PoEduo - C++阶段班【Po学校】-07static与const与类的化学变化- 课堂笔记

1-->static与类　　

#define _CRT_SECURE_NO_WARNINGS
#include <iostream>

using namespace std;

class AA
{
public:
    AA(int a, int b)
    {
        m_a = a;
        m_b = b;
    }

    int getC()
    {
        m_c++;
        return m_c;
    }

    //静态的成员方法
    static int& getCC()
    {
        return m_c;
    }

private:
    //static修饰的静态成员变量
    static int m_c;
    int m_a;
    int m_b;
};

//静态成员变量的初始化，一定类的外边。
int AA::m_c = 100;

int main(void)
{
    AA a1(10, 20);
    AA a2(100, 200);

    cout << a1.getC() << endl;//101
    cout << a2.getC() << endl;//102

    //a1.getCC() = 200;
    AA::getCC() = 200;

    cout << a1.getC() << endl;//201
    cout << a2.getC() << endl;//202

    return 0;
}

示例2

#define _CRT_SECURE_NO_WARNINGS
#include <iostream>

using namespace std;

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

    int volume()
    {
        int v = len *width*hight;

        cout << "高度是" << hight << endl;
        cout << "体积是" <<  v << endl;

        return v;
    }

    static void changeHight(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::changeHight(300);

    b1.volume();
    b2.volume();

    return 0;
}

示例  学生求平均分数

#define _CRT_SECURE_NO_WARNINGS
#include <iostream>

using namespace std;

class Student
{
public:
    Student(int id, double score)
    {
        //创建一个学生
        m_id = id;
        m_score = score;

        m_count++;//累加一个个数
        sum_score += score;
    }

    static int getCount()
    {
        return m_count;
    }

    static double getAvg()
    {
        return sum_score / m_count;
    }

    ~Student() {
        m_count--;
        sum_score -= m_score;
    }

private:
    int m_id;
    double m_score;//得分

    //统计学生个数的静态成员变量
    static int m_count;

    //统计学生总分数的静态成员变量
    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 << "学生总人数: " << Student::getCount() << endl;
    cout << "学生的平均分: " << Student::getAvg() << endl;

    return 0;
}

static不占用内存空间

#include    <iostream>
using namespace    std;

class    C1
{
public:
    int    i;        //4
    int    j;        //4
    int    k;        //4
};    //12

class    C2
{
public:
    int    i;
    int    j;
    int    k;
    static int    m;                                                                            //4
public:
    int    getK()    const    { return    k; }        //4
    void    setK(int    val)    { k = val; }    //4
};

struct    S1
{
    int    i;
    int    j;
    int    k;
};    //12

struct    S2
{
    int    i;
    int    j;
    int    k;
    static int    m;
};    //12?

int    main()
{
    cout << "c1    :    " << sizeof(C1) << endl;
    cout << "c1    :    " << sizeof(C2) << endl;

    C2 c1, c2;

    //c1.getK();  //为什么会return c1.k
    //c2.getK(); // 为什么会return c2.k

    cout << " ----- " << endl;

    cout << "c1    :    " << sizeof(S1) << endl;
    cout << "c1    :    " << sizeof(S2) << endl;

    return 0;
}

#include <stdio.h>

class Test
{
private:
    int mCount;
public:
    Test() : mCount(0)
    {
        mCount++;
    }
    ~Test()
    {
        --mCount;
    }
    int getCount()
    {
        return mCount;
    }
};

Test gTest;

int main()
{
    Test t1;
    Test t2;

    printf("count = %d\n", gTest.getCount());
    printf("count = %d\n", t1.getCount());
    printf("count = %d\n", t2.getCount());

    return 0;
}

再看示例：

#include <stdio.h>

class Test
{
private:
    static int cCount;
public:
    Test()
    {
        cCount++;
    }
    ~Test()
    {
        --cCount;
    }
    int getCount()
    {
        return cCount;
    }
};

int Test::cCount = 0;

Test gTest;

int main()
{
    Test t1;
    Test t2;

    printf("count = %d\n", gTest.getCount());
    printf("count = %d\n", t1.getCount());
    printf("count = %d\n", t2.getCount());

    Test* pt = new Test();

    printf("count = %d\n", pt->getCount());

    delete pt;

    printf("count = %d\n", gTest.getCount());

    return 0;
}

示例

#include <stdio.h>

class Test
{
public:
    static int cCount;
public:
    Test()
    {
        cCount++;
    }
    ~Test()
    {
        --cCount;
    }
    int getCount()
    {
        return cCount;
    }
};

int Test::cCount = 0;

int main()
{
    printf("count = %d\n", Test::cCount);

    Test::cCount = 1000;

    printf("count = %d\n", Test::cCount);

    return 0;
}

再看  示例：

#include <stdio.h>

class Demo
{
private:
    int i;
public:
    int getI();
    static void StaticFunc(const char* s);
    static void StaticSetI(Demo& d, int v);
};

int Demo::getI()
{
    return i;
}

void Demo::StaticFunc(const char* s)
{
    printf("StaticFunc: %s\n", s);
}

void Demo::StaticSetI(Demo& d, int v)
{
    d.i = v;
}

int main()
{
    Demo::StaticFunc("main Begin...");

    Demo d;

    Demo::StaticSetI(d, 10);

    printf("d.i = %d\n", d.getI());

    Demo::StaticFunc("main End...");

    return 0;
}

示例3

#include <stdio.h>

class Test
{
private:
    static int cCount;
public:
    Test()
    {
        cCount++;
    }
    ~Test()
    {
        --cCount;
    }
    static int GetCount()
    {
        return cCount;
    }
};

int Test::cCount = 0;

int main()
{
    printf("count = %d\n", Test::GetCount());

    Test t1;
    Test t2;

    printf("count = %d\n", t1.GetCount());
    printf("count = %d\n", t2.GetCount());

    Test* pt = new Test();

    printf("count = %d\n", pt->GetCount());

    delete pt;

    printf("count = %d\n", Test::GetCount());

    return 0;
}