第六章：静态成员与友元

静态成员变量

这里面要回顾一下构造函数和析构函数？自己理解不是太深刻啊

class student
{
    static int num;
public:
    student()
    {
        num++;
    }
    ~student()
    {
        num--;
    }
    static int getnum()
    {
        return num;
    }
};
int student ::num=0;//在这里全局初始化
int main()
{
    student t1;
    student t2;
    student *t3 = new student;

    cout<<"student num1 ="<<student :: getnum()<<endl;//类的名字加：：，静态成员变量

    cout<<"student num2 = "<<t1.getnum()<<endl;

    delete t3;
}

友元：声明函数

class student
{
    int m_id;
public:
    student(int id)
    {
        m_id=id;
    }
    friend void test(student t);//声明void test函数可以使用类里面的成员变量
};
void test(student t)
{
    cout<<"id="<<t.m_id<<endl;
}
int main()
{
    
    student t1(10);
    test(t1);
}

友元：声明类

class student
{
    int m_id;
public:
    student(int id)
    {
        m_id=id;
    }
    friend class A;//声明A类的里面
};
class A
{
public:
    void test(student t)
    {
        cout<<"id="<<t.m_id<<endl;
    }
};
int main()
{
    
    student t1(10);
    A t2;
    t2.test(t1);
}

友元：声明类里面的函数

using namespace std;
class student;//student的声明
class A
{
public:
    void test(student t);   //函数声明
};

class student
{
    int m_id;
public:
    student(int id)
    {
        m_id=id;
    }
    friend void A::test(student t);//函数友元声明
};
void A::test(student t)
{
    cout<<"id="<<t.m_id<<endl;
}


int main()
{
    
    student t1(10);
    A t2;
    t2.test(t1);
}