生活中你的家有客厅(Public),有你的卧室(Private)
客厅所有来的客人都可以过去,但是你的卧室是私有的,也就是说只有你能进去
但是呢,你也可以允许你的好闺蜜好基友进去
在程序里,有些私有属性也想让类外特殊的一些函数或类进行访问,就需要用到友元的技术
- 友元的目的就是让一个函数或者类访问另一个类中的私有成员
- 友元的关键字为 friend
友元的三种实现:
- 全局函数做友元
- 类做友元
- 成员函数做友元
1. 全局函数做友元
friend void goodGay(Building *building);
#include <iostream>
#include <string>
using namespace std;
class Building{
//告诉编译器goodGay全局函数是Building好朋友,可以访问Building中的私有成员
friend void goodGay(Building *building);
public:
Building(){
m_SittingRoom = "客厅";
m_BedRoom = "卧室";
}
string m_SittingRoom; //客厅
private:
string m_BedRoom; //卧室
};
//全局函数
void goodGay(Building *building){
cout << "好基友的全局函数 正在访问:" << building->m_SittingRoom << endl;
cout << "好基友的全局函数 正在访问:" << building->m_BedRoom << endl;
}
void test01(){
Building building;
goodGay(&building);
}
int main(){
test01();
return 0;
}
2. 类做友元
friend class GoodGay;
#include <iostream>
#include <string>
using namespace std;
class Building{
friend class GoodGay;
public:
Building();
string m_SittingRoom; //客厅
private:
string m_BedRoom; //卧室
};
class GoodGay{
public:
GoodGay();
void visit(); //参观函数,访问Building中的属性
Building *building;
};
//类外写成员函数
Building::Building(){
m_SittingRoom = "客厅";
m_BedRoom = "卧室";
}
GoodGay::GoodGay(){
//创建建筑物对象
building = new Building;
}
void GoodGay::visit(){
cout << "好基友正在访问:" << building->m_SittingRoom << endl;
cout << "好基友正在访问:" << building->m_BedRoom << endl;
}
void test01(){
GoodGay gg;
gg.visit();
}
int main(){
test01();
return 0;
}
3. 成员函数做友元
friend void GoodGay::visit();
#include <iostream>
#include <string>
using namespace std;
class Building;
class GoodGay{
public:
GoodGay();
void visit(); //让visit函数可以访问Building中私有成员
void visit2(); //让visit2函数不可以访问Building中私有成员
Building *building;
};
class Building{
friend void GoodGay::visit();
public:
Building();
string m_SittingRoom; //客厅
private:
string m_BedRoom; //卧室
};
//类外写成员函数
Building::Building(){
m_SittingRoom = "客厅";
m_BedRoom = "卧室";
}
GoodGay::GoodGay(){
//创建建筑物对象
building = new Building;
}
void GoodGay::visit(){
cout << "visit正在访问:" << building->m_SittingRoom << endl;
cout << "visit正在访问:" << building->m_BedRoom << endl;
}
void GoodGay::visit2(){
cout << "visit2正在访问:" << building->m_SittingRoom << endl;
//cout << "visit2正在访问:" << building->m_BedRoom << endl;
}
void test01(){
GoodGay gg;
gg.visit();
gg.visit2();
}
int main(){
test01();
return 0;
}