C++知识记录（1）

友元

https://baike.baidu.com/item/友元/6150794?fr=aladdin
一种定义在类外部的普通函数或类，但它需要在类体内进行说明，为了与该类的成员函数加以区别，在说明时前面加以关键字friend。友元不是成员函数，但是它可以访问类中的私有成员。

#include<iostream>
#include<cmath>
using namespace std;
class Point
{
public:
    Point(double xx, double yy)
    {
        x = xx;
        y = yy;
    };
    void Getxy();
    friend double Distance(Point &a, Point &b);
private:
    double x, y;
};
void Point::Getxy()
{
    cout << "(" << x << "," << y << ")" << endl;
}
double Distance(Point &a, Point &b)
{
    double dx = a.x - b.x;
    double dy = a.y - b.y;
    return sqrt(dx*dx + dy*dy);
}
int main(void)
{
    Point p1(3.0, 4.0), p2(6.0, 8.0);
    p1.Getxy();
    p2.Getxy();
    double d = Distance(p1, p2);
    cout << "Distance is" << d << endl;
    return 0;
}

引用&

通过引用，在函数调用的过程中，会实际修改指针的指向。改变指针这样会引入一个非法引用。
system函数需要包括cstdlib库

#include<iostream>
#include <cstdlib>
using namespace std;
void function2(int* &p) {
	int x = 12;
	p = &x;
	cout << "p: " << p << endl;
	cout << "*p: " << *p << endl;
}int main() {
	int* a = new int;
	int b = 1;
	*a = b;
	cout << "a:" << a << endl;
	cout << "*a:" << *a << endl;
	function2(a);
	cout << "a:" << a << endl;
	cout << "*a:" << *a << endl;
	delete a;
	system("pause");
}