C++ - 结构指针数组

Question

我有一个小程序，其中有两个结构： Person - 由 id 和基本方法组成 Ppl - 由一组人组成，他们有一些操作数组的方法。

struct Person {
 const int id;
 Person();
 Person(int a);
};

Person::Person(int a) : id(a) {}

这是 Person 结构及其方法。

const int MAX = 5;

设置数组长度限制

struct Ppl {
 static int current;   //Keeps track of current position in array
 Person *ppls[MAX];    //Creates an array of Person structure pointers
 void add(int a);      //Adds a person with id to the next available position
 //void remove(int b);
 int searchid(int c);  //Searches ppls for an id c.
 Ppl();                //Constructor
};

int Ppl::current = 0;  //Initializing static variable

void Ppl::add(int a) {
 int ret = searchid(a);          //Determine if id a already exists in ppls
 //cout << "Ret: " << ret << endl;
 if (ret > MAX) {                //If value returned is > MAX, value exists
  cout << "User " << a << " already exists" << endl;
 } else {                        //else, add it to the next available spot
  Person p(a);
  ppls[current] = &p;
  cout << "Added user: " << a << " at index: " << current << endl;
  current++;
 }
}

Ppl::Ppl() {
 current = 0;
 int i = 0;
 while (i < MAX) {    //Sets all Person pointers to NULL on initialization
  ppls[i] = NULL;
  cout << "ppls[" << i << "] is NULL" << endl;
  i++;
 }
}

int Ppl::searchid(int c) {
 int i = 0;
 bool found = false;
 while(i < MAX) {
  if (ppls[i] == NULL) {  //If NULL, then c wasn't found in array because
   break;                 //there is no NULL between available spots.
  } else {
    if (ppls[i]->id == c) {
     found = true;
   }
  }
  i++;
 }
 if (found == true) {
  return 10;     //A number higher than MAX
 } else {
  return 1;      //A number lower than MAX
 }
}

主要功能是：

int main() {
 Ppl people;
 people.add(21);
 cout << people.ppls[0]->id << endl;
 people.add(7);
 cout << people.ppls[0]->id << " ";
 cout << people.ppls[1]->id << endl;
 people.add(42);
 cout << people.ppls[0]->id << " ";
 cout << people.ppls[1]->id << " ";
 cout << people.ppls[2]->id << endl;
 people.add(21);
 cout << people.ppls[0]->id << " ";
 cout << people.ppls[1]->id << " ";
 cout << people.ppls[2]->id << " ";
 cout << people.ppls[3]->id << endl;
}

我得到的输出是：

ppls[0] is NULL
ppls[1] is NULL
ppls[2] is NULL
ppls[3] is NULL
ppls[4] is NULL
Added user: 21 at index: 0
21
Added user: 7 at index: 1
7 0
Added user: 42 at index: 2
42 0 0
Added user: 21 at index: 3
21 0 0 0

为什么要将所有新条目添加到数组的开头，而将其余条目保持为 NULL？ 为什么没有检测到已经添加了 21。

我一直在疯狂地试图弄清楚这一点。任何帮助将非常感激。 非常感谢社区。​​p>

编辑 我已对其进行了修复，以便它将元素添加到数组中并识别 id 何时存在。

我通过添加析构函数对 Ppl 结构进行了更改：

Ppl::~Ppl() {
 int i = 0;
 while (i < MAX) {
  delete ppls[i];
  i++;
 }
}

并通过更改添加方法。

void Ppl::add(int a) {
 int ret = searchid(a);
 //cout << "Ret: " << ret << endl;
 if (ret > MAX) {
  cout << "User " << a << " already exists" << endl;
 } else {
  **Person *p = new Person(a);
  ppls[current] = p;**
  cout << "Added user: " << a << " at index: " << current << endl;
  current++;
 }
}

所以现在的输出是

ppls[0] is NULL
ppls[1] is NULL
ppls[2] is NULL
ppls[3] is NULL
ppls[4] is NULL
Added user: 21 at index: 0
21
Added user: 7 at index: 1
21 7
Added user: 42 at index: 2
21 7 42
User 21 already exists
Segmentation fault (core dumped)

什么是分段错误，我该如何解决？ 再次感谢

Answer 1

Person p(a);
ppls[current] = &p;

是个问题。您正在存储指向局部变量的指针。您的程序会受到未定义行为的影响。

使用

Person* p = new Person(a);
ppls[current] = p;

确保删除Ppl的析构函数中的对象。

改进建议

尚不清楚您使用此程序的目标是什么，但您可以通过使用使您的生活变得更简单

std::vector<Person> ppls;

而不是

Person *ppls[MAX];