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Lab 12 — Heap Sort & Smart Pointers
Checkpoint 0:
If you haven’t done so already, please complete your course evaluation for Data Structures (your honest & anonymous feedback is very important!). Have the webpage receipt saying "completed" open in your browser to receive credit for Checkpoint 0.
Checkpoint 1:
Two youtube videos will be provided when the lab starts. Both demonstrate the heap sort algorithm, please determine which one is better. There is only one of them whose runtime complexity is O(n logn), what is the run time complexity of the other one?
To complete this checkpoint, Answer the question and explain to the TA/mentor what the problem exists in the bad one.
Checkpoint 2:
estimate: 20-40 minutes
The theme for this checkpoint are the helium filled balloons for the Macy’s Thanksgiving Day parade. These balloons are held in place by one or more ropes held by people on the ground. Alternately, balloons may be connected to other balloons that are held by people! People can swap which balloon they are holding on to, but if everyone holding on to the ropes for a particular balloon lets go, we will have a big problem! Download, compile, and run these files: ds_smart_pointers.h and main_smart_pointers.cpp. Use Dr. Memory or Valgrind to inspect the initial code for memory errors and/or memory leaks.
Carefully examine the example allocations in the main function of the provided code. Draw simple pictures to help keep track of which objects have been allocated with new, and which variables currently point to each dynamically allocated object.
To fix the memory leaks in this program, you will need to add explicit deallocation for the non-smart pointer examples (marked CHECKPOINT 2A and 2B). For comparison, the code includes simple examples of smart pointers for these objects as well! When we know that just one person will hold on to a Balloon at a time (one owner) we can use a dsAutoPtr (see also the STL auto_ptr or STL unique_ptr, depending on your version of g++/clang/STL). When multiple people might hold ropes to the same Balloon, we should use a dsSharedPtr (see also STL shared_ptr or Boost shared_ptr). A shared pointer uses reference counting! When the last person disconnects from a Balloon using a shared pointer, the Balloon is automatically deleted.
Re-compile & re-run with the memory debugger to confirm you have fixed the simple leaks. For the final piece of this checkpoint (marked CHECKPOINT 2C), you must also re-write the interconnected balloon example to use shared pointers. You will need to modify the Balloon class to use dsSharedPointer as well.
To complete this checkpoint: Explain to your TA the code you needed to add and/or modify to correct the memory leaks in the provided code. Show your TA the result of the memory debugger on your finished implementation.
Checkpoint 3:
estimate: 20-40 minutes
For the last checkpoint, let’s consider cyclic balloon structures. A simple example is included at the bottom of main_smart_pointer.cpp. Draw a diagram of this structure on paper. The provided code has memory leaks for this example. We could try to re-write this example to use the shared smart pointer. However, a reference counting smart pointer will still have a problem on this cyclic example. Why?
Instead, let’s write a helper function to explicitly deallocate a general cyclic structure of Balloon objects. (We will not use smart pointers for this checkpoint). You should switch back to the original Balloon class (if you modified it for Checkpoint 2). You will first need to collect all nodes that are reachable from the provided argument. Make sure that you do not enter an infinite loop when tracing through the structure! You may find std::set helpful. Once you have identified all of the nodes that are accessible from the input argument, you can call delete on each Node. Write additional test cases to confirm that your code is debugged.
To complete this checkpoint: Show the TA cyclic memory diagram, your debugged implementation, and the memory debugger output to confirm that your code has no memory errors or leaks.