第五章  静态成员与友元

一、填空题

    1、一个类的头文件如下所示，num初始化值为5，程序产生对象T，且修改num为10，并使用show()函数输出num的值10。

#include <iostream.h>

class Test

{ private:

static int num;

public:

Test(int);

void show();

};

__int test::num = 5;_______

Test::Test(int n)

{ num=n; }

void Test::show()

{ cout<<num<<endl; }

void main()

{ Test t(10);

__t.show();_______

}

  2 、在下面程序的底画线处填上适当的字句，使该程序执行结果为40。

#include <iostream>

class Test

{ public:

____static int x__;

Test (int i=0)

{x=i+x;}

int Getnum()

{return Test::x+7;}

};

Int test::x=33_______;

void main()

{Test test;

cout<<test.Getnum()<<endl;

}

3 、下列程序运行的结果是___________

     #include <iostream.h>

#include <string.h>

#include <iomanip.h>

class student

{char name［8］;

int deg;

char level［7］;

friend class process; // 说明友元类

public:

student(char na［］,int d)

{ strcpy(name,na);

deg=d;

}

};

class process

{ public:

void trans(student &s)

{int i=s.deg/10;

switch(i)

{case 9:

strcpy(s.level, "优");break;

case 8:

strcpy(s.level,"良");break;

case 7:

strcpy(s.level,"中");break;

case 6:

strcpy(s.level,"及格");break;

default:

strcpy(s.level,"不及格");

}

}

void show(student &s)

{cout<<setw(10)<<s.name<<setw(4)<<s.deg<<setw(8)<<s.level<<endl;}

};

void main()

{ student st［］={student("张三",78),student("李四",92),student("王五

",62),student("孙六",88)};

process p;

cout<<"结 果:"<<"姓名"<<setw(6)<<"成绩"<<setw(8)<<"等级"<<endl;

for(int i=0;i<4;i++)

{ p.trans(st［i］);

p.show(st［i］);}

}

 姓名   成绩   等级

 张三   78     中

 李四   92     优

 王五   62     及格

 孙六   88     良

二、编程题

1.

 (1) 编写一个类，声明一个数据成员和一个静态数据成员。让构造函数初始化数据成员，并把静态数据成员加1，让析构函数把静态数据成员减1。

 (2) 根据(1)编写一个应用程序，创建三个对象，然后显示它们的数据成员和静态数据成员，再析构每个对象，并显示它们对静态数据成员的影响。

 (3) 修改(2)，让静态成员函数访问静态数据成员，并让静态数据成员是保户的。

#include <iostream>using namespace std;class AAA
{friend int static_re();
private:int a;
protected:static int b;
public:AAA(int a):a(a){b++;}~AAA(){b--;cout << "b=" << b << endl;}int get_int(){return a;}static int get_static(){return b;}
};int static_re()
{AAA::b=100;cout << "B:" << AAA::b << endl;return AAA::b;
}int AAA::b=0;int main()
{AAA a1(10),a2(20),a3(30);cout << "a1_int=" << a1.get_int() << endl;cout << "a2_int=" << a2.get_int() << endl;cout << "a3_int=" << a3.get_int() << endl;cout << "static=" << a1.get_static() << endl;static_re();return 0;
}

2.

 (1) 下述代码有何错误，改正它。

#include < iostream> using namespace std; class Animal; void SetValue(Animal&, int); void SetValue(Animal&, int, int); class Animal { public:     friend void SetValue(Animal&, int); protected: int itsWeight; int itsAge; }; void SetValue(Animal& ta, int tw) {     a.itsWeight = tw;   ---》ta.itsWeight=tw; } void SetValue(Animal& ta, int tw, int tn) {     ta.itsWeight = tw;     ta.itsAge = tn; } int main() {     Animal peppy;     SetValue(peppy, 5);     SetValue(peppy, 7,9);   return 0; }

 (2) 将上面程序中的友员改成普通函数，为此增加访问类中保护数据的成员函数。

#include <iostream>class Animal {
public:void SetValue(int);void SetValue(int, int);protected:int itsWeight;int itsAge;};void Animal::SetValue(int tw) {itsWeight = tw;}void Animal::SetValue(int tw, int tn) {itsWeight = tw;itsAge = tn;}int main() {Animal peppy;peppy.SetValue(5);peppy.SetValue(7, 9);return 0;}

3. 重新编写下述程序，使函数Leisure()成为类Car和类Boat的函数。作为重新编程，在类Car和类Boat中，增加函数set()。

#include < iostream> using namespace std; class Boat; class Car { public:     Car(int j){size = j;}     friend int Leisure(int time, Car& aobj, Boat& bobi); protected:     int size; class Boat { public:     Boat(int j){size = j;}     friend int Leisure(int time, Car& aobj, Boat& bobj); protected:     int size; }; int Leisure(int time, Car& aobj, Boat& bobi) {     return time * aobj.size *bobi.size; } ｝; int main() {     Car c1(2);     Boat bl(3);     int time = 4;     cout << Leisure(time, cl,bl);   return 0; }

 

 #include <iostream>
using namespace std;class Boat;class Car {
public:Car(int j) { size = j; }int Leisure(int time, Boat& bobj);void set(int j) { size = j; }private:int size;
};class Boat {
public:Boat(int j) { size = j; }int Leisure(int time, Car& aobj);void set(int j) { size = j; }private:int size;
};int Car::Leisure(int time, Boat& bobj) {return time * size * bobj.size;
}int Boat::Leisure(int time, Car& aobj) {return time * size * aobj.size;
}int main() {Car c1(2);Boat bl(3);int time = 4;cout << c1.Leisure(time, bl) << endl;return 0;
}