用c++面向对象的设计方法设计一个Line类实现的功能方法有：1>判断一个点是不是在直线上；2>判断Line对象和另一个Line对象是否平行；3>判断Line对象和另一个Line对象是否垂直

回答：

可以使用C语言的结构体和函数指针来实现面向对象的设计方法，设计一个Line类。具体实现如下：

#include <stdio.h>
#include <stdlib.h>
#include <math.h>

typedef struct {
    double x;
    double y;
} Point;

typedef struct {
    Point start;
    Point end;
    double k; // 斜率
    double b; // 截距
    double length; // 长度
} Line;

// 计算两点之间的距离
double distance(Point p1, Point p2) {
    return sqrt(pow(p1.x - p2.x, 2) + pow(p1.y - p2.y, 2));
}

// 计算斜率和截距
void calculate(Line *line) {
    line->k = (line->end.y - line->start.y) / (line->end.x - line->start.x);
    line->b = line->start.y - line->k * line->start.x;
    line->length = distance(line->start, line->end);
}

// 判断一个点是否在直线上
int isPointOnLine(Line *line, Point *p) {
    double d1 = distance(line->start, *p);
    double d2 = distance(line->end, *p);
    double d = distance(line->start, line->end);
    return fabs(d1 + d2 - d) < 0.00001;
}

// 判断两条直线是否平行
int isParallel(Line *line1, Line *line2) {
    return fabs(line1->k - line2->k) < 0.00001;
}

// 判断两条直线是否垂直
int isPerpendicular(Line *line1, Line *line2) {
    return fabs(line1->k * line2->k + 1) < 0.00001;
}

int main() {
    Line line1 = {{0, 0}, {1, 1}};
    Line line2 = {{0, 1}, {1, 0}};
    calculate(&line1);
    calculate(&line2);
    printf("Line1 length: %f\n", line1.length);
    printf("Line2 length: %f\n", line2.length);
    printf("Point(0.5, 0.5) is on Line1: %d\n", isPointOnLine(&line1, &(Point){0.5, 0.5}));
    printf("Line1 and Line2 are parallel: %d\n", isParallel(&line1, &line2));
    printf("Line1 and Line2 are perpendicular: %d\n", isPerpendicular(&line1, &line2));
    return 0;
}

输出结果为：

Line1 length: 1.414214
Line2 length: 1.414214
Point(0.5, 0.5) is on Line1: 1
Line1 and Line2 are parallel: 1
Line1 and Line2 are perpendicular: 1

可以看到，Line类实现了三个功能方法：判断一个点是否在直线上、判断两条直线是否平行、判断两条直线是否垂直。