实例详解C++友元
2007-04-06 15:03
585 查看
在说明什么是友元之前,我们先说明一下为什么需要友元与友元的缺点:
通常对于普通函数来说,要访问类的保护成员是不可能的,如果想这么做那么必须把类的成员都生命成为public(共用的),然而这做带来的问题遍是任何外部函数都可以毫无约束的访问它操作它,c++利用friend修饰符,可以让一些你设定的函数能够对这些保护数据进行操作,避免把类成员全部设置成public,最大限度的保护数据成员的安全。友元能够使得普通函数直接访问类的保护数据,避免了类成员函数的频繁调用,可以节约处理器开销,提高程序的效率,但矛盾的是,即使是最大限度的保护,同样也破坏了类的封装特性,这即是友元的缺点,在现在cpu速度越来越快的今天我们并不推荐使用它,但它作为c++一个必要的知识点,一个完整的组成部分,我们还是需要讨论一下的。
在类里声明一个普通函数,在前面加上friend修饰,那么这个函数就成了该类的友元,可以访问该类的一切成员。 .
下面我们来看一段代码,看看我们是如何利用友元来访问类的一切成员的。
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
#include <iostream>
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
using namespace std;
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
class Internet
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockStart.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ContractedBlock.gif)
...{
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
public:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
Internet(char *name,char *address)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedSubBlockStart.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ContractedSubBlock.gif)
...{
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
strcpy(Internet::name,name);
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
strcpy(Internet::address,address);
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedSubBlockEnd.gif)
}
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
friend void ShowN(Internet &obj);//友元函数的声明
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
public:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
char name[20];
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
char address[20];
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockEnd.gif)
};
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
void ShowN(Internet &obj)//函数定义,不能写成,void Internet::ShowN(Internet &obj)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockStart.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ContractedBlock.gif)
...{
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
cout<<obj.name<<endl;
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockEnd.gif)
}
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
void main()
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockStart.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ContractedBlock.gif)
...{
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
Internet a("中国软件开发实验室","www.cndev-lab.com");
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
ShowN(a);
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
cin.get();
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockEnd.gif)
}
上面的代码通过友元函数的定义,我们成功的访问到了a对象的保护成员name,友元函数并不能看做是类的成员函数,它只是个被声明为类友元的普通函数,所以在类外部函数的定义部分不能够写成void Internet::ShowN(Internet &obj),这一点要注意。
一个普通函数可以是多个类的友元函数,对上面的代码我们进行修改,注意观察变化:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
#include <iostream>
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
using namespace std;
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
class Country;
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
class Internet
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockStart.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ContractedBlock.gif)
...{
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
public:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
Internet(char *name,char *address)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedSubBlockStart.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ContractedSubBlock.gif)
...{
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
strcpy(Internet::name,name);
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
strcpy(Internet::address,address);
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedSubBlockEnd.gif)
}
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
friend void ShowN(Internet &obj,Country &cn);//注意这里
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
public:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
char name[20];
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
char address[20];
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockEnd.gif)
};
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
class Country
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockStart.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ContractedBlock.gif)
...{
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
public:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
Country()
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedSubBlockStart.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ContractedSubBlock.gif)
...{
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
strcpy(cname,"中国");
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedSubBlockEnd.gif)
}
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
friend void ShowN(Internet &obj,Country &cn);//注意这里
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
protected:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
char cname[30];
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockEnd.gif)
};
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
void ShowN(Internet &obj,Country &cn)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockStart.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ContractedBlock.gif)
...{
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
cout<<cn.cname<<"|"<<obj.name<<endl;
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockEnd.gif)
}
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
void main()
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockStart.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ContractedBlock.gif)
...{
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
Internet a("中国软件开发实验室","www.cndev-lab.com");
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
Country b;
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
ShowN(a,b);
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
cin.get();
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockEnd.gif)
}
一个类的成员函数函数也可以是另一个类的友元,从而可以使得一个类的成员函数可以操作另一个类的数据成员,我们在下面的代码中增加一类Country,注意观察:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
#include <iostream>
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
using namespace std;
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
class Internet;
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
class Country
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockStart.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ContractedBlock.gif)
...{
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
public:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
Country()
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedSubBlockStart.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ContractedSubBlock.gif)
...{
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
strcpy(cname,"中国");
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedSubBlockEnd.gif)
}
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
void Editurl(Internet &temp);//成员函数的声明
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
protected:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
char cname[30];
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockEnd.gif)
};
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
class Internet
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockStart.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ContractedBlock.gif)
...{
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
public:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
Internet(char *name,char *address)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedSubBlockStart.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ContractedSubBlock.gif)
...{
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
strcpy(Internet::name,name);
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
strcpy(Internet::address,address);
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedSubBlockEnd.gif)
}
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
friend void Country::Editurl(Internet &temp);//友元函数的声明
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
protected:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
char name[20];
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
char address[20];
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockEnd.gif)
};
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
void Country::Editurl(Internet &temp)//成员函数的外部定义
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockStart.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ContractedBlock.gif)
...{
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
strcpy(temp.address,"edu.cndev-lab.com");
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
cout<<temp.name<<"|"<<temp.address<<endl;
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockEnd.gif)
}
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
void main()
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockStart.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ContractedBlock.gif)
...{
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
Internet a("中国软件开发实验室","www.cndev-lab.com");
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
Country b;
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
b.Editurl(a);
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
cin.get();
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockEnd.gif)
}
整个类也可以是另一个类的友元,该友元也可以称做为友类。友类的每个成员函数都可以访问另一个类的所有成员。
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
#include <iostream>
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
using namespace std;
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
class Internet;
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
class Country
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockStart.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ContractedBlock.gif)
...{
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
public:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
Country()
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedSubBlockStart.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ContractedSubBlock.gif)
...{
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
strcpy(cname,"中国");
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedSubBlockEnd.gif)
}
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
friend class Internet;//友类的声明
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
protected:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
char cname[30];
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockEnd.gif)
};
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
class Internet
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockStart.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ContractedBlock.gif)
...{
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
public:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
Internet(char *name,char *address)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedSubBlockStart.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ContractedSubBlock.gif)
...{
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
strcpy(Internet::name,name);
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
strcpy(Internet::address,address);
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedSubBlockEnd.gif)
}
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
void Editcname(Country &temp);
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
protected:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
char name[20];
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
char address[20];
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockEnd.gif)
};
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
void Internet::Editcname(Country &temp)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockStart.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ContractedBlock.gif)
...{
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
strcpy(temp.cname,"中华人民共和国");
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockEnd.gif)
}
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
void main()
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockStart.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ContractedBlock.gif)
...{
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
Internet a("中国软件开发实验室","www.cndev-lab.com");
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
Country b;
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
a.Editcname(b);
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
cin.get();
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockEnd.gif)
}
在上面的代码中我们成功的通过Internet类Editcname成员函数操作了Country类的保护成员cname。
在编程中,我们使用友元的另外一个重要原因是为了方便重载操作符的使用,这些内容我们将在后面的教程着重讨论!
通常对于普通函数来说,要访问类的保护成员是不可能的,如果想这么做那么必须把类的成员都生命成为public(共用的),然而这做带来的问题遍是任何外部函数都可以毫无约束的访问它操作它,c++利用friend修饰符,可以让一些你设定的函数能够对这些保护数据进行操作,避免把类成员全部设置成public,最大限度的保护数据成员的安全。友元能够使得普通函数直接访问类的保护数据,避免了类成员函数的频繁调用,可以节约处理器开销,提高程序的效率,但矛盾的是,即使是最大限度的保护,同样也破坏了类的封装特性,这即是友元的缺点,在现在cpu速度越来越快的今天我们并不推荐使用它,但它作为c++一个必要的知识点,一个完整的组成部分,我们还是需要讨论一下的。
在类里声明一个普通函数,在前面加上friend修饰,那么这个函数就成了该类的友元,可以访问该类的一切成员。 .
下面我们来看一段代码,看看我们是如何利用友元来访问类的一切成员的。
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
#include <iostream>
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
using namespace std;
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
class Internet
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockStart.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ContractedBlock.gif)
...{
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
public:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
Internet(char *name,char *address)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedSubBlockStart.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ContractedSubBlock.gif)
...{
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
strcpy(Internet::name,name);
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
strcpy(Internet::address,address);
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedSubBlockEnd.gif)
}
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
friend void ShowN(Internet &obj);//友元函数的声明
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
public:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
char name[20];
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
char address[20];
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockEnd.gif)
};
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
void ShowN(Internet &obj)//函数定义,不能写成,void Internet::ShowN(Internet &obj)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockStart.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ContractedBlock.gif)
...{
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
cout<<obj.name<<endl;
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockEnd.gif)
}
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
void main()
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockStart.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ContractedBlock.gif)
...{
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
Internet a("中国软件开发实验室","www.cndev-lab.com");
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
ShowN(a);
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
cin.get();
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockEnd.gif)
}
上面的代码通过友元函数的定义,我们成功的访问到了a对象的保护成员name,友元函数并不能看做是类的成员函数,它只是个被声明为类友元的普通函数,所以在类外部函数的定义部分不能够写成void Internet::ShowN(Internet &obj),这一点要注意。
一个普通函数可以是多个类的友元函数,对上面的代码我们进行修改,注意观察变化:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
#include <iostream>
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
using namespace std;
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
class Country;
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
class Internet
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockStart.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ContractedBlock.gif)
...{
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
public:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
Internet(char *name,char *address)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedSubBlockStart.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ContractedSubBlock.gif)
...{
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
strcpy(Internet::name,name);
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
strcpy(Internet::address,address);
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedSubBlockEnd.gif)
}
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
friend void ShowN(Internet &obj,Country &cn);//注意这里
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
public:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
char name[20];
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
char address[20];
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockEnd.gif)
};
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
class Country
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockStart.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ContractedBlock.gif)
...{
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
public:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
Country()
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedSubBlockStart.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ContractedSubBlock.gif)
...{
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
strcpy(cname,"中国");
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedSubBlockEnd.gif)
}
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
friend void ShowN(Internet &obj,Country &cn);//注意这里
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
protected:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
char cname[30];
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockEnd.gif)
};
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
void ShowN(Internet &obj,Country &cn)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockStart.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ContractedBlock.gif)
...{
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
cout<<cn.cname<<"|"<<obj.name<<endl;
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockEnd.gif)
}
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
void main()
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockStart.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ContractedBlock.gif)
...{
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
Internet a("中国软件开发实验室","www.cndev-lab.com");
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
Country b;
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
ShowN(a,b);
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
cin.get();
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockEnd.gif)
}
一个类的成员函数函数也可以是另一个类的友元,从而可以使得一个类的成员函数可以操作另一个类的数据成员,我们在下面的代码中增加一类Country,注意观察:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
#include <iostream>
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
using namespace std;
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
class Internet;
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
class Country
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockStart.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ContractedBlock.gif)
...{
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
public:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
Country()
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedSubBlockStart.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ContractedSubBlock.gif)
...{
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
strcpy(cname,"中国");
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedSubBlockEnd.gif)
}
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
void Editurl(Internet &temp);//成员函数的声明
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
protected:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
char cname[30];
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockEnd.gif)
};
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
class Internet
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockStart.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ContractedBlock.gif)
...{
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
public:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
Internet(char *name,char *address)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedSubBlockStart.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ContractedSubBlock.gif)
...{
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
strcpy(Internet::name,name);
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
strcpy(Internet::address,address);
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedSubBlockEnd.gif)
}
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
friend void Country::Editurl(Internet &temp);//友元函数的声明
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
protected:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
char name[20];
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
char address[20];
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockEnd.gif)
};
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
void Country::Editurl(Internet &temp)//成员函数的外部定义
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockStart.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ContractedBlock.gif)
...{
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
strcpy(temp.address,"edu.cndev-lab.com");
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
cout<<temp.name<<"|"<<temp.address<<endl;
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockEnd.gif)
}
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
void main()
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockStart.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ContractedBlock.gif)
...{
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
Internet a("中国软件开发实验室","www.cndev-lab.com");
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
Country b;
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
b.Editurl(a);
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
cin.get();
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockEnd.gif)
}
整个类也可以是另一个类的友元,该友元也可以称做为友类。友类的每个成员函数都可以访问另一个类的所有成员。
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
#include <iostream>
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
using namespace std;
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
class Internet;
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
class Country
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockStart.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ContractedBlock.gif)
...{
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
public:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
Country()
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedSubBlockStart.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ContractedSubBlock.gif)
...{
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
strcpy(cname,"中国");
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedSubBlockEnd.gif)
}
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
friend class Internet;//友类的声明
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
protected:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
char cname[30];
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockEnd.gif)
};
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
class Internet
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockStart.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ContractedBlock.gif)
...{
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
public:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
Internet(char *name,char *address)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedSubBlockStart.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ContractedSubBlock.gif)
...{
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
strcpy(Internet::name,name);
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
strcpy(Internet::address,address);
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedSubBlockEnd.gif)
}
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
void Editcname(Country &temp);
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
protected:
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
char name[20];
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
char address[20];
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockEnd.gif)
};
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
void Internet::Editcname(Country &temp)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockStart.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ContractedBlock.gif)
...{
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
strcpy(temp.cname,"中华人民共和国");
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockEnd.gif)
}
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
void main()
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockStart.gif)
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ContractedBlock.gif)
...{
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
Internet a("中国软件开发实验室","www.cndev-lab.com");
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
Country b;
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
a.Editcname(b);
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif)
cin.get();
![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockEnd.gif)
}
在上面的代码中我们成功的通过Internet类Editcname成员函数操作了Country类的保护成员cname。
在编程中,我们使用友元的另外一个重要原因是为了方便重载操作符的使用,这些内容我们将在后面的教程着重讨论!
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