[section:facade Iterator Facade] 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. [h2 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. [table Core Interface [ [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. [h2 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. `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. [h2 `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://www.open-std.org/JTC1/SC22/WG21/docs/papers/2003/n1550.htm .. _`issue 299`: http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#299 .. _`operator arrow`: [h2 `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. [section:facade_reference Reference] template < class Derived , class Value , class CategoryOrTraversal , class Reference = Value& , class Difference = ptrdiff_t > class iterator_facade { public: typedef remove_const::type value_type; typedef Reference reference; typedef Value\* pointer; typedef Difference difference_type; typedef /* see below__ \*/ iterator_category; reference operator\*() const; /* see below__ \*/ operator->() const; /* see below__ \*/ operator[](difference_type n) const; Derived& operator++(); Derived operator++(int); Derived& operator--(); Derived operator--(int); Derived& operator+=(difference_type n); Derived& operator-=(difference_type n); Derived operator-(difference_type n) const; protected: typedef iterator_facade iterator_facade\_; }; // Comparison operators template typename enable_if_interoperable::type // exposition operator ==(iterator_facade const& lhs, iterator_facade const& rhs); template typename enable_if_interoperable::type operator !=(iterator_facade const& lhs, iterator_facade const& rhs); template typename enable_if_interoperable::type operator <(iterator_facade const& lhs, iterator_facade const& rhs); template typename enable_if_interoperable::type operator <=(iterator_facade const& lhs, iterator_facade const& rhs); template typename enable_if_interoperable::type operator >(iterator_facade const& lhs, iterator_facade const& rhs); template typename enable_if_interoperable::type operator >=(iterator_facade const& lhs, iterator_facade const& rhs); // Iterator difference template /* see below__ \*/ operator-(iterator_facade const& lhs, iterator_facade const& rhs); // Iterator addition template Derived operator+ (iterator_facade const&, typename Derived::difference_type n); template Derived operator+ (typename Derived::difference_type n, iterator_facade const&); __ `iterator category`_ __ `operator arrow`_ __ brackets_ __ minus_ .. _`iterator category`: The `iterator_category` member of `iterator_facade` is .. parsed-literal:: *iterator-category*\ (CategoryOrTraversal, reference, value_type) where *iterator-category* is defined as follows: .. include:: facade_iterator_category.rst The `enable_if_interoperable` template used above is for exposition purposes. The member operators should only be in an overload set provided the derived types `Dr1` and `Dr2` are interoperable, meaning that at least one of the types is convertible to the other. The `enable_if_interoperable` approach uses SFINAE to take the operators out of the overload set when the types are not interoperable. The operators should behave *as-if* `enable_if_interoperable` were defined to be: template enable_if_interoperable_impl {}; template enable_if_interoperable_impl { typedef T type; }; template struct enable_if_interoperable : enable_if_interoperable_impl< is_convertible::value || is_convertible::value , T > {}; [h2 Requirements] The following table describes the typical valid expressions on `iterator_facade`\ 's `Derived` parameter, depending on the iterator concept(s) it will model. The operations in the first column must be made accessible to member functions of class `iterator_core_access`. In addition, `static_cast(iterator_facade*)` shall be well-formed. In the table below, `F` is `iterator_facade`, `a` is an object of type `X`, `b` and `c` are objects of type `const X`, `n` is an object of `F::difference_type`, `y` is a constant object of a single pass iterator type interoperable with `X`, and `z` is a constant object of a random access traversal iterator type interoperable with `X`. .. _`core operations`: .. topic:: `iterator_facade` Core Operations [table Core Operations [ [Expression] [Return Type] [Assertion/Note] [Used to implement Iterator Concept(s)] ] [ [`c.dereference()`] [`F::reference`] [] [Readable Iterator, Writable Iterator] ] [ [`c.equal(y)`] [convertible to bool] [true iff `c` and `y` refer to the same position] [Single Pass Iterator] ] [ [`a.increment()`] [unused] [] [Incrementable Iterator] ] [ [`a.decrement()`] [unused] [] [Bidirectional Traversal Iterator] ] [ [`a.advance(n)`] [unused] [] [Random Access Traversal Iterator] ] [ [`c.distance_to(z)`] [convertible to `F::difference_type`] [equivalent to `distance(c, X(z))`.] [Random Access Traversal Iterator] ] ] [h2 Operations] The operations in this section are described in terms of operations on the core interface of `Derived` which may be inaccessible (i.e. private). The implementation should access these operations through member functions of class `iterator_core_access`. reference operator*() const; [*Returns:] `static_cast(this)->dereference()` operator->() const; (see below__) __ `operator arrow`_ [*Returns:] If `reference` is a reference type, an object of type `pointer` equal to: `&static_cast(this)->dereference()` Otherwise returns an object of unspecified type such that, `(*static_cast(this))->m` is equivalent to `(w = **static_cast(this), w.m)` for some temporary object `w` of type `value_type`. .. _brackets: *unspecified* operator[](difference_type n) const; [*Returns:] an object convertible to `value_type`. For constant objects `v` of type `value_type`, and `n` of type `difference_type`, `(*this)[n] = v` is equivalent to `*(*this + n) = v`, and `static_cast((*this)[n])` is equivalent to `static_cast(*(*this + n))` Derived& operator++(); [*Effects:] static_cast(this)->increment(); return *static_cast(this); Derived operator++(int); [*Effects:] Derived tmp(static_cast(this)); ++*this; return tmp; Derived& operator--(); [*Effects:] static_cast(this)->decrement(); return *static_cast(this); Derived operator--(int); [*Effects:] Derived tmp(static_cast(this)); --*this; return tmp; Derived& operator+=(difference_type n); [*Effects:] static_cast(this)->advance(n); return *static_cast(this); Derived& operator-=(difference_type n); [*Effects:] static_cast(this)->advance(-n); return *static_cast(this); Derived operator-(difference_type n) const; [*Effects:] Derived tmp(static_cast(this)); return tmp -= n; template Derived operator+ (iterator_facade const&, typename Derived::difference_type n); template Derived operator+ (typename Derived::difference_type n, iterator_facade const&); [*Effects:] Derived tmp(static_cast(this)); return tmp += n; template typename enable_if_interoperable::type operator ==(iterator_facade const& lhs, iterator_facade const& rhs); [*Returns:] [pre if `is_convertible::value` then `((Dr1 const&)lhs).equal((Dr2 const&)rhs)`. Otherwise, `((Dr2 const&)rhs).equal((Dr1 const&)lhs)`. ] template typename enable_if_interoperable::type operator !=(iterator_facade const& lhs, iterator_facade const& rhs); [*Returns:] [pre if `is_convertible::value` then `!((Dr1 const&)lhs).equal((Dr2 const&)rhs)`. Otherwise, `!((Dr2 const&)rhs).equal((Dr1 const&)lhs)`. ] template typename enable_if_interoperable::type operator <(iterator_facade const& lhs, iterator_facade const& rhs); [*Returns:] [pre if `is_convertible::value` then `((Dr1 const&)lhs).distance_to((Dr2 const&)rhs) < 0`. Otherwise, `((Dr2 const&)rhs).distance_to((Dr1 const&)lhs) > 0`. ] template typename enable_if_interoperable::type operator <=(iterator_facade const& lhs, iterator_facade const& rhs); [*Returns:] [pre if `is_convertible::value` then `((Dr1 const&)lhs).distance_to((Dr2 const&)rhs) <= 0`. Otherwise, `((Dr2 const&)rhs).distance_to((Dr1 const&)lhs) >= 0`. ] template typename enable_if_interoperable::type operator >(iterator_facade const& lhs, iterator_facade const& rhs); [*Returns:] [pre if `is_convertible::value` then `((Dr1 const&)lhs).distance_to((Dr2 const&)rhs) > 0`. Otherwise, `((Dr2 const&)rhs).distance_to((Dr1 const&)lhs) < 0`. ] template typename enable_if_interoperable::type operator >=(iterator_facade const& lhs, iterator_facade const& rhs); [*Returns:] [pre if `is_convertible::value` then `((Dr1 const&)lhs).distance_to((Dr2 const&)rhs) >= 0`. Otherwise, `((Dr2 const&)rhs).distance_to((Dr1 const&)lhs) <= 0`. ] .. _minus: template typename enable_if_interoperable::type operator -(iterator_facade const& lhs, iterator_facade const& rhs); [*Return Type:] [pre if `is_convertible::value` then `difference` shall be `iterator_traits::difference_type`. Otherwise `difference` shall be `iterator_traits::difference_type` ] [*Returns:] [pre if `is_convertible::value` then `-((Dr1 const&)lhs).distance_to((Dr2 const&)rhs)`. Otherwise, `((Dr2 const&)rhs).distance_to((Dr1 const&)lhs)`. ] [endsect] [include facade_tutorial.qbk] [endsect]