已知三维上的两点坐标，确定一条直线，输入x，y，首先判断x,y是否在直线水平面投影上，若在求z，根据上述要求，写一段C++代码，要求将点封装为结构体

以下是C++代码（结构体Point包含三个成员变量x、y、z）：

#include <iostream>
#include <cmath>
using namespace std;

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

struct Line {
    Point p1;
    Point p2;
};

bool isOnHorizontal(Line line, Point point) {
    double x1 = line.p1.x, y1 = line.p1.y, x2 = line.p2.x, y2 = line.p2.y;
    double px = point.x, py = point.y;
    double k = (y2 - y1) / (x2 - x1);
    double b = y1 - k * x1;
    double h = k * px + b;
    return fabs(h - py) < 1e-6;
}

bool isOnLine(Line line, Point point) {
    double x1 = line.p1.x, y1 = line.p1.y, z1 = line.p1.z;
    double x2 = line.p2.x, y2 = line.p2.y, z2 = line.p2.z;
    double px = point.x, py = point.y;
    double k = (z2 - z1) / sqrt(pow(x2 - x1, 2) + pow(y2 - y1, 2));
    double b = z1 - k * sqrt(pow(px - x1, 2) + pow(py - y1, 2));
    point.z = k * sqrt(pow(px - x1, 2) + pow(py - y1, 2)) + b;
    return isOnHorizontal(line, point);
}

int main() {
    Point point = {2, 3, 0};
    Line line = {{1, 1, 1}, {4, 4, 4}};
    if (isOnLine(line, point)) {
        cout << "The point is on the line." << endl;
        cout << "z = " << point.z << endl;
    } else {
        cout << "The point is not on the line." << endl;
    }
    return 0;
}

首先定义了两个结构体Point和Line，分别表示点和线。isOnHorizontal函数判断点是否在直线的水平面投影上，isOnLine函数根据点的x、y坐标计算出z坐标，并判断点是否在直线上。在main函数中测试了一个点是否在指定的直线上，如果在则输出z坐标，否则输出不在直线上的信息。