28-友元

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#include <stdio.h>#include <math.h>class Point{    double x;    double y;public:    Point(double x, double y)    {        this->x = x;        this->y = y;    }    double getX()    {        return x;    }    double getY()    {        return y;    }    friend double func(Point& p1, Point& p2);};double func(Point& p1, Point& p2){    double ret = 0;    ret = (p2.y - p1.y) * (p2.y - p1.y) +          (p2.x - p1.x) * (p2.x - p1.x);    ret = sqrt(ret);    return ret;}int main(){    Point p1(1, 2);    Point p2(10, 20);    printf("p1(%f, %f)\n", p1.getX(), p1.getY());    printf("p2(%f, %f)\n", p2.getX(), p2.getY());    printf("|(p1, p2)| = %f\n", func(p1, p2));    return 0;}p1(1.000000, 2.000000)p2(10.000000, 20.000000)|(p1, p2)| = 20.124612

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#include <stdio.h>class ClassC{    const char* n;public:    ClassC(const char* n)    {        this->n = n;    }    friend class ClassB;};class ClassB{    const char* n;public:    ClassB(const char* n)    {        this->n = n;    }    void getClassCName(ClassC& c)    {        printf("c.n = %s\n", c.n);    }    friend class ClassA;};class ClassA{    const char* n;public:    ClassA(const char* n)    {        this->n = n;    }    void getClassBName(ClassB& b)    {        printf("b.n = %s\n", b.n);    }    /*    void getClassCName(ClassC& c)    {        printf("c.n = %s\n", c.n);    }    */};int main(){    ClassA A("A");    ClassB B("B");    ClassC C("C");    A.getClassBName(B);    B.getClassCName(C);    return 0;}b.n=Bc.n=C

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