靜態成員

靜態變量的初始化(如何定義)

#include<iostream>
using namespace std;
class Student
{
    static int num;
public:
    Student()
    {
        num++;
    }
    ~Student()
    {
        num--;
    }
    int getnum()
    {
        return num;
    }
};
int Student::num=0;
int main()
{
    Student t1;
    Student t2;
    Student *t3=new Student;
    cout<<"Student num1 = "<<t2.getnum()<<endl;
    delete t3;
    cout<<"Student num2 = "<<t2.getnum()<<endl;
}
//結果為:
//Student num1=3;
//Student num2=2;

• static int getnum()//靜態成員函數

#include<iostream>
using namespace std;
class Student
{
    static int num;
public:
    Student()
    {
        num++;
    }
    ~Student()
    {
        num--;
    }
    static int getnum()
    {
        return num;
    }
};
int Student::num=0;
int main()
{
    cout<<"Student num0 = "<<Student::getnum()<<endl;
    Student t1;
    Student t2;
    Student *t3=new Student;
    cout<<"Student num1 = "<<Student::getnum()<<endl;
    delete t3;
    cout<<"Student num2 = "<<Student::getnum()<<endl;
}
//結果為：
//num0=0;
//num1=3;
//num2=2;

• 靜態成員函數只能操作靜態成員變量

1. 第一種

#include<iostream>
using namespace std;
class Student
{
    static int num;
    int id;
public:
    Student()
    {
        num++;
    }
    ~Student()
    {
        num--;
    }
    int getnum()
    {
        int id=0;
        return num;
    }
};
int Student::num=0;
int main()
{
//  cout<<"Student num0 = "<<Student::getnum()<<endl;
    Student t1;
    Student t2;
    Student *t3=new Student;
    cout<<"Student num1 = "<<t2.getnum()<<endl;
    delete t3;
    cout<<"Student num2 = "<<t2.getnum()<<endl;
}

2. 第二種( 靜態成員函數只能操作靜態成員變量)(推薦此用法)

#include<iostream>
using namespace std;
class Student
{
    static int num;
    int id;
public:
    Student()
    {
        num++;
    }
    ~Student()
    {
        num--;
    }
    int getid()
    {
        id=0;
        return id;
    }
    static int getnum()
    {
        return num;
    }
};
int Student::num=0;//靜態成員變量的初始化,一定要寫在外面
int main()
{
//  cout<<"Student num0 = "<<Student::getnum()<<endl;
    Student t1;
    Student t2;
    Student *t3=new Student;
    cout<<"Student num1 = "<<Student::getnum()<<endl;
    delete t3;
    cout<<"Student num2 = "<<Student::getnum()<<endl;
}

總結

• 靜態數據成員用得比較多的地方
• 一個靜態成員函數不能訪問非靜態成員變量

友元

#include<iostream>
using namespace std;
class Student
{
    int m_id;
public:
    Student(int id)
    {
        m_id=id;
    }
    friend void test(Student t);//所以要加聲明;
};
void test(Student t)
{
    cout<<"id = "<<t.m_id<<endl;//是錯誤的,m_id是私有的,不能再外部訪問
}
int main()
{
    Student t1(10);
    test(t1);
}
//結果為:id=10;

友類

#include<iostream>
using namespace std;
class Student
{
    int m_id;
public:
    Student(int id)
    {
        m_id=id;
    }
    friend class A;
};
class A
{
public:
    void test(Student t)
    {
        cout<<"id = "<<t.m_id<<endl;
    }
};

int main()
{
    Student t1(10);
    A a;
    a.test(t1);
}
//結果為:id = 10;

• 友元不能傳遞

#include<iostream>
using namespace std;
class Student
{
    int m_id;
public:
    Student(int id)
    {
        m_id=id;
    }
    friend class B;
    friend class A;
};
class B
{
    friend class A;
}
class A
{
public:
    void test(Student t)
    {
        cout<<"id = "<<t.m_id<<endl;
    }
};

int main()
{
    Student t1(10);
    A a;
    a.test(t1);
}

//結果為:
//id=10;

• 詭異的程序

#include<iostream>
using namespace std;
class Student;
class A
{
public:
    void test(Student t);
};
class Student
{
    int m_id;
public:
    Student(int id)
    {
        m_id=id;
    }
    friend void A::test(Student t);
};

void A::test(Student t)
{
    cout<<"id = "<<t.m_id<<endl;
}
int main()
{
    Student t1(10);
    A a;
    a.test(t1);
}

//結果為:
//id=10

友元函數

需要友元的原因

1. 有時候，普通函數需要直接訪問一個類的保護或私有數據成員。
2. 需要友元的另一個原因是為了方便重載操作符的使用。
3. 友元函數不是成員函數，它是類的朋友，因而能夠訪問類的全部成員。
4. 在類的內部，只能聲明它的函數原型，加上friend關鍵字.

友元的使用

1. 在類里聲明一個普通函數，標上關鍵字friend，就成了該類的友元，可以訪問該類的一切成員。
2. 一個類的成員函數可以是另一個類的友元。