C++Primer Plus 第十四章代码重用：编程练习,第5题
C++Primer Plus 第十四章代码重用：编程练习,第5题
前言
5.
下面是一些类声明:
//emp.h--header file for abstr emp class and children
#include <iostream>
#include <string>
class abstr_emp
private :
std::string fname;// abstr emp's first name//abstr emp's last name
std::string lname;
std::string job;
public:
abstr_emp();
abstr_emp(const std::string &fn,const std::string & lnconst std::string&j);
virtual void showAll()const;// labels and shows all data
virtual void SetA1l();// prompts user for values
friend std::ostream &operator<<(std::ostream &os,const abstremp&e);//just displays first and last name
virtual ~abstr_emp()=0;
//virtual base class
class employee:public abstr_emp
{
public :
employee();
employee(const std::string &fn,const std::string&ln,const std::string&j);
virtual void ShowAll()const;
virtual void SetA1l();
}
class manager:virtual public abstr_emp
{
private :
int inchargeof;//number of abstr emps manaqed
protected:
int InCharge0f()const(return inchargeof;)// output
int & InChargeof()return {inchargeof};// input
public :
manager();
manager(const std::string &fn,const std::string & ln,const std::string&j,int ico=0);
manager(constabstremp&e，intico);
manager(const manager&m);
virtual void showAll()const;
virtual void SetA1l();
}
class fink:virtual public abstr_emp
{
private:
//to whom fink reports
std::string reportsto;
protected:
const std::string ReportsTo()const {return reportsto};
std::string &ReportsTo() {return reportsto};
public :
fink();
fink(const std::string & fn,const std::string & ln,const std::string&j,const std::string &rpo);
fink(const abstremp &e,const std::string &rpo);
fink(const fink & e);
virtual void ShowAll()const;
virtual void SetAl1();
}
class highfink: public manager,public fink // management finkpublic:
{
highfink();
highfink(const std::string &fn,const std::string & lnconst std::string&j,const std::string&rpo,int ico);
highfink(const abstr emp &e,const std::string & rpo,int ico);
highfink(const fink & f，int ico);
highfink(const manager &m,const std::string & rpo);
highfink(const highfink & h);
virtual void ShowAll()const;
virtual void setAl1();
}
注意，该类层次结构使用了带虚基类的 M,所以要牢记这种情况下用于构造函数初始化列表的特殊规则。还需要注意的是，有些方法被声明为保护的。这可以简化一些highfnk方法的代码(例如，如果highfink::ShowAll( )只是调用 fink::ShowAll()和manager::ShwAll( )，则它将调用 abstr_emp::ShowAl( )两次)。请提供类方法的实现，并在一个程序中对这些类进行测试。下面是一个小型测试程序:
// pe14-5.cPp
// useemp1.cpp)-using the abstremp classes
#include <iostream>
using namespace std;
#include "emp.h"
int main(void)
{
employee em("Trip","Harris","Thumper" );
cout <<em< endl;
em.ShowAll();
manager ma("Amorphia","Spindragon"，"Nuancer"，5);
Cout <<ma <endl;
ma .ShowAl1();
fink fi("Matt","Oggs","Oiler","Juno Barr");
cout << fi << endl;
fi.showA11();
highfink hf(ma,"Curly Kew");//recruitment?
hf.showA11();
cout <<"Press a key for next phase:\n";
cin.get();
highfink hf2;
hf2.setA11();
cout <<"Using an abstr emp *pointer:\n";
abstr emp *tri[4]={&em, &fi,&hf, &hf2};
for(inti=0;i<4;i++)
tri[i]->showA11();
return 0;
}
- 为什么没有定义赋值运算符?
- 为什么要将ShowA11()和setA11()定义为虚的?
- 为什么要将 abstr_emp 定义为虚基类?
- 为什么 highfink类没有数据部分?
- 为什么只需要一个operator<<()版本?
- 如果使用下面的代码替换程序的结尾部分，将会发生什么情况?
abstr emp tri[4]={em,fi,hf,hf2);
for(inti=0;i<4;i++)
tri[i].ShowA11();
提示：以下是本篇文章正文内容，下面案例可供参考
一、方法
#include <iostream>
#include <limits>
#include <string>
using namespace std;
// ========================================
class abstr_emp
{
private:
std::string fname; // abstr_emp's first name
std::string lname; // abstr_emp's last name
std::string job;
public:
//abstr_emp();
explicit abstr_emp(const std::string & fn = "", const std::string & ln = "", const std::string & j = "");
virtual void ShowAll() const; // labels and shows all data
virtual void SetAll(); // prompts user for values
friend std::ostream & operator<<(std::ostream & os, const abstr_emp & e); // just displays first and last name
virtual ~abstr_emp() = 0; // virtual base class
};
abstr_emp::abstr_emp (const std::string & fn, const std::string & ln, const std::string & j)
: fname(fn), lname(ln), job(j)
{
;
}
abstr_emp::~abstr_emp ()
{
;
}
void
abstr_emp::ShowAll () const
{
cout << "name: " << fname << ' ' << lname << '\n'
<< "job: " << job << endl;
}
void
abstr_emp::SetAll ()
{
cout << "enter first name: ";
cin >> fname;
cin.clear(); // 清空输入缓冲区错误标志位
cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n'); // 清空输入缓冲区内容
cout << "enter second name: ";
cin >> lname;
cin.clear(); // 清空输入缓冲区错误标志位
cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n'); // 清空输入缓冲区内容
cout << "enter job: ";
cin >> job;
cin.clear(); // 清空输入缓冲区错误标志位
cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n'); // 清空输入缓冲区内容
}
std::ostream &
operator<< (std::ostream & os, const abstr_emp & e)
{
e.ShowAll();
return (os);
}
// ========================================
class employee : public abstr_emp
{
public:
//employee();
explicit employee(const std::string & fn = "", const std::string & ln = "", const std::string & j = "");
virtual void ShowAll() const;
virtual void SetAll();
};
employee::employee (const std::string & fn, const std::string & ln, const std::string & j)
: abstr_emp(fn, ln, j)
{
;
}
void
employee::ShowAll () const
{
abstr_emp::ShowAll();
}
void
employee::SetAll ()
{
abstr_emp::SetAll();
}
// ========================================
class manager: virtual public abstr_emp
{
private:
int inchargeof; // number of abstr_emps managed
protected:
int InChargeOf() const { return inchargeof; } // output
int & InChargeOf(){ return inchargeof; } // input
public:
//manager();
explicit manager(const std::string & fn = "", const std::string & ln = "", const std::string & j = "", int ico = 0);
manager(const abstr_emp & e, int ico);
//manager(const manager & m);
virtual void ShowAll() const;
virtual void SetAll();
};
manager::manager (const std::string & fn, const std::string & ln, const std::string & j, int ico)
: abstr_emp(fn, ln, j), inchargeof(ico)
{
;
}
manager::manager (const abstr_emp & e, int ico)
: abstr_emp(e), inchargeof(ico)
{
;
}
void
manager::ShowAll () const
{
abstr_emp::ShowAll();
cout << "incharge of " << inchargeof << " employees" << endl;
}
void
manager::SetAll ()
{
abstr_emp::SetAll();
cout << "enter the number of managed: ";
cin >> inchargeof;
cin.clear(); // 清空输入缓冲区错误标志位
cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n'); // 清空输入缓冲区内容
}
// ========================================
class fink: virtual public abstr_emp
{
private:
std::string reportsto; // to whom fink reports
protected:
const std::string ReportsTo() const { return reportsto; }
std::string & ReportsTo(){ return reportsto; }
public:
//fink();
fink(const std::string & fn = "", const std::string & ln = "", const std::string & j = "", const std::string & rpo = "");
fink(const abstr_emp & e, const std::string & rpo);
//fink(const fink & e);
virtual void ShowAll() const;
virtual void SetAll();
};
fink::fink (const std::string & fn, const std::string & ln, const std::string & j, const std::string & rpo)
: abstr_emp(fn, ln, j), reportsto(rpo)
{
;
}
fink::fink (const abstr_emp & e, const std::string & rpo)
: abstr_emp(e), reportsto(rpo)
{
;
}
void
fink::ShowAll () const
{
abstr_emp::ShowAll();
cout << "report to " << reportsto << endl;
}
void
fink::SetAll ()
{
abstr_emp::SetAll();
cout << "enter to whom fink reports: ";
cin >> reportsto;
cin.clear(); // 清空输入缓冲区错误标志位
cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n'); // 清空输入缓冲区内容
}
// ========================================
class highfink: public manager, public fink // management fink
{
public:
//highfink();
explicit highfink(const std::string & fn = "", const std::string & ln = "", const std::string & j = "", const std::string & rpo = "", int ico = 0);
//highfink(const abstr_emp & e, const std::string & rpo, int ico);
highfink(const fink & f, int ico);
highfink(const manager & m, const std::string & rpo);
//highfink(const highfink & h);
virtual void ShowAll() const;
virtual void SetAll();
};
highfink::highfink (const std::string & fn, const std::string & ln, const std::string & j, const std::string & rpo, int ico)
: abstr_emp(fn, ln, j), manager(fn, ln, j, ico), fink(fn, ln, j, rpo)
{
;
}
highfink::highfink(const fink & f, int ico)
: abstr_emp(f), manager(f, ico), fink(f)
{
;
}
highfink::highfink (const manager & m, const std::string & rpo)
: abstr_emp(m), manager(m), fink(m, rpo)
{
;
}
void
highfink::ShowAll() const
{
manager::ShowAll();
cout << "report to " << ReportsTo() << endl;
}
void
highfink::SetAll()
{
manager::SetAll();
cout << "enter to whom fink reports: ";
cin >> ReportsTo();
cin.clear(); // 清空输入缓冲区错误标志位
cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n'); // 清空输入缓冲区内容
}
// ========================================
int main(void)
{
employee em("Trip", "Harris", "Thumper");
cout << em << endl;
//em.ShowAll();
manager ma("Amorphia", "Spindragon", "Nuancer", 5);
cout << ma << endl;
//ma.ShowAll();
fink fi("Matt", "Oggs", "Oiler", "Juno Barr");
cout << fi << endl;
//fi.ShowAll();
highfink hf(ma, "Curly Kew"); // recruitment?
cout << hf << endl;
//hf.ShowAll();
cout << endl;
cout << "Press a key for next phase:\n";
cin.get();
highfink hf2;
hf2.SetAll();
cout << endl;
cout << "Using an abstr_emp * pointer:\n";
abstr_emp * tri[4] = {&em, &fi, &hf, &hf2};
for (int i = 0; i < 4; i++)
tri[i]->ShowAll();
return 0;
}
二、解答
Why is no assignment operator defined?
——abstr_emp、employee、manager、fink、highfink 等五个类内部并未涉及动态内存分配，并非深拷贝，因此，无须定义赋值运算符、拷贝构造函数、析构函数。
Why are ShowAll() and SetAll() virtual?
为什么要将 ShowAll() 和 SetAll() 定义为虚的？
——各级派生类均有各自新增的数据成员，显示、设置新增数据成员必须由派生类自行负责，无法直接复用基类的 ShowAll() 和 SetAll() 成员函数，在这两类操作上，派生类需要体现出类的多态特性。
Why is abstr_emp a virtual base class?
——若 abstr_emp 按一般方式派生出 manager、fink，而 manager、fink 派生出 highfink，highfink 将包含两份 abstr_emp 的数据成员，必须将 abstr_emp 通过 virtual 方式派生出 manager、fink，才能正常地只包含一份 abstr_emp 的数据成员。
Why does the highfink class have no data section?
——highfink 通过 manager、fink 多重继承而来，无须新增数据成员。
Why is only one version of operator<<() needed?
——friend std::ostream & operator<<(std::ostream & os, const abstr_emp & e) 的形参是基类引用，各派生类实现了各自的 virtual ShowAll() 函数，因此，operator<<() 在内部通过多态特性调用基类和派生类各自的 ShowAll() 实现输出。但是，书上的原型声明 virtual void ShowAll() const 有误，应改为 virtual std::ostream& ShowAll(std::ostream& os) const;
What would happen if the end of the program were replaced with this code?
abstr_emp tri[4] = {em, fi, hf, hf2};
for (int i = 0; i < 4; i++)
tri[i].ShowAll();
——语法错误，由于 abstr_emp 中含有纯虚成员函数，所以 abstr_emp 为抽象类，抽象类无法创建对象。
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