C++ - Array of struct pointers - c++

Question

I have a small program in which I have two structs:
Person - Consists of id, and basic methods
Ppl - Consists of an array of people with some methods to operate on the array.
struct Person {
const int id;
Person();
Person(int a);
};
Person::Person(int a) : id(a) {}
This is the Person struct with its methods.
const int MAX = 5;
Sets limit on array length
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
}
}
The main function is:
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;
}
The output that I get is:
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
Why is it adding all new entries to the beginning of the array while keeping the rest NULL?
Why isn't it detecting that 21 was already added.
I have been going crazy trying to figure this out. Any help would be much appreciated.
Thanks a lot community.
EDIT
I have fixed it so that it adds the elements to the array and recognizes when an id exists.
I made changes to the Ppl struct by adding a destructor:
Ppl::~Ppl() {
int i = 0;
while (i < MAX) {
delete ppls[i];
i++;
}
}
and by changing the add method.
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++;
}
}
So the output now is
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)
What is a segmentation fault and how can I fix it?
Thanks again

Answer 1

Person p(a);
ppls[current] = &p;
is a problem. You are storing a pointer to a local variable. Your program is subject to undefined behavior.
Use
Person* p = new Person(a);
ppls[current] = p;
Make sure to delete the objects in the destructor of Ppl.
Suggestion for improvement
It's not clear what's your objective with this program but you can make your life a lot simpler by using
std::vector<Person> ppls;
instead of
Person *ppls[MAX];