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:
_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］);}
}
结 果: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) 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) { 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; } |
void SetValue(Animal& ta, int tw, int tn)无法访问类中被保护的成员，应在类中加上friend void SetValue(Animal& ta, int tw, int tn)；
(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。
void AnimalSetValue(Animal& ta, int tw)
{
ta.itsWeight = tw;
}
- 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()。
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| #include < iostream> using namespace std; class Boat; class Car { public: Car(int j){size = j;} void set(int j){size=j;} friend int Leisure(int time, Car& aobj, Boat& bobi); protected: int size; class Boat { public: Boat(int j){size = j;} void set(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; } |