DCEL 数据结构为特定向量元素抛出垃圾值？

Question

#include<bits/stdc++.h>
using namespace std;

class Vertex;
class Edge;
class Face;

class Vertex{
    public:
    float x;
    float y;
    Edge* edge; 

    Vertex(float x, float y) : x(x), y(y), edge(NULL) {}
};

class Edge{
    public:
    Vertex origin;
    Edge* twin;
    Edge* prev;
    Edge* next;
    Face* right;

    Edge(Vertex origin): origin(origin), twin(NULL), prev(NULL), next(NULL), right(NULL) {}
};

class Face{
    public:
    Edge* edge;

    Face(Edge* edge): edge(edge){}
};

class DCEL{
    public:
    vector<Vertex> vertices;
    vector<Edge> edges;
    vector<Face> faces;

    void addVertex(Vertex &vertex){
        vertices.push_back(vertex);
    }

    void addEdge(Vertex *origin, Vertex *destination){
        Edge e1 = Edge(*origin);
        Edge e2 = Edge(*destination);
        // origin->edge = &e1;
        // destination->edge = &e2;
        edges.push_back(e1);
        edges.push_back(e2);
        edges[edges.size()-1].twin = &edges[edges.size()-2];
        edges[edges.size()-2].twin = &edges[edges.size()-1];
        printEdges();
    }

    void addFace(Edge *edge){
        Face f1(edge);
        faces.push_back(f1);

    }

    void printVertices (){
        cout << "Vertices: " << endl;
        cout << "Count: "<< vertices.size() << endl;
        for (auto v: vertices){
            cout << "(" << v.x << ", " << v.y << ")" << endl;
        }
        cout << endl;
    }

    void printEdges (){
        cout << "Edges: " << endl;
        cout << "Count: " << edges.size() << endl;
        for (auto e: edges){
            cout << "(" << e.origin.x << ", " << e.origin.y << ")";
            cout << " <-> (" << e.twin->origin.x << ", " << e.twin->origin.y << ")" << endl;
        }
        cout << endl;
    }

    void printFaces (){
        cout << "Faces: " << endl;
        cout << "Count: "<< faces.size() << endl; // TODO: to be changed
        for (auto f: faces){
            cout << "(" << f.edge->origin.x << ", " << f.edge->origin.y << ")" << endl;
        }
        cout << endl;
    }

    void print(){
        cout << "-----" << endl;
        printVertices();
        printEdges();
        printFaces();
        cout << "-----" << endl;
    }

};

int main(){

    DCEL dcel;
    Vertex v1(0.0, 0.0);
    Vertex v2(1.0, 0.0);
    Vertex v3(1.0, 1.0);
    Vertex v4(0.0, 1.0);
    // Vertex v5(0.5, 0.5);

    dcel.addVertex(v1);
    dcel.addVertex(v2);
    dcel.addVertex(v3);
    dcel.addVertex(v4);
    // dcel.addVertex(v5);

    dcel.addEdge(&v1, &v2);
    dcel.addEdge(&v2, &v3);
    dcel.addEdge(&v3, &v4);
    dcel.addEdge(&v4, &v1);

    dcel.addFace(&dcel.edges[0]);

    cout << endl;
    dcel.print();
}

以上是我实现的代码。 每次执行

addEdge

调试它时，我都会打印边缘列表。

我得到的输出真的很荒谬

当 addEdge 第一次执行时（即 (0,0) ）我得到了正确的值但随后它突然改变（在 VSCode 调试器中它显示当我试图将它推入向量时发生这种变化）

Edges: 
Count: 2
(0, 0) <-> (1, 0)
(1, 0) <-> **(0, 0)**

Edges: 
Count: 4
(0, 0) <-> (1, 0)
(1, 0) <-> **(1.54853e+21, 7.00649e-45)**
(1, 0) <-> (1, 1)
(1, 1) <-> (1, 0)

Edges: 
Count: 6
(0, 0) <-> (1, 0)
(1, 0) <-> **(1.54853e+21, 7.00649e-45)**
(1, 0) <-> (1, 1)
(1, 1) <-> (1, 0)
(1, 1) <-> (0, 1)
(0, 1) <-> (1, 1)

Edges: 
Count: 8
(0, 0) <-> (1, 0)
(1, 0) <-> **(1.54853e+21, 7.00649e-45)**
(1, 0) <-> (1, 1)
(1, 1) <-> (1, 0)
(1, 1) <-> (0, 1)
(0, 1) <-> (1, 1)
(0, 1) <-> (0, 0)
(0, 0) <-> (0, 1)


-----
Vertices: 
Count: 4
(0, 0)
(1, 0)
(1, 1)
(0, 1)

Edges: 
Count: 8
(0, 0) <-> (1, 0)
(1, 0) <-> **(1.54853e+21, 7.00649e-45)**
(1, 0) <-> (1, 1)
(1, 1) <-> (1, 0)
(1, 1) <-> (0, 1)
(0, 1) <-> (1, 1)
(0, 1) <-> (0, 0)
(0, 0) <-> (0, 1)

Faces: 
Count: 1
(0, 0)

-----

Answer 1

发生这种情况的原因是因为 push_back 没有按照您的想法进行。实际上，push_back 使用默认的边缘复制构造函数制作并存储边缘的新副本——这对指针造成了严重破坏。

快速修复是提供一个复制构造函数，它用指针做正确的事情。

你还需要看看你是如何存储顶点的。考虑

main() {
    DCEL dcel;
    Vertex v1(0.0, 0.0);
    Vertex v2(1.0, 0.0);
    Vertex v3(1.0, 1.0);

    dcel.addVertex(v1);
    dcel.addVertex(v2);
    dcel.addVertex(v3);


    dcel.addEdge(&v1, &v2);
    dcel.addEdge(&v1, &v3);
}

v1存放在哪里？好吧，有一个由 main 函数构造并作为局部变量存储在那里的原始函数。然后 V1 的副本存储在 DCEL::vertices 向量中。两条边的 Edge::Origin 属性中也存储了两个副本。如果需要，我们应该更新这 4 个副本中的哪一个？

你的设计充满了这类问题。它需要重新思考。我的建议：如果你使用存储在 DCEL 向量中的顶点和边的索引而不是指针，你的代码会更安全。

像这样的东西：

#include <bits/stdc++.h>
using namespace std;

class Vertex;
class Edge;
class Face;

class Vertex
{
public:
    float x;
    float y;
    Edge *edge;

    Vertex(float x, float y) : x(x), y(y), edge(NULL) {}

};

class Edge
{
public:
    int origin;
    int twin;
    Edge *prev;
    Edge *next;
    Face *right;

    Edge(int o) : origin(o), prev(NULL), next(NULL), right(NULL) {}

};

class Face
{
public:
    Edge *edge;

    Face(Edge *edge) : edge(edge) {}
};

class DCEL
{
public:
    vector<Vertex> vertices;
    vector<Edge> edges;
    vector<Face> faces;

    int addVertex(int x, int y)
    {
        vertices.push_back(Vertex(x,y));
        return vertices.size()-1;
    }

    void addEdge(int origin,int destination)
    {
        edges.push_back( Edge(origin));
        int e1 = edges.size()-1;
        edges.push_back( Edge(destination));
        int e2 = edges.size()-1;
        edges[e1].twin = e2;
        edges[e2].twin = e1;
        printEdges();
    }

    void addFace(Edge *edge)
    {
        Face f1(edge);
        faces.push_back(f1);
    }

    void printVertices()
    {
        cout << "Vertices: " << endl;
        cout << "Count: " << vertices.size() << endl;
        for (auto v : vertices)
        {
            cout << "(" << v.x << ", " << v.y << ")" << endl;
        }
        cout << endl;
    }

    void printEdges()
    {
        cout << "Edges: " << endl;
        cout << "Count: " << edges.size() << endl;
        for (auto e : edges)
        {
            cout << "(" << vertices[e.origin].x << ", " << vertices[e.origin].y << ")";
            cout << " <-> (" << vertices[edges[e.twin].origin].x << ", " << vertices[edges[e.twin].origin].y << ")" << endl;
        }
        cout << endl;
    }

    // void printFaces()
    // {
    //     cout << "Faces: " << endl;
    //     cout << "Count: " << faces.size() << endl; // TODO: to be changed
    //     for (auto f : faces)
    //     {
    //         cout << "(" << f.edge->origin.x << ", " << f.edge->origin.y << ")" << endl;
    //     }
    //     cout << endl;
    // }

    void print()
    {
        cout << "-----" << endl;
        printVertices();
        printEdges();
        //printFaces();
        cout << "-----" << endl;
    }
};

int main()
{

    DCEL dcel;

    int v1 = dcel.addVertex(0,0);
    int v2 = dcel.addVertex(1,0);
    int v3 = dcel.addVertex(1,1);
    int v4 = dcel.addVertex(0,1);

    dcel.addEdge(v1, v2);
    dcel.addEdge(v2, v3);
    dcel.addEdge(v3, v4);
    dcel.addEdge(v4, v1);

    dcel.addFace(&dcel.edges[0]);

    cout << endl;
    dcel.print();
}

输出

Edges: 
Count: 2
(0, 0) <-> (1, 0)
(1, 0) <-> (0, 0)

Edges:
Count: 4
(0, 0) <-> (1, 0)
(1, 0) <-> (0, 0)
(1, 0) <-> (1, 1)
(1, 1) <-> (1, 0)

Edges:
Count: 6
(0, 0) <-> (1, 0)
(1, 0) <-> (0, 0)
(1, 0) <-> (1, 1)
(1, 1) <-> (1, 0)
(1, 1) <-> (0, 1)
(0, 1) <-> (1, 1)

Edges:
Count: 8
(0, 0) <-> (1, 0)
(1, 0) <-> (0, 0)
(1, 0) <-> (1, 1)
(1, 1) <-> (1, 0)
(1, 1) <-> (0, 1)
(0, 1) <-> (1, 1)
(0, 1) <-> (0, 0)
(0, 0) <-> (0, 1)


-----
Vertices:
Count: 4
(0, 0)
(1, 0)
(1, 1)
(0, 1)

Edges:
Count: 8
(0, 0) <-> (1, 0)
(1, 0) <-> (0, 0)
(1, 0) <-> (1, 1)
(1, 1) <-> (1, 0)
(1, 1) <-> (0, 1)
(0, 1) <-> (1, 1)
(0, 1) <-> (0, 0)
(0, 0) <-> (0, 1)