Question

Assume i have

struct node{
  int val;
  node* left;
};

Now, I have a priority_queue<node> sth;
What does the following do:

node temp = sth.top();// does it perform copy
sth.pop();
temp.left = sth.top(); // is this allowed?

How do I pop an element from the queue and store it in temp.left?

Answer 1

node temp = sth.top();// does it perform copy

Yes, this makes a copy of the top element and stores it in temp. temp.left points to the same place as top node’s left pointer. Note that top() actually returns a reference but this line asks for a copy. node &temp = sth.top(); would declare a reference to the top element.

sth.pop();

Removes the top node from the queue. You may have just broken your data structure here. Any pointers to that node object (e.g., left pointers of other nodes in the queue) are now invalid.

temp.left = sth.top(); // is this allowed?

No, this won’t compile. temp.left is a pointer and top() returns a reference to an object. At a minimum, you need to get the address of that object: temp.left = &sth.top();