Android NDK基础20：C++_类大小_友元函数_运算符重载

类大小

//类的大小
class A {
public:
    int i;
    int j;
    int k;
    static int m;
};

class B {
public:
    int i;
    int j;
    int k;

    void printf() {
        cout << "打印" << endl;
    }
};

void main() {
    cout << sizeof(A) << endl;  //12
    cout << sizeof(B) << endl;  //12

    //C/C++ 内存分区：栈、堆、全局（静态、全局）、常量区（字符串）、程序代码区
    //普通属性与结构体相同的内存布局

    //JVM 内存分区：
    //JVM Stack（基本数据类型、对象引用）
    //Native Method Stack（本地方法栈）
    //方法区

    system("pause");
}

友元函数

//友元函数
class A {
    //友元函数
    friend void modify_i(A *p, int a);
private:
    int i;
public:
    A(int i) {
        this->i = i;
    }

    void print() {
        cout << i << endl;
    }
};

//友元函数的实现，在友元函数中可以访问私有的属性
void modify_i(A *p, int a) {
    p->i = a;
}

void main() {
    A* a = new A(10);
    a->print();

    modify_i(a, 20);
    a->print();

    system("pause");
}

//友元类
class A {        
    //友元类
    friend class B;
private:
    int i;
public:
    A(int i) {
        this->i = i;
    }

    void print() {
        cout << i << endl;
    }    
};

class B {
public:
    //B这个友元类可以访问A类的任何成员
    void accessAny() {
        a.i = 30;        
    }
private:
    A a;
};

运算符重载

//运算符重载
class Point {
public:
    int x;
    int y;
public:
    Point(int x = 0, int y = 0) {
        this->x = x;
        this->y = y;
    }

    void print() {
        cout << x << "," << y << endl;
    }
};

//重载+号
Point operator+(Point &p1, Point &p2) {
    Point p(p1.x + p2.x, p1.y + p2.y);
    return p;
}

//重载-号
Point operator-(Point &p1, Point &p2) {
    Point p(p1.x - p2.x, p1.y - p2.y);
    return p;
}

void main() {
    Point p1(10, 20);
    Point p2(20, 10);

    Point p3 = p1 + p2;

    p3.print();

    system("pause");
}

//成员函数，运算符重载
class Point {
public:
    int x;
    int y;
public:
    Point(int x = 0, int y = 0) {
        this->x = x;
        this->y = y;
    }

    //成员函数，运算符重载
    Point operator+(Point &p2) {
        Point tmp(this->x + p2.x, this->y + p2.y);
        return tmp;
    }

    void print() {
        cout << x << "," << y << endl;
    }
};

void main() {
    Point p1(10, 20);
    Point p2(20, 10);

    //运算符的重载，本质还是函数调用
    //p1.operator+(p2)
    Point p3 = p1 + p2;

    p3.print();

    system("pause");
}

//当属性私有时，通过友元函数完成运算符重载
class Point {
    friend Point operator+(Point &p1, Point &p2);
private:
    int x;
    int y;
public:
    Point(int x = 0, int y = 0) {
        this->x = x;
        this->y = y;
    }

    void print() {
        cout << x << "," << y << endl;
    }
};

Point operator+(Point &p1, Point &p2) {
    Point p(p1.x + p2.x, p1.y + p2.y);
    return p;
}

void main() {
    Point p1(10, 20);
    Point p2(20, 10);

    //运算符的重载，本质还是函数调用
    //p1.operator+(p2)
    Point p3 = p1 + p2;

    p3.print();

    system("pause");
}