static變數和函式

靜態成員變數不在物件的記憶體區，在靜態區，所以sizeof()時計算不包含靜態變數的大小；

類中的函式也不在物件中。

#define _CRT_SECURE_NO_WARNINGS
#include <iostream>
#include<cstdlib>

using namespace std;

class AA {
public:
    AA(int a, int b) {
        m_a = a;
        m_b = b;
    }
    
    //返回public許可權的靜態變數m_c
    int
 
 get_c() {
        m_c++;
        return m_c;
    }
    
    //static int m_c;//靜態成員變數

    //類直接可訪問的方法
    static int& get_pvc() {
        return m_c;
    }//此處為了把返回值當左值，定義返回型別為引用
private:
    int m_a;
    int m_b;
    static int m_c;//靜態成員變數
};

//靜態成員變數的初始化一定在類的外部，屬於整個類共有
int AA::m_c = 100;

int main(void) {
    AA a1(
 
10, 20);
    AA a2(100, 200);
    cout << a1.get_c() << endl;
    cout << a2.get_c() << endl;
    AA::get_pvc() = 200;
    cout << a1.get_c() << endl;
    cout << a2.get_c() << endl;
    system("pause");
    return 0;
}

練習：

#define _CRT_SECURE_NO_WARNINGS
#include 
 
<iostream>
#include<cstdlib>

using namespace std;

class Box {
public:
    Box(int l, int w) {
        len = l;
        width = w;
    }

    int volume() {
        cout << "高度是" << hight << endl;
        int v = hight * len*width;
        cout << "體積是" << v << endl;
        return v;
    }

    static void change_hight(int h) {
        hight = h;
    }
private:
    int len;
    int width;
    static int hight;
};
int Box::hight = 100;

int main(void) {
    Box b1(10, 20);
    Box b2(100, 200);
    b1.volume();
    b2.volume();

    Box::change_hight(300);
    b1.volume();
    b2.volume();

    system("pause");
    return 0;
}

#define _CRT_SECURE_NO_WARNINGS
#include <iostream>
#include<cstdlib>

using namespace std;

class Student {
public:
    Student(int id,double score) {
        m_id = id;
        m_score = score;
        m_count++;//增加一個學生
        sum_score += score;//累加一個分數
    }

    static int get_count() {
        return m_count;
    }

    static double get_avg() {
        return sum_score / m_count;
    }
     
    //***通過解構函式將靜態變量回復到初始值
    ~Student() {
        m_count--;
        sum_score -= m_score;
    }
private:
    int m_id;
    double m_score;

    //統計學生個數的靜態成員變數
    static int m_count;

    //統計學生總分數的靜態成員變數
    static double sum_score;

};
int Student::m_count = 0;
double Student::sum_score = 0.0;

int main(void) {
    Student s1(1, 80);
    Student s2(2, 90);
    Student s3(3, 100);

    cout << "學生的總人數：" << Student::get_count() << endl;
    cout << "平均分為：" << Student::get_avg() << endl;

    system("pause");
    return 0;
}