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indirect_iterator
adapts
an iterator by applying an extra dereference
inside of operator*()
.
For example, this iterator adaptor makes it possible to view a container
of pointers (e.g. list<foo*>
)
as if it were a container of the pointed-to type (e.g. list<foo>
). indirect_iterator
depends on two auxiliary traits, pointee
and indirect_reference
, to
provide support for underlying iterators whose value_type
is not an iterator.
This example prints an array of characters, using indirect_iterator
to access the array of characters through an array of pointers. Next indirect_iterator
is used with the transform
algorithm to copy the characters
(incremented by one) to another array. A constant indirect iterator is used
for the source and a mutable indirect iterator is used for the destination.
The last part of the example prints the original array of characters, but
this time using the make_indirect_iterator
helper function.
char characters[] = "abcdefg"; const int N = sizeof(characters)/sizeof(char) - 1; // -1 since characters has a null char char* pointers_to_chars[N]; // at the end. for (int i = 0; i < N; ++i) pointers_to_chars[i] = &characters[i]; // Example of using indirect_iterator boost::indirect_iterator<char**, char> indirect_first(pointers_to_chars), indirect_last(pointers_to_chars + N); std::copy(indirect_first, indirect_last, std::ostream_iterator<char>(std::cout, ",")); std::cout << std::endl; // Example of making mutable and constant indirect iterators char mutable_characters[N]; char* pointers_to_mutable_chars[N]; for (int j = 0; j < N; ++j) pointers_to_mutable_chars[j] = &mutable_characters[j]; boost::indirect_iterator<char* const*> mutable_indirect_first(pointers_to_mutable_chars), mutable_indirect_last(pointers_to_mutable_chars + N); boost::indirect_iterator<char* const*, char const> const_indirect_first(pointers_to_chars), const_indirect_last(pointers_to_chars + N); std::transform(const_indirect_first, const_indirect_last, mutable_indirect_first, std::bind1st(std::plus<char>(), 1)); std::copy(mutable_indirect_first, mutable_indirect_last, std::ostream_iterator<char>(std::cout, ",")); std::cout << std::endl; // Example of using make_indirect_iterator() std::copy(boost::make_indirect_iterator(pointers_to_chars), boost::make_indirect_iterator(pointers_to_chars + N), std::ostream_iterator<char>(std::cout, ",")); std::cout << std::endl;
The output is:
a,b,c,d,e,f,g, b,c,d,e,f,g,h, a,b,c,d,e,f,g,
The source code for this example can be found here.
template < class Iterator , class Value = use_default , class CategoryOrTraversal = use_default , class Reference = use_default , class Difference = use_default > class indirect_iterator { public: typedef /* see below */ value_type; typedef /* see below */ reference; typedef /* see below */ pointer; typedef /* see below */ difference_type; typedef /* see below */ iterator_category; indirect_iterator(); indirect_iterator(Iterator x); template < class Iterator2, class Value2, class Category2 , class Reference2, class Difference2 > indirect_iterator( indirect_iterator< Iterator2, Value2, Category2, Reference2, Difference2 > const& y , typename enable_if_convertible<Iterator2, Iterator>::type* = 0 // exposition ); Iterator const& base() const; reference operator*() const; indirect_iterator& operator++(); indirect_iterator& operator--(); private: Iterator m_iterator; // exposition };
The member types of indirect_iterator
are defined according to the following pseudo-code, where V
is iterator_traits<Iterator>::value_type
if (Value is use_default) then typedef remove_const<pointee<V>::type>::type value_type; else typedef remove_const<Value>::type value_type; if (Reference is use_default) then if (Value is use_default) then typedef indirect_reference<V>::type reference; else typedef Value& reference; else typedef Reference reference; if (Value is use_default) then typedef pointee<V>::type* pointer; else typedef Value* pointer; if (Difference is use_default) typedef iterator_traits<Iterator>::difference_type difference_type; else typedef Difference difference_type; if (CategoryOrTraversal is use_default) typedef iterator-category ( iterator_traversal<Iterator>::type,reference
,value_type
) iterator_category; else typedef iterator-category ( CategoryOrTraversal,reference
,value_type
) iterator_category;
The expression *v
,
where v
is an object of
iterator_traits<Iterator>::value_type
, shall be valid expression and
convertible to reference
.
Iterator
shall model the
traversal concept indicated by iterator_category
.
Value
, Reference
,
and Difference
shall be chosen
so that value_type
, reference
, and difference_type
meet the requirements indicated by iterator_category
.
Note: there are further requirements on the iterator_traits<Iterator>::value_type
if the Value
parameter is
not use_default
, as implied
by the algorithm for deducing the default for the value_type
member.
In addition to the concepts indicated by iterator_category
and by iterator_traversal<indirect_iterator>::type
,
a specialization of indirect_iterator
models the following concepts, Where v
is an object of iterator_traits<Iterator>::value_type
:
Readable Iterator if reference(*v)
is convertible to value_type
.
Writable Iterator if reference(*v)
= t
is a valid expression (where t
is an object of type indirect_iterator::value_type
)
Lvalue Iterator if reference
is a reference type.
indirect_iterator<X,V1,C1,R1,D1>
is interoperable with indirect_iterator<Y,V2,C2,R2,D2>
if and only if X
is interoperable
with Y
.
In addition to the operations required by the concepts described above, specializations
of indirect_iterator
provide
the following operations:
indirect_iterator();
Requires: Iterator
must be Default Constructible.
Effects:
Constructs an instance of indirect_iterator
with a default-constructed m_iterator
.
indirect_iterator(Iterator x);
Effects: Constructs an instance of indirect_iterator
with m_iterator
copy constructed from x
.
template < class Iterator2, class Value2, unsigned Access, class Traversal , class Reference2, class Difference2 > indirect_iterator( indirect_iterator< Iterator2, Value2, Access, Traversal, Reference2, Difference2 > const& y , typename enable_if_convertible<Iterator2, Iterator>::type* = 0 // exposition );
Requires: Iterator2
is implicitly convertible to Iterator
.
Effects: Constructs an instance of indirect_iterator
whose m_iterator
subobject is constructed from y.base()
.
Iterator const& base() const;
Returns: m_iterator
reference operator*() const;
Returns: **m_iterator
indirect_iterator& operator++();
Effects: ++m_iterator
Returns:
*this
indirect_iterator& operator--();
Effects: --m_iterator
Returns:
*this