Q: How do I compare strings to see which comes earlier in ABC order?
A: C++ string objects support the standard comparison operators like <, <=, >, >=, ==, and != .
Q: How can I implement operator << for printing a priority queue? It seems like the operator would need access to the private data inside of the priority queue object.
A: The << operator in our assignment is declared with a special keyword called friend that makes it so that this operator is able to directly access the private data inside the priority queue if needed.
Q: What am I supposed to do in a destructor?
A: Free up any dynamic memory that you previously allocated with the new keyword.
Q: What is the difference between a destructor and the clear method? Don't they do the same thing, deleting all elements from the queue?
A: A clear method is called explicitly when a client wants to wipe the elements and then start over and use the same list to store something else. A destructor is called implicitly by C++ when an object is being thrown away forever; it won't ever be used to store anything else after that. The implementations might be similar, but their external purpose is different.
Q: If I have an array priority queue storing {"a":1, "b":4, "b":1, "c":3}, and the client calls pq.changePriority("b", 2); , which "b" should I change? "b":4 or "b":1?
A: It should be the first occurrence you find when traversing the array from left-to-right. So the "b":4 one is the one to change in that case.
Q: What is the difference between PQNode and PQNode* ? Which one should I use?
A: You literally never want to create a variable of type PQNode ; you want only PQNode* . The former is an object, the latter is a pointer to an object. You always want pointers to PQNode objects in this assignment because objects created with new live longer; they are not cleaned up when the current function exits.
Q: How do I declare a PQNode?
A: Since the linked list PQ needs to keep its memory around dynamically, you should use the new keyword. Like this: 
PQNode* node = new PQNode();

Or, you can pass any of the value, priority, and next values on construction: 

PQNode* node = new PQNode(value, priority, next);

You *must* declare all your nodes as pointers; do not declare them as regular objects like the following code, because it will break when the list node goes out of scope: 

// do not do this!
PQNode node;           // bad bad bad
node.data = 42;        // bad bad bad
node.next = nullptr;   // bad bad bad
...
Q: For the linked list PQ, how do I make sure that the strings stay in sorted order?
A: You have to do this yourself. You can compare strings to each other using the standard comparison operators: >= =< > < == !=
Q: Whenever I try to test my linked list PQ, the program "unexpectedly finishes." Why?
A: This is a very common bug when using pointers. It means you tried to dereference (->) a null pointer or garbage pointer. Run the program in Debug Mode (F5) and find out the exact line number of the error. Then trace through the code, draw pictures, and try to figure out how the pointer on that line could be NULL or garbage. Often it involves something like checking a value like current->next->data before checking whether current or current->next are NULL.
Q: Is there a difference between "deleting" and "freeing" memory?
A: We use the terms somewhat interchangeably. But what we mean is that you must call 'delete' on your dynamically allocated memory. There is a function named 'free' in C++, but we don't want you to use that.