派生来的异常类层次结构，则应按什么样的顺序放置catch块？
应按从子孙到祖先的顺序排列catch语句块。
6. 对于本章定义的Grand、Superb和Magnificent类，假设pg为Grand *指针，并将其中某个类的对象的地址赋给了它，而ps为Superb *指针，则下面两个代码示例的行为有什么不同？
if (ps = dynamic_cast<Superb *>(pg))
ps->say(); // sample #1
if (typeid(*pg) == typeid(Superb))
(Superb *) pg)->say(); // sample #2
对于dynamic_cast<Superb *>(pg)，如果pg指向一个Superb对象或从Superb派生而来的任何类的对象，则if条件为true。具体地说，如果pg指向Magnificent对象，则if条件也为true。
对于typeid(*pg) == typeid(Superb)，仅当指向Superb对象时，if条件才为true，如果指向的是从Superb派生出来的对象，则if条件不为true。
7. static_cast运算符与dynamic_cast运算符有什么不同？
dynamic_cast运算符只允许沿类层次结构向上转换，而static_cast运算符允许向上转换和向下转换。static_cast运算符还允许枚
举类型和整型之间以及数值类型之间的转换。
编程练习
1. 对Tv和Remote类做如下修改：
- a. 让它们互为友元；
- b. 在`Remote`类中添加一个状态变量成员，该成员描述遥控器是处于常规模式还是互动模式；
- c. 在`Remote`中添加一个显示模式的方法；
- d. 在Tv类中添加一个对`Remote`中新成员进行切换的方法，该方法应仅当`TV`处于打开状态时才能运行。
编写一个小程序来测试这些新特性。
tv.h:
代码语言：javascript复制#ifndef TV_H_
#define TV_H_
#include <iostream>
class Remote; // forward declaration
class Tv {
public:
friend class Remote;
enum { Off, On };
enum { MinVal, MaxVal = 20 };
enum { Antenna, Cable };
enum { TV, DVD };
private:
int state;
int volume;
int maxchannel;
int channel;
int mode;
int input;
public:
Tv(int s = Off, int mc = 125)
: state(s),
volume(5),
maxchannel(mc),
channel(2),
mode(Cable),
input(DVD) {}
void onoff() { state = (state == On) ? Off : On; }
bool ison() const { return state == On; }
bool volup();
bool voldown();
void chanup();
void chandown();
void set_mode() { mode = (mode == Antenna) ? Cable : Antenna; }
void set_input() { input = (input == TV) ? DVD : TV; }
void show_settings() const;
void set_remote_mode(Remote& r) const;
};
class Remote {
public:
friend class Tv;
enum { Normal, Interactive };
private:
int mode;
int gmode;
public:
Remote(int m = Tv::TV) : mode(m), gmode(Normal) {}
bool volup(Tv& t) { return t.volup(); }
bool voldown(Tv& t) { return t.voldown(); }
void onoff(Tv& t) { t.onoff(); }
void chanup(Tv& t) { t.chanup(); }
void chandown(Tv& t) { t.chandown(); }
void set_chan(Tv& t, int c) { t.channel = c; }
void set_mode(Tv& t) { t.set_mode(); }
void set_input(Tv& t) { t.set_input(); }
void show_remote_mode() const;
void set_remote_mode();
};
inline void Remote::show_remote_mode() const {
std::cout << (gmode == Normal ? "Normal" : "Interactive") << std::endl;
}
inline void Remote::set_remote_mode() {
gmode = (gmode == Normal) ? Interactive : Normal;
}
#endif // TV_H_
tv.cpp:
代码语言：javascript复制#include "tv.h"
bool Tv::volup() {
if (volume < MaxVal) {
volume ;
return true;
} else
return false;
}
bool Tv::voldown() {
if (volume > MinVal) {
volume--;
return true;
} else
return false;
}
void Tv::chanup() {
if (channel < maxchannel)
channel ;
else
channel = 1;
}
void Tv::chandown() {
if (channel > 1)
channel--;
else
channel = maxchannel;
}
void Tv::show_settings() const {
std::cout << "TV is " << (state == On ? "On" : "Off") << std::endl;
if (state == On) {
std::cout << "Volume setting = " << volume << std::endl;
std::cout << "Channel setting = " << channel << std::endl;
std::cout << "Mode = " << (mode == Antenna ? "Antenna" : "Cable")
<< std::endl;
std::cout << "Input = " << (input == TV ? "TV" : "DVD") << std::endl;
}
}
void Tv::set_remote_mode(Remote& r) const {
if (state == On) r.set_remote_mode();
}
main.cpp:
代码语言：javascript复制#include <iostream>
#include "tv.h"
int main() {
Tv t;
t.show_settings();
Remote r;
r.show_remote_mode();
r.onoff(t);
t.show_settings();
r.voldown(t);
t.show_settings();
t.set_remote_mode(r);
r.show_remote_mode();
return 0;
}
2. 修改程序清单15.11，使两种异常类型都是从头文件<stdexcept>提供的logic_error类派生出来的类。让每个what()方法都报告函数名和问题的性质。异常对象不用存储错误的参数值，而只需支持what()方法。
exc_mean.h:
代码语言：javascript复制#ifndef EXC_MEAN_H_
#define EXC_MEAN_H_
#include <stdexcept>
class bad_hmean : public std::logic_error {
public:
bad_hmean() : std::logic_error("hmean() invalid arguments: a = -bn") {}
};
class bad_gmean : public std::logic_error {
public:
bad_gmean() : std::logic_error("gmean() arguments should be >= 0n") {}
};
#endif // EXC_MEAN_H_
error4.cpp:
代码语言：javascript复制// error4.cpp ?using exception classes
#include <cmath> // or math.h, unix users may need -lm flag
#include <iostream>
#include "exc_mean.h"
// function prototypes
double hmean(double a, double b);
double gmean(double a, double b);
int main() {
using std::cin;
using std::cout;
using std::endl;
double x, y, z;
cout << "Enter two numbers: ";
while (cin >> x >> y) {
try { // start of try block
z = hmean(x, y);
cout << "Harmonic mean of " << x << " and " << y << " is " << z << endl;
cout << "Geometric mean of " << x << " and " << y << " is " << gmean(x, y)
<< endl;
cout << "Enter next set of numbers <q to quit>: ";
} // end of try block
catch (bad_hmean& he) // start of catch block
{
cout << he.what();
cout << "Try again.n";
continue;
} catch (bad_gmean& ge) {
cout << ge.what();
cout << "Sorry, you don't get to play any more.n";
break;
} // end of catch block
}
cout << "Bye!n";
return 0;
}
double hmean(double a, double b) {
if (a == -b) throw bad_hmean();
return 2.0 * a * b / (a b);
}
double gmean(double a, double b) {
if (a < 0 || b < 0) throw bad_gmean();
return std::sqrt(a * b);
}
3. 这个练习与编程练习2相同，但异常类是从一个这样的基类派生而来的：它是从logic_error派生而来的，并存储两个参数值。异常类应该有一个这样的方法：报告这些值以及函数名。程序使用一个catch块来捕获基类异常，其中任何一种从该基类异常派生而来的异常都将导致循环结束。
exc_mean.h:
代码语言：javascript复制#ifndef EXC_MEAN_H_
#define EXC_MEAN_H_
#include <iostream>
#include <stdexcept>
#include <string>
class bad_base : public std::logic_error {
private:
double v1;
double v2;
std::string fun_name;
public:
bad_base(double a = 0, double b = 0, std::string name = "none")
: std::logic_error("hmean() invalid arguments: a = -bn"),
v1(a),
v2(b),
fun_name(name) {}
void mesg() {
std::cout << v1 << ", " << v2 << ", " << fun_name << std::endl;
}
};
class bad_hmean : public bad_base {
public:
bad_hmean(double a = 0, double b = 0, std::string name = "none")
: bad_base(a, b, name) {}
};
class bad_gmean : public bad_base {
public:
bad_gmean(double a = 0, double b = 0, std::string name = "none")
: bad_base(a, b, name) {}
};
#endif // EXC_MEAN_H_
error4.cpp:
代码语言：javascript复制// error4.cpp ?using exception classes
#include <cmath> // or math.h, unix users may need -lm flag
#include <iostream>
#include "exc_mean.h"
// function prototypes
double hmean(double a, double b);
double gmean(double a, double b);
int main() {
using std::cin;
using std::cout;
using std::endl;
double x, y, z;
cout << "Enter two numbers: ";
while (cin >> x >> y) {
try { // start of try block
z = hmean(x, y);
cout << "Harmonic mean of " << x << " and " << y << " is " << z << endl;
cout << "Geometric mean of " << x << " and " << y << " is " << gmean(x, y)
<< endl;
cout << "Enter next set of numbers <q to quit>: ";
} // end of try block
catch (bad_hmean& he) // start of catch block
{
he.mesg();
cout << "Try again.n";
continue;
} catch (bad_gmean& ge) {
ge.mesg();
cout << "Sorry, you don't get to play any more.n";
break;
} // end of catch block
catch (bad_base& ge) {
ge.mesg();
cout << "Sorry, you don't get to play any more.n";
break;
} // end of catch block
}
cout << "Bye!n";
return 0;
}
double hmean(double a, double b) {
if (a == -b) throw bad_hmean(a, b, "hmean()");
return 2.0 * a * b / (a b);
}
double gmean(double a, double b) {
if (a < 0 || b < 0) throw bad_gmean(a, b, "gmean()");
return std::sqrt(a * b);
}
4. 程序清单15.16在每个try后面都使用两个catch块，以确保nbad_index异常导致方法label_val( )被调用。请修改该程序，在每个try块后面只使用一个catch块，并使用RTTI来确保合适时调用label_val( )。
sales.h:
代码语言：javascript复制#ifndef SALES_H_
#define SALES_H_
// sales.h -- exceptions and inheritance
#include <stdexcept>
#include <string>
class Sales {
public:
enum { MONTHS = 12 }; // could be a static const
class bad_index : public std::logic_error {
private:
int bi; // bad index value
public:
explicit bad_index(int ix,
const std::string& s = "Index error in Sales objectn");
int bi_val() const { return bi; }
virtual ~bad_index() throw() {}
};
explicit Sales(int yy = 0);
Sales(int yy, const double* gr, int n);
virtual ~Sales() {}
int Year() const { return year; }
virtual double operator[](int i) const;
virtual double& operator[](int i);
private:
double gross[MONTHS];
int year;
};
class LabeledSales : public Sales {
public:
class nbad_index : public Sales::bad_index {
private:
std::string lbl;
public:
nbad_index(const std::string& lb, int ix,
const std::string& s = "Index error in LabeledSales objectn");
const std::string& label_val() const { return lbl; }
virtual ~nbad_index() throw() {}
};
explicit LabeledSales(const std::string& lb = "none", int yy = 0);
LabeledSales(const std::string& lb, int yy, const double* gr, int n);
virtual ~LabeledSales() {}
const std::string& Label() const { return label; }
virtual double operator[](int i) const;
virtual double& operator[](int i);
private:
std::string label;
};
#endif // SALES_H_
sales.cpp:
代码语言：javascript复制// sales.cpp -- Sales implementation
#include "sales.h"
using std::string;
Sales::bad_index::bad_index(int ix, const string& s)
: std::logic_error(s), bi(ix) {}
Sales::Sales(int yy) {
year = yy;
for (int i = 0; i < MONTHS; i) gross[i] = 0;
}
Sales::Sales(int yy, const double* gr, int n) {
year = yy;
int lim = (n < MONTHS) ? n : MONTHS;
int i;
for (i = 0; i < lim; i) gross[i] = gr[i];
// for i > n and i < MONTHS
for (; i < MONTHS; i) gross[i] = 0;
}
double Sales::operator[](int i) const {
if (i < 0 || i >= MONTHS) throw bad_index(i);
return gross[i];
}
double& Sales::operator[](int i) {
if (i < 0 || i >= MONTHS) throw bad_index(i);
return gross[i];
}
LabeledSales::nbad_index::nbad_index(const string& lb, int ix, const string& s)
: Sales::bad_index(ix, s) {
lbl = lb;
}
LabeledSales::LabeledSales(const string& lb, int yy) : Sales(yy) { label = lb; }
LabeledSales::LabeledSales(const string& lb, int yy, const double* gr, int n)
: Sales(yy, gr, n) {
label = lb;
}
double LabeledSales::operator[](int i) const {
if (i < 0 || i >= MONTHS) throw nbad_index(Label(), i);
return Sales::operator[](i);
}
double& LabeledSales::operator[](int i) {
if (i < 0 || i >= MONTHS) throw nbad_index(Label(), i);
return Sales::operator[](i);
}
use_sales.cpp:
代码语言：javascript复制// use_sales.cpp -- nested exceptions
#include <iostream>
#include <typeinfo>
#include "sales.h"
int main() {
using std::cin;
using std::cout;
using std::endl;
double vals1[12] = {1220, 1100, 1122, 2212, 1232, 2334,
2884, 2393, 3302, 2922, 3002, 3544};
double vals2[12] = {12, 11, 22, 21, 32, 34, 28, 29, 33, 29, 32, 35};
Sales sales1(2011, vals1, 12);
LabeledSales sales2("Blogstar", 2012, vals2, 12);
cout << "First try block:n";
try {
int i;
cout << "Year = " << sales1.Year() << endl;
for (i = 0; i < 12; i) {
cout << sales1[i] << ' ';
if (i % 6 == 5) cout << endl;
}
cout << "Year = " << sales2.Year() << endl;
cout << "Label = " << sales2.Label() << endl;
for (i = 0; i <= 12; i) {
cout << sales2[i] << ' ';
if (i % 6 == 5) cout << endl;
}
cout << "End of try block 1.n";
} catch (Sales::bad_index &bad) {
cout << bad.what();
if (typeid(bad) == typeid(LabeledSales::nbad_index)) {
cout << "Company: " << ((LabeledSales::nbad_index &)bad).label_val()
<< endl;
}
cout << "bad index: " << bad.bi_val() << endl;
}
cout << "nNext try block:n";
try {
sales2[2] = 37.5;
sales1[20] = 23345;
cout << "End of try block 2.n";
} catch (Sales::bad_index &bad) {
cout << bad.what();
if (typeid(bad) == typeid(LabeledSales::nbad_index)) {
cout << "Company: " << ((LabeledSales::nbad_index &)bad).label_val()
<< endl;
}
cout << "bad index: " << bad.bi_val() << endl;
}
cout << "donen";
return 0;
}
