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.
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