indirect_iterator synopsis

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 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. We use the abbreviation V=iterator_traits<Iterator>::value_type.:

if (Value is use_default) then
    typedef iterator_traits<V>::value_type value_type;
else
    typedef remove_const<Value>::type value_type;

if (Reference is use_default) then
    if (Value is use_default) then
        typedef iterator_traits<V>::reference reference;
    else
        typedef Value& reference;
else
    typedef Reference reference;

if (Value is use_default) then
    typedef ?? pointer;
else
    typedef Value* pointer;

if (Difference is use_default)
    typedef iterator_traits<Iterator>::difference_type difference_type;
else
    typedef Difference difference_type;

The member iterator_category is a type that satisfies the requirements of the concepts modeled by the indirect_iterator as specified in the models section.

indirect_iterator requirements

The Iterator argument shall meet the requirements of Readable Iterator. The CategoryOrTraversal argument shall be one of the standard iterator tags or use_default. If CategoryOrTraversal is an iterator tag, the template parameter Iterator argument shall meet the traversal requirements corresponding to the iterator tag.

The expression *v, where v is an object of type iterator_traits<Iterator>::value_type, must be a valid expression and must be convertible to indirect_iterator::reference. Also indirect_iterator::reference must be convertible to indirect_iterator::value. 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.

indirect_iterator models

If CategoryOrTraversal is a standard iterator tag, indirect_iterator is a model of the iterator concept corresponding to the tag, otherwise indirect_iterator satisfies the requirements of the most refined standard traversal concept that is satisfied by the Iterator argument.

indirect_iterator models Readable Iterator. If indirect_iterator::reference(*v) = t is a valid expression (where t is an object of type indirect_iterator::value_type) then indirect_iterator models Writable Iterator. If indirect_iterator::reference is a reference then indirect_iterator models Lvalue Iterator.

indirect_iterator operations

In addition to the operations required by the concepts modeled by indirect_iterator, indirect_iterator provides the following operations.

indirect_iterator();

indirect_iterator(Iterator 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
);

Iterator base() const;

reference operator*() const;

indirect_iterator& operator++();

indirect_iterator& operator--();

Example

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.