adding smart pointers lab

labs/13_smart_memory/README.md

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+# Lab 12 — Garbage Collection & Smart Pointers
+
+## Checkpoint 1:
+
+*estimate: 20-40 minutes*
+
+For the first checkpoint, download, compile, and run these files: [stop_and_copy.h](stop_and_copy.h), [stop_and_copy.cpp](stop_and_copy.cpp), [main_stop_and_copy.cpp](main_stop_and_copy.cpp) 
+
+In Lecture 25, we stepped through the Stop and Copy garbage collection algorithm on a small example.
+Examine the output of the program to see a computer simulation of this same example. Verify that the
+program behaves as we predicted in lecture.
+
+Continuing with the same example, 3 more nodes have been added and the garbage collector must be run
+again. Draw a “box and pointer” diagram (the more usual human-friendly version of the interconnected
+node structure with few or no crossing pointer arrows) of the memory accessible from the root pointer after
+these 3 nodes are added and work through the Stop and Copy algorithm for this example on paper. When
+you are finished, uncomment the simulation and check your work.
+
+**To complete this checkpoint**: Show (and explain to) one of the TAs your box and pointer diagram.
+
+## 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](ds_smart_pointers.h) and [main_smart_pointers.cpp](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-rewrite 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.