CPP-Union的主要用法特性

//
// Created by hewei on 2021/4/3.
//

#include <iostream>

 union Matrix
 {
     struct
     {
         double _a11, _a12, _a13;
         double _a21, _a22, _a23;
         double _a31, _a32, _a33;
     };

     double _element[3][3];
 };

/**
 * Union结构中，是共享存储空间的，所以实际上_element和_a11,...,_a33是同一块区域的数据
 * 上述Union结构可以对3x3的矩阵通过_element实现批量处理；
 * 也可以通过_a11,...,_a33实现单独元素的处理，避免了一个一个取值的麻烦
 *
*/

 int main() {
     Matrix matrix;
     for (int i = 0; i <= 2; ++i) {
         for (int j = 0; j <= 2; ++j) {
             std::cin >> matrix._element[i][j];
         }
     }
     double v = matrix._a11*matrix._a22*matrix._a33 + matrix._a12*matrix._a23*matrix._a31
             + matrix._a13*matrix._a21*matrix._a32 - matrix._a11*matrix._a23*matrix._a32 +
             matrix._a12*matrix._a21*matrix._a33 - matrix._a13*matrix._a22*matrix._a31;
     std::cout << "v: " << v << std::flush;
 }

//
// Created by hewei on 2021/4/3.
//

#include <iostream>

enum FIGURE_TYPE {
    LINE, RECTANGLE, ELLIPSE
};

struct Line {
    FIGURE_TYPE t;
    int x1, y1, x2, y2;
};

struct Rectangle {
    FIGURE_TYPE t;
    int left, top, right, bottom;
};

struct Ellipse {
    FIGURE_TYPE t;
    int x, y, r;
};

union FIGURE {
    FIGURE_TYPE t;
    Line line;
    Rectangle rect;
    Ellipse ellipse;
};

void draw_line(Line line);

void draw_rect(Rectangle rect);

void draw_ellipse(Ellipse ellipse);


int main() {
    FIGURE figure;
    figure.ellipse = {ELLIPSE, 3, 3, 3};
    figure.rect = {RECTANGLE, 2, 2, 2, 2};
    figure.line = {LINE, 1, 1, 1, 1};
    int t;
    while (std::cin >> t) {
        switch (t) {
            case LINE:
                std::cout << figure.line.x1 << std::endl;
                draw_line(figure.line);
                break;
            case RECTANGLE:
                std::cout << figure.rect.top << std::endl;
                draw_rect(figure.rect);
                break;
            case ELLIPSE:
                std::cout << figure.ellipse.x << std::endl;
                draw_ellipse(figure.ellipse);
                break;
            default:
                std::cerr << "No Match Pattern!" << std::flush;
                return 0;
        }
    }
}


void draw_line(Line line) {
    std::cout << "Draw Line" << std::endl;
}

void draw_rect(Rectangle rect) {
    std::cout << "Draw Rect" << std::endl;
}

void draw_ellipse(Ellipse ellipse) {
    std::cout << "Draw Ellipse" << std::endl;
}

struct Shape{
	FIGURE_TYPE type;
	union {
		Line line;
		Rectangle rect;
		Ellipse ellipse;
	} obj;
};

就不说废话了，自己理解吧！强蔷蔷！！！！

wwww，awsl！