我在以下代码（指针）上收到 SIGSEGV 错误，不知道哪里出错了

Question

这是我正在构建的代码（对我的作业进行解释）：

/*
  Proposal:
    Create a list-based graph representation.

    It will need to support the following operations.
      1. Ask the user how many Vertices there are.

      2. Ask the user to label those Vertices, ie "A", "B", "C"...

      3. Define the graph as an array of linked lists based on the number of
      Vertices that hold labels. The node type would support the label and weight.

      4. Repeatedly ask the user to define edges between two Vertices with the weight of the edge. Add these edges to the list.

      5. Have a list method that will list out all of the edges in the graph
      with their weights.

  Sample Run:
  N/A
*/
#include <iostream>
#include <cstdlib>
#include <string>


// ========== Struct Definition(s) ==========
struct AdjListNode {
  int dest;
  std::string label;
  int weight;

  struct AdjListNode* next;
};


struct AdjList {
  struct AdjListNode *head;
};


// ===== Class Definition(s) ==========
class Graph {
  private:

    int num;
    struct AdjList* array;

  public:
    std::string *a;

    Graph(int);

    void addEdge(int, int, int, std::string, std::string);
    void label(std::string,int);
    void output();

    AdjListNode* newAdjListNode(int point, int weight, std::string labelname) {
      AdjListNode* newNode = new AdjListNode;

      newNode->dest = point;
      newNode->weight = weight;
      newNode->label = labelname;
      newNode->next = NULL;

      return newNode;
    }
};


// ========== Function Prototype(s) ==========
// In Class(es)


// ========== Main Function ==========
int main() {
  int num, distance, choice, head, tail;
  std::string input;

  std::cout << "Enter the number of vertices: ";
  std::cin >> num;

  Graph graph(num);

  std::cout << "\nPlease label the vertices ('A', 'B', 'C', etc.)\n";

  for (int i = 0; i < num; i++) {
    std::cout << "Label for vertex " << (i+1) << ": ";
    std::cin >> input;

    graph.label(input, i);
  }

  for (int i = 0; i < num; i++) {
    std::cout << "\nEnter two edges between " << "OUTPUT i and i+1 vertices (e.g. 1 5): ";
    std::cin >> head >> tail;

    std::cout << "Enter the weight of this edge: ";
    std::cin >> distance;

    graph.addEdge(head, tail, distance, graph.a[head], graph.a[tail]);
  }

  graph.output();

  return 0;
}


// ========== Function Definition(s) ==========
Graph::Graph(int num) {
  this->num = num;

  array = new AdjList [num];
  a = new std::string [num];

  for (int i = 0; i < num; ++i) {
    array[i].head = NULL;
  }
}


void Graph::addEdge(int a, int b, int weight, std::string bname, std::string tname) {
  if( a >= num || b >= num || a < 0 || b < 0) {
    std::cout << " Input is invalid\n";
  }

  else {
    AdjListNode* newNode = newAdjListNode(b,weight, tname);

    newNode->next = array[a].head;
    array[a].head = newNode;
    
    newNode = newAdjListNode(a,weight, bname);
    newNode->next = array[b].head;
    array[b].head = newNode;
  }
}


void Graph::label(std::string b, int i) {
  a[i] = b;
}


void Graph::output() {
  std::string input;

  for (int i = 0; i < num; i++) {
    AdjListNode* list = array[i].head;
    std::cout << "\nAdjacency list of the vertex " << i << " with labeled name: " << a[i];

    while (list) {
      std::cout << list->dest << " with the labeled name " << list->label << " and the weight being " << list->weight << "\n";
      list = list->next;
    }

    std::cout << "\n";
  }
}

点击此行时我收到错误：graph.addEdge(head, tail, distance, graph.a[head], graph.a[tail]);，但是，我怀疑问题出在我的类定义及其设置方式中。

有人可以在这里给我一些帮助吗？

我正在运行代码here (onlinegdb)

阻止我的是我稍微了解 SIGSEGV 错误是什么，但我只是看不到我错误地尝试访问内存的位置。调试器显示graph 具有以下内容 -> {num = 2, array = 0x614c20, a = 0x614c48}，但内存中的这些点应该可以访问，因为我保留它们，不是吗？

澄清graph 值 - 我正在输入 2 个顶点，对于前 2 个边，我正在输入带有权重的 1 和 5 5

这是我运行程序时的确切 I/O：

Enter the number of vertices: 2                                                                                                                                     
                                                                                                                                                                    
Please label the vertices ('A', 'B', 'C', etc.)                                                                                                                     
Label for vertex 1: A                                                                                                                                               
Label for vertex 2: B                                                                                                                                               
                                                                                                                                                                    
Enter two edges between OUTPUT i and i+1 vertices (e.g. 1 5): 1 5                                                                                                   
Enter the weight of this edge: 2                                                                                                                                    
Segmentation fault (core dumped)                                                                                                                                    
                                                                                                                                                                    
                                                                                                                                                                    
...Program finished with exit code 139                                                                                                                              
Press ENTER to exit console.

Answer 1

graph.a 有两个元素，但 tail 设置为 5 和 graph.a[tail] 是越界访问。它会导致未定义的行为。

您应该避免动态内存分配并改用std::vector。 std::vector 提供std::vector::at 来帮助查找这些错误。你用性能来支付它。 std::vector::at 增加了范围检查，速度有点慢，但搜索错误会让你变慢。

#include <iostream>
#include <string>
#include <vector>

struct AdjListNode {
  int dest;
  std::string label;
  int weight;

  AdjListNode* next;
};

struct AdjList {
  AdjListNode *head;
};

class Graph {
  private:

    int num;
    std::vector<AdjList> array;

  public:
    std::vector<std::string> a;

    Graph(int);

    void addEdge(int, int, int, std::string, std::string);
    void label(std::string,int);
    void output();

    AdjListNode* newAdjListNode(int point, int weight, std::string labelname) {
      AdjListNode* newNode = new AdjListNode;

      newNode->dest = point;
      newNode->weight = weight;
      newNode->label = labelname;
      newNode->next = nullptr;

      return newNode;
    }
};

int main() {
  int num, distance, head, tail;
  std::string input;

  std::cout << "Enter the number of vertices: ";
  std::cin >> num;

  Graph graph(num);

  std::cout << "\nPlease label the vertices ('A', 'B', 'C', etc.)\n";

  for (int i = 0; i < num; i++) {
    std::cout << "Label for vertex " << (i+1) << ": ";
    std::cin >> input;

    graph.label(input, i);
  }

  for (int i = 0; i < num; i++) {
    std::cout << "\nEnter two edges between " << "OUTPUT i and i+1 vertices (e.g. 1 5): ";
    std::cin >> head >> tail;

    std::cout << "Enter the weight of this edge: ";
    std::cin >> distance;

    graph.addEdge(head, tail, distance, graph.a.at(head), graph.a.at(tail));
  }

  graph.output();

  return 0;
}

Graph::Graph(int num) : array(num), a(num) {
  this->num = num;

  for (int i = 0; i < num; ++i) {
    array.at(i).head = nullptr;
  }
}

void Graph::addEdge(int a, int b, int weight, std::string bname, std::string tname) {
  if( a >= num || b >= num || a < 0 || b < 0) {
    std::cout << " Input is invalid\n";
  } else {
    AdjListNode* newNode = newAdjListNode(b,weight, tname);

    newNode->next = array.at(a).head;
    array.at(a).head = newNode;
    
    newNode = newAdjListNode(a,weight, bname);
    newNode->next = array.at(b).head;
    array.at(b).head = newNode;
  }
}

void Graph::label(std::string b, int i) {
  a.at(i) = b;
}

void Graph::output() {
  std::string input;

  for (int i = 0; i < num; i++) {
    AdjListNode* list = array.at(i).head;
    std::cout << "\nAdjacency list of the vertex " << i << " with labeled name: " << a.at(i);

    while (list) {
      std::cout << list->dest << " with the labeled name " << list->label << " and the weight being " << list->weight << "\n";
      list = list->next;
    }

    std::cout << "\n";
  }
}

抛出异常：

terminate called after throwing an instance of 'std::out_of_range'
  what():  vector::_M_range_check: __n (which is 5) >= this->size() (which is 2)