.. Version 1.1 of this ReStructuredText document corresponds to n1530_, the paper accepted by the LWG for TR1. .. Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003. While the iterator interface is rich, there is a core subset of the interface that is necessary for all the functionality. We have identified the following core behaviors for iterators: * dereferencing * incrementing * decrementing * equality comparison * random-access motion * distance measurement In addition to the behaviors listed above, the core interface elements include the associated types exposed through iterator traits: ``value_type``, ``reference``, ``difference_type``, and ``iterator_category``. Iterator facade uses the Curiously Recurring Template Pattern (CRTP) [Cop95]_ so that the user can specify the behavior of ``iterator_facade`` in a derived class. Former designs used policy objects to specify the behavior, but that approach was discarded for several reasons: 1. the creation and eventual copying of the policy object may create overhead that can be avoided with the current approach. 2. The policy object approach does not allow for custom constructors on the created iterator types, an essential feature if ``iterator_facade`` should be used in other library implementations. 3. Without the use of CRTP, the standard requirement that an iterator's ``operator++`` returns the iterator type itself would mean that all iterators built with the library would have to be specializations of ``iterator_facade<...>``, rather than something more descriptive like ``indirect_iterator``. Cumbersome type generator metafunctions would be needed to build new parameterized iterators, and a separate ``iterator_adaptor`` layer would be impossible. Usage ----- The user of ``iterator_facade`` derives his iterator class from a specialization of ``iterator_facade`` and passes the derived iterator class as ``iterator_facade``\ 's first template parameter. The order of the other template parameters have been carefully chosen to take advantage of useful defaults. For example, when defining a constant lvalue iterator, the user can pass a const-qualified version of the iterator's ``value_type`` as ``iterator_facade``\ 's ``Value`` parameter and omit the ``Reference`` parameter which follows. The derived iterator class must define member functions implementing the iterator's core behaviors. The following table describes expressions which are required to be valid depending on the category of the derived iterator type. These member functions are described briefly below and in more detail in the iterator facade requirements. +------------------------+-------------------------------+ |Expression |Effects | +========================+===============================+ |``i.dereference()`` |Access the value referred to | +------------------------+-------------------------------+ |``i.equal(j)`` |Compare for equality with ``j``| +------------------------+-------------------------------+ |``i.increment()`` |Advance by one position | +------------------------+-------------------------------+ |``i.decrement()`` |Retreat by one position | +------------------------+-------------------------------+ |``i.advance(n)`` |Advance by ``n`` positions | +------------------------+-------------------------------+ |``i.distance_to(j)`` |Measure the distance to ``j`` | +------------------------+-------------------------------+ .. Should we add a comment that a zero overhead implementation of iterator_facade is possible with proper inlining? In addition to implementing the core interface functions, an iterator derived from ``iterator_facade`` typically defines several constructors. To model any of the standard iterator concepts, the iterator must at least have a copy constructor. Also, if the iterator type ``X`` is meant to be automatically interoperate with another iterator type ``Y`` (as with constant and mutable iterators) then there must be an implicit conversion from ``X`` to ``Y`` or from ``Y`` to ``X`` (but not both), typically implemented as a conversion constructor. Finally, if the iterator is to model Forward Traversal Iterator or a more-refined iterator concept, a default constructor is required. Iterator Core Access -------------------- ``iterator_facade`` and the operator implementations need to be able to access the core member functions in the derived class. Making the core member functions public would expose an implementation detail to the user. The design used here ensures that implementation details do not appear in the public interface of the derived iterator type. Preventing direct access to the core member functions has two advantages. First, there is no possibility for the user to accidently use a member function of the iterator when a member of the value_type was intended. This has been an issue with smart pointer implementations in the past. The second and main advantage is that library implementers can freely exchange a hand-rolled iterator implementation for one based on ``iterator_facade`` without fear of breaking code that was accessing the public core member functions directly. In a naive implementation, keeping the derived class' core member functions private would require it to grant friendship to ``iterator_facade`` and each of the seven operators. In order to reduce the burden of limiting access, ``iterator_core_access`` is provided, a class that acts as a gateway to the core member functions in the derived iterator class. The author of the derived class only needs to grant friendship to ``iterator_core_access`` to make his core member functions available to the library. .. This is no long uptodate -thw .. Yes it is; I made sure of it! -DWA ``iterator_core_access`` will be typically implemented as an empty class containing only private static member functions which invoke the iterator core member functions. There is, however, no need to standardize the gateway protocol. Note that even if ``iterator_core_access`` used public member functions it would not open a safety loophole, as every core member function preserves the invariants of the iterator. ``operator[]`` -------------- The indexing operator for a generalized iterator presents special challenges. A random access iterator's ``operator[]`` is only required to return something convertible to its ``value_type``. Requiring that it return an lvalue would rule out currently-legal random-access iterators which hold the referenced value in a data member (e.g. |counting|_), because ``*(p+n)`` is a reference into the temporary iterator ``p+n``, which is destroyed when ``operator[]`` returns. .. |counting| replace:: ``counting_iterator`` Writable iterators built with ``iterator_facade`` implement the semantics required by the preferred resolution to `issue 299`_ and adopted by proposal n1550_: the result of ``p[n]`` is an object convertible to the iterator's ``value_type``, and ``p[n] = x`` is equivalent to ``*(p + n) = x`` (Note: This result object may be implemented as a proxy containing a copy of ``p+n``). This approach will work properly for any random-access iterator regardless of the other details of its implementation. A user who knows more about the implementation of her iterator is free to implement an ``operator[]`` that returns an lvalue in the derived iterator class; it will hide the one supplied by ``iterator_facade`` from clients of her iterator. .. _n1550: http://anubis.dkuug.dk/JTC1/SC22/WG21/docs/papers/2003/n1550.html .. _`issue 299`: http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/lwg-active.html#299 .. _`operator arrow`: ``operator->`` -------------- The ``reference`` type of a readable iterator (and today's input iterator) need not in fact be a reference, so long as it is convertible to the iterator's ``value_type``. When the ``value_type`` is a class, however, it must still be possible to access members through ``operator->``. Therefore, an iterator whose ``reference`` type is not in fact a reference must return a proxy containing a copy of the referenced value from its ``operator->``. The return types for ``iterator_facade``\ 's ``operator->`` and ``operator[]`` are not explicitly specified. Instead, those types are described in terms of a set of requirements, which must be satisfied by the ``iterator_facade`` implementation. .. [Cop95] [Coplien, 1995] Coplien, J., Curiously Recurring Template Patterns, C++ Report, February 1995, pp. 24-27.