全局函数作为友元

#include <iostream>
using namespace std;
#include <string>
class Building
{
  friend void GoodGay(Building* building); //全局函数作为友元
public:
  Building()
  {
    m_sittingroom = "客厅";
    m_bedroom = "卧室";
  }

public:
  string m_sittingroom;
private:
  string m_bedroom;
};
  
void GoodGay(Building* building)
{
  cout << "好基友正在访问:" << building->m_sittingroom << endl;
  //可以访问 private权限
  cout << "好基友正在访问:" << building->m_bedroom << endl; 
}

int main()
{
  Building building;
  GoodGay(&building);
  return 0;
}

类作为友元

#include <iostream>
using namespace std;

class Building; //先声明 后实现

class GoodGay
{
public:
  GoodGay();

  void visit();

  Building * m_building;
};
class Building  //实现类
{
  friend class GoodGay;   //类作为友元
public:
  Building();
  string m_sittingroom;
private:
  string m_bedroom;
};

Building::Building()    //构造函数
{
  this->m_sittingroom = "客厅";
  this->m_bedroom = "卧室";
}
GoodGay::GoodGay()    //构造函数
{
  this->m_building = new Building;
}

void GoodGay::visit()   //成员函数
{
  cout << "好基友正在访问:" << this->m_building->m_sittingroom << endl;
  cout << "好基友正在访问:" << this->m_building->m_bedroom << endl;
}
int main()
{
  GoodGay gg;
  gg.visit();

  return 0;
}

成员函数作为友元

#include <iostream>
using namespace std;

class Building; //先声明 后实现

class GoodGay
{
public:
  GoodGay();

  void visit();

  void visit2();

  Building* m_building;
};
class Building  //实现类
{
  //让GoodGay类中的 visit2成员函数 作为友元
  friend void GoodGay::visit2();
public:
  Building();
  string m_sittingroom;
private:
  string m_bedroom;
};

Building::Building()    //构造函数
{
  this->m_sittingroom = "客厅";
  this->m_bedroom = "卧室";
}
GoodGay::GoodGay()    //构造函数
{
  this->m_building = new Building;
}

void GoodGay::visit()   //成员函数
{
  cout << "好基友正在访问:" << this->m_building->m_sittingroom << endl;
  //cout << "好基友正在访问:" << this->m_building->m_bedroom << endl;
}
void GoodGay::visit2()   //成员函数
{
  cout << "好基友正在访问:" << this->m_building->m_sittingroom << endl;
  cout << "好基友正在访问:" << this->m_building->m_bedroom << endl;
}
int main()
{
  GoodGay gg;
  gg.visit2();

  return 0;
}