Pointer to Pointer to Structure (Priority Queue) - c++

Question

I'm a beginner (in C++, I'm coming from C (6 months experience)) and I'm trying to create a priority queue, but something is not working.
When I start the program and compile it, there are no errors. But there is nothing printed on the screen and the program crashes.
So here is the code:
PriorityQueue.h
using namespace std;
class PriorityQueue{
private:
struct pqentry_t{
string value;
float priority;
};
pqentry_t **_pqentry;
int _size;
int _next;
public:
PriorityQueue();
~PriorityQueue();
void insert(string value, float priority);
void printQueue();
};
PriorityQueue.cpp
#include <iostream>
#include <string>
#include "PriorityQueue.h"
#define SIZE 8
using namespace std;
PriorityQueue::PriorityQueue(){
_size = SIZE;
_next = 0;
_pqentry = new pqentry_t*[_size];
}
PriorityQueue::~PriorityQueue(){
delete[] _pqentry;
}
void PriorityQueue::insert(string value, float priority){
_pqentry[_next]->value = value; // this is probably not working
_pqentry[_next]->priority = priority;
_next++;
}
void PriorityQueue::printQueue(){
for (int i = 0; i < _next; i++){
cout << "Value: " << _pqentry[i]->value << endl
<< "Priority: " << _pqentry[i]->priority << endl;
}
cout << endl;
}
main.cpp
#include <iostream>
#include <string>
#include "PriorityQueue.h"
using namespace std;
int main()
{
PriorityQueue pq;
pq.insert("Banana", 23);
pq.printQueue();
}
I think, I know where the error is, in the PriorityQueue.cpp, here:
_pqentry[_next]->value = value;
_pqentry[_next]->priority = priority;
But I don't know what's wrong and I can't fix it. The compiler says there are no errors.
I hope, you can help me. Thanks in advance!

Answer 1

You did allocate the _pqentry member but you need to allocate each entry of this array as well, for example:
_pqentry[_next] = new pqentry_t;
before writing to it.
And do not forget to delete those :)

Answer 2

It looks like you are creating an array of pointers to pqentry_t in your constructor, but your insert method is expecting it to be an array of _pqentry structures themselves. You are not allocating space for the pqentry_t elements themselves and so when you try to dereference them in your insert method, the program crashes.
Try changing the definition of _pqentry in the class to pqentry_t *_pqentry and the allocation in the constructor to new pqentry_t[size]. This will allow your insert and printQueue methods to access the entries of _pqentry as they are written.