If you need to write an abstract for an academic or scientific paper, don't panic! Your abstract is simply a short, standalone summary of the work or paper that others can use as an overview.  An abstract describes what you do in your essay, whether it’s a scientific experiment or a literary analysis paper. It should help your reader understand the paper and help people searching for this paper decide whether it suits their purposes prior to reading. To write an abstract, finish your paper first, then type a summary that identifies the purpose, problem, methods, results, and conclusion of your work. After you get the details down, all that's left is to format it correctly. Since an abstract is only a summary of the work you've already done, it's easy to accomplish!
I often write or revise abstracts last. The ideal time for me is after working through the entire article or proposal. Not immediately after, when I'm tired and may be tempted to dash off something quick; but just long enough for it all to percolate and brew up a clear vision of what I've accomplished. If such vision appears, I just do my best-or possibly decide the article really lacks coherence and needs yet another thorough overhaul! If the vision does appear, I try to capture a good snapshot for the harried potential reader, hoping at least one of us will benefit.
The ItrBase class is the top class of the hierarchy. Itr is simply a wrapper on a pointer to an ItrBase, so that it can be allocated on the heap –the actual implementation of the class deriving from ItrBase can have an arbitrary size. Note how the Itr copy and assignment operators are implemented via the ItrBase::clone() method, so that Itr behaves as a scalar type. Last but not least, the (non-virtual) ItrBase::operator== equality operator first checks for type equality before calling the (virtual) equality method equal on the virtual subclass. The reason ItrBase is not a pure virtual is that it can conveniently be used to denote an empty range, ., the range (ItrBase(), ItrBase()) is empty.