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Thinking in C++
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Cleanup responsibility with pointers

To make the Stash and Stack containers flexible (able to hold any type of object), they will hold void pointers. This means that when a pointer is returned from the Stash or Stack object, you must cast it to the proper type before using it; as seen above, you must also cast it to the proper type before deleting it or you’ll get a memory leak.

The other memory leak issue has to do with making sure that delete is actually called for each object pointer held in the container. The container cannot “own” the pointer because it holds it as a void* and thus cannot perform the proper cleanup. The user must be responsible for cleaning up the objects. This produces a serious problem if you add pointers to objects created on the stack and objects created on the heap to the same container because a delete-expression is unsafe for a pointer that hasn’t been allocated on the heap. (And when you fetch a pointer back from the container, how will you know where its object has been allocated?) Thus, you must be sure that objects stored in the following versions of Stash and Stack are made only on the heap, either through careful programming or by creating classes that can only be built on the heap.

It’s also important to make sure that the client programmer takes responsibility for cleaning up all the pointers in the container. You’ve seen in previous examples how the Stack class checks in its destructor that all the Link objects have been popped. For a Stash of pointers, however, another approach is needed.

Thinking in C++
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire