+
+Header +boost/iterator_adaptors.hpp
+ +The file boost/iterator_adaptors.hpp +includes the main iterator_adaptors class and several other classes +for constructing commonly used iterator adaptors.
+ +-
+
- iterator_adaptors. +
- iterator_adaptor. +
- transform_iterator +
- indirect_iterators +
- reverse_iterators +
- integer_range +
Dave
+Abrahams started the library, coming up with the idea to use
+policy classes and how to handle the const/non-const iterator
+interactions. He also contributed the indirect_iterators and
+reverse_iterators classes.
+
+Jeremy Siek
+contributed transform_iterator, integer_range,
+and this documentation.
+
+
The Iterator Adaptors Class
+ +Implementing standard conforming iterators is a non-trivial task. +There are some fine-points such as iterator/const_iterator +interactions and there are the myriad of operators that should be +implemented but are easily forgotten such as +operator->(). The purpose of the +iterator_adaptors class is to make it easier to implement an +iterator class, and even easier to extend and adapt existing iterator +types. The iterator_adaptors class itself is not an adaptor +class but a type generator. It generates a pair of adaptor classes, +one class for the mutable iterator and one class for the const +iterator. The definition of the iterator_adaptors class is as +follows: + ++
+
+template <class Iterator,
+ class ConstIterator,
+ class Traits = std::iterator_traits<Iterator>,
+ class ConstTraits = std::iterator_traits<ConstIterator>,
+ class Policies = default_iterator_policies>
+struct iterator_adaptors
+{
+ typedef ... iterator;
+ typedef ... const_iterator;
+};
+ |
The Iterator and ConstIterator template parameters +are the iterator types that you want to adapt. The Traits and +ConstTraits must be iterator traits classes. The traits +parameters default to the specialization of the +std::iterator_traits class for the adapted iterators. If you +want the traits for your new iterator adaptor (value_type, +iterator_category, etc.) to be the same as the adapted +iterator then use the default, otherwise create your own traits +classes and pass them in [1]. + + +
The Policies class that you pass in will become the heart of +the iterator adaptor. The policy class determines how your new adaptor +class will behave. The Policies class must implement 3, 4, or +7 of the core iterator operations depending on whether you wish the +new iterator adaptor class to be a + +ForwardIterator, + +BidirectionalIterator, or +RandomAccessIterator. Make sure that the +iterator_category type of the traits class you pass in +matches the category of iterator that you want to create. The default +policy class, default_iterator_policies, implements all 7 of +the core operations in the usual way. If you wish to create an +iterator adaptor that only changes a few of the iterator's behaviors, +then you can have your new policy class inherit from +default_iterator_policies to avoid retyping the usual +behaviours. You should also look at default_iterator_policies +as the "boiler-plate" for your own policy classes. The +following is definition of the default_iterator_policies +class: + + +
+
+
+struct default_iterator_policies
+{
+ // required for a ForwardIterator
+ template <class Reference, class Iterator>
+ Reference dereference(type<Reference>, const Iterator& x) const
+ { return *x; }
+
+ template <class Iterator>
+ void increment(Iterator& x) const
+ { ++x; }
+
+ template <class Iterator1, class Iterator2>
+ bool equal(Iterator1& x, Iterator2& y) const
+ { return x == y; }
+
+ // required for a BidirectionalIterator
+ template <class Iterator>
+ void decrement(Iterator& x) const
+ { --x; }
+
+ // required for a RandomAccessIterator
+ template <class Iterator, class DifferenceType>
+ void advance(Iterator& x, DifferenceType n) const
+ { x += n; }
+
+ template <class Difference, class Iterator1, class Iterator2>
+ Difference distance(type<Difference>, Iterator1& x, Iterator2& y) const
+ { return y - x; }
+
+ template <class Iterator1, class Iterator2>
+ bool less(Iterator1& x, Iterator2& y) const
+ { return x < y; }
+};
+ |
+The generated iterator adaptor types will have the following +constructors. + +
+
++iterator(const Iterator& i, const Policies& p = Policies()) + +const_iterator(const ConstIterator& i, const Policies& p = Policies()) + |
The Iterator Adaptor Class
+ +This is the class used inside of the iterator_adaptors type +generator. Use this class directly (instead of using +iterator_adaptors) when there is no difference between the +const and non-const versions of the iterator type. Often this is +because there is only a const (read-only) version of the iterator, as +is the case for std::set's iterators. Use the same type for +the Iterator and NonconstIterator template +arguments. + ++
++template <class Iterator, + class Policies = default_iterator_policies, + class NonconstIterator = Iterator, + class Traits = std::iterator_traits<Iterator> > +struct iterator_adaptor; + |
+Next we will look at some iterator adaptors that are examples of how +to use the iterator adaptors class, and that are useful iterator +adaptors in their own right. + +
The Transform Iterator Class
+ +It is often useful to automatically apply some function to the value +returned by dereferencing (operator*()) an iterator. The +transform_iterators class makes it easy to create an iterator +adaptor that does just that. + +First let us consider what the Policies class for the transform +iterator should look like. We are only changing one of the iterator +behaviours, so we will inherit from +default_iterator_policies. In addition, we will need a +function object to apply, so we will have a template parameter and a +data member for the function object. The function will take one +argument (the dereferenced value) and we will need to know the +result_type of the function, so +AdaptableUnaryFunction is the corrent concept to choose for the +function object type. Now for the heart of our iterator adaptor, we +implement the dereference method, applying the function +object to *i. The type<Reference> class is +there to tell you what the reference type of the iterator is, which is +handy when writing generic iterator adaptors such as this one [2]. + + ++
+
+ template <class AdaptableUnaryFunction>
+ struct transform_iterator_policies : public default_iterator_policies
+ {
+ transform_iterator_policies(const AdaptableUnaryFunction& f) : m_f(f) { }
+
+ template <class Reference, class Iterator>
+ Reference dereference(type<Reference>, const Iterator& i) const
+ { return m_f(*i); }
+
+ AdaptableUnaryFunction m_f;
+ };
+ |
+
+
+ template <class AdaptableUnaryFunction, class IteratorTraits>
+ struct transform_iterator_traits {
+ typedef typename AdaptableUnaryFunction::result_type value_type;
+ typedef value_type reference;
+ typedef value_type* pointer;
+ typedef typename IteratorTraits::difference_type difference_type;
+ typedef typename IteratorTraits::iterator_category iterator_category;
+ };
+ |
+
+
+template <class AdaptableUnaryFunction,
+ class Iterator,
+ class Traits = std::iterator_traits<Iterator>
+ >
+struct transform_iterator
+{
+ typedef transform_iterator_traits<AdaptableUnaryFunction,Traits>
+ TransTraits;
+ typedef iterator_adaptor<Iterator, TransTraits,
+ transform_iterator_policies<AdaptableUnaryFunction> >::type type;
+};
+ |
+The following is a simple example of how to use the +transform_iterators class to iterate through a range of +numbers, multiplying each of them by 2 when they are dereferenced. + +
+
+
+#include <functional>
+#include <iostream>
+#include <boost/iterator_adaptors.hpp>
+
+int
+main(int, char*[])
+{
+ int x[] = { 1, 2, 3, 4, 5, 6, 7, 8 };
+
+ typedef std::binder1st< std::multiplies<int> > Function;
+ typedef boost::transform_iterator<Function, int*,
+ boost::iterator<std::random_access_iterator_tag, int>
+ >::type doubling_iterator;
+
+ doubling_iterator i(x, std::bind1st(std::multiplies<int>(), 2)),
+ i_end(x + sizeof(x)/sizeof(int), std::bind1st(std::multiplies<int>(), 2));
+
+ std::cout << "multiplying the array by 2:" << std::endl;
+ while (i != i_end)
+ std::cout << *i++ << " ";
+ std::cout << std::endl;
+
+ return 0;
+}
+ |
The Indirect Iterators Class
+ +It is not all that uncommon to create data structures that consist of +pointers to pointers. For such a structure it might be nice to have an +iterator that applies a double-dereference inside the +operator*(). The implementation of this is similar to the +transform_iterators[3]. We first create a +policies class which does a double-dereference in the +dereference() method. We then create a traits class, this +time also including a template parameter for the traits of the second +level iterators as well as the first. Lastly we wrap this up in the +type generator indirect_iterators, using +iterator_adaptors to do most of the work. + ++
+
+ struct indirect_iterator_policies : public default_iterator_policies
+ {
+ template <class Reference, class Iterator>
+ Reference dereference(type<Reference>, const Iterator& x) const
+ { return **x; }
+ };
+
+ template <class IndirectIterator,
+ class IndirectTraits = std::iterator_traits<IndirectIterator>,
+ class Traits =
+ std::iterator_traits<typename IndirectTraits::value_type>
+ >
+ struct indirect_traits
+ {
+ typedef typename IndirectTraits::difference_type difference_type;
+ typedef typename Traits::value_type value_type;
+ typedef typename Traits::pointer pointer;
+ typedef typename Traits::reference reference;
+ typedef typename IndirectTraits::iterator_category iterator_category;
+ };
+
+ template <class IndirectIterator, class ConstIndirectIterator,
+ class IndirectTraits =
+ std::iterator_traits<IndirectIterator>,
+ class ConstIndirectTraits =
+ std::iterator_traits<ConstIndirectIterator>,
+ class Traits =
+ std::iterator_traits<typename IndirectTraits::value_type>
+ >
+ struct indirect_iterators
+ {
+ typedef typename IndirectTraits::value_type Iterator;
+ typedef typename Traits::value_type ValueType;
+ typedef iterator_adaptors<IndirectIterator, ConstIndirectIterator,
+ indirect_traits<IndirectIterator, IndirectTraits, Traits>,
+ indirect_traits<ConstIndirectIterator, ConstIndirectTraits, Traits>,
+ indirect_iterator_policies
+ > Adaptors;
+ typedef typename Adaptors::iterator iterator;
+ typedef typename Adaptors::const_iterator const_iterator;
+ };
+ |
The Reverse Iterators Class
+ ++Yes, there is already a reverse_iterator adaptor class +defined in the C++ Standard, but using the iterator_adaptors +class we can re-implement this classic adaptor in a more succinct and +elegant fashion. Also, this makes for a good example of using +iterator_adaptors that is in familiar territory. + +
+The first step is to create the Policies class. As in the +std::reverse_iterator class, we need to flip all the +operations of the iterator. Increment will become decrement, advancing +by n will become retreating by n, etc. + +
+
+
+struct reverse_iterator_policies
+{
+ template <class Reference, class Iterator>
+ Reference dereference(type<Reference>, const Iterator& x) const
+ { return *boost::prior(x); }
+ // this is equivalent to { Iterator tmp = x; return *--tmp; }
+
+ template <class Iterator>
+ void increment(Iterator& x) const
+ { --x; }
+
+ template <class Iterator>
+ void decrement(Iterator& x) const
+ { ++x; }
+
+ template <class Iterator, class DifferenceType>
+ void advance(Iterator& x, DifferenceType n) const
+ { x -= n; }
+
+ template <class Difference, class Iterator1, class Iterator2>
+ Difference distance(type<Difference>, Iterator1& x, Iterator2& y) const
+ { return x - y; }
+
+ template <class Iterator1, class Iterator2>
+ bool equal(Iterator1& x, Iterator2& y) const
+ { return x == y; }
+
+ template <class Iterator1, class Iterator2>
+ bool less(Iterator1& x, Iterator2& y) const
+ { return y < x; }
+};
+ |
+
+
+template <class Iterator, class ConstIterator,
+ class Traits = std::iterator_traits<Iterator>,
+ class ConstTraits = std::iterator_traits<ConstIterator>
+ >
+struct reverse_iterators
+{
+ typedef iterator_adaptors<Iterator,ConstIterator,Traits,ConstTraits,
+ reverse_iterator_policies> Adaptor;
+ typedef typename Adaptor::iterator iterator;
+ typedef typename Adaptor::const_iterator const_iterator;
+};
+ |
+
+
+class my_container {
+ ...
+ typedef ... iterator;
+ typedef ... const_iterator;
+
+ typedef reverse_iterators<iterator, const_iterator> RevIters;
+ typedef typename RevIters::iterator reverse_iterator;
+ typedef typename RevIters::const_iterator const_reverse_iterator;
+ ...
+};
+ |
The Integer Range Class
+ +The iterator_adaptors class can not only be used for adapting +iterators, but it can also be used to take a non-iterator type and use +it to build an iterator. An especially simple example of this is +turning an integer type into an iterator, a counting iterator. The +builtin integer types of C++ are almost iterators. They have +operator++(), operator--(), etc. The one operator +they are lacking is the operator*(), which we will want to +simply return the current value of the integer. The following few +lines of code implement the policy and traits class for the counting +iterator. + ++
+
+struct counting_iterator_policies : public default_iterator_policies
+{
+ template <class IntegerType>
+ IntegerType dereference(type<IntegerType>, const IntegerType& i) const
+ { return i; }
+};
+template <class IntegerType>
+struct counting_iterator_traits {
+ typedef IntegerType value_type;
+ typedef IntegerType reference;
+ typedef value_type* pointer;
+ typedef std::ptrdiff_t difference_type;
+ typedef std::random_access_iterator_tag iterator_category;
+};
+ |
+
+
+template <class IntegerType>
+struct integer_range {
+ typedef typename iterator_adaptor<IntegerType,
+ counting_iterator_traits<IntegerType>,
+ counting_iterator_policies >::type iterator;
+ typedef iterator const_iterator;
+ typedef IntegerType value_type;
+ typedef std::ptrdiff_t difference_type;
+ typedef IntegerType reference;
+ typedef IntegerType* pointer;
+ typedef IntegerType size_type;
+
+ integer_range(IntegerType start, IntegerType finish)
+ : m_start(start), m_finish(finish) { }
+
+ iterator begin() const { return iterator(m_start); }
+ iterator end() const { return iterator(m_finish); }
+ size_type size() const { return m_finish - m_start; }
+ bool empty() const { return m_finish == m_start; }
+ void swap(integer_range& x) {
+ std::swap(m_start, x.m_start);
+ std::swap(m_finish, x.m_finish);
+ }
+protected:
+ IntegerType m_start, m_finish;
+};
+ |
+The following is an example of how to use the +integer_range class to count from 0 to 4. + +
+
++boost::integer_range<int> r(0,5); + +cout << "counting to from 0 to 4:" << endl; +std::copy(r.begin(), r.end(), ostream_iterator<int>(cout, " ")); +cout << endl; + |
Challenge
+ ++There is an unlimited number of ways the the +iterator_adaptors class can be used to create iterators. One +interesting exercise would be to re-implement the iterators of +std::list and std::slist using +iterator_adaptors, where the adapted Iterator types +would be node pointers. + + +
Notes
+ ++[1] +If your compiler does not support partial specialization and hence +does not have a working std::iterator_traits class, you will +not be able to use the defaults and will need to supply your own +Traits and ConstTraits classes. + +
+[2] +The reference type could also be obtained from +std::iterator_traits, but that is not portable on compilers +that do not support partial specialization. + +
+[3] +It would have been more elegant to implement indirect_iterators +using transform_iterators, but for subtle reasons that would require +the use of boost::remove_cv which is not portable. + +
Implementation Notes
+ +The code is somewhat complicated because there are three iterator +adaptor class: forward_iterator_adaptor, +bidirectional_iterator_adaptor, and +random_access_iterator_adaptor. The alternative would be to +just have one iterator adaptor equivalent to the +random_access_iterator_adaptor. The reason for going with +the three adaptors is that according to 14.5.3p5 in the C++ Standard, +friend functions defined inside a template class body are instantiated +when the template class is instantiated. This means that if we only +used the one iterator adaptor, then if the adapted iterator did not +meet all of the requirements for a + +RandomAccessIterator then a compiler error should occur. Many +current compilers in fact do not instantiate the friend functions +unless used, so we could get away with the one iterator adaptor in +most cases. However, out of respect for the standard this implementation +uses the three adaptors. + + + ++
Revised 17 Jun 2000
+© Copyright Jeremy Siek 2000. Permission to copy, use, +modify, sell and distribute this document is granted provided this copyright +notice appears in all copies. This document is provided "as is" +without express or implied warranty, and with no claim as to its suitability for +any purpose.
+ + + + diff --git a/tie.html b/tie.html new file mode 100644 index 0000000..a583f84 --- /dev/null +++ b/tie.html @@ -0,0 +1,138 @@ + + + +
+
++ +
+tie +
+ ++
+template <class A, class B> +tied<A,B> tie(A& a, B& b); ++ +
+This is a utility function that makes it more convenient to work with +a function which returns a pair. The effect of the tie() +function is to allow the assignment of the two values of the pair to +two separate variables. The idea for this comes from Jaakko +Järvi's Binders [1]. + +
+ +
Where Defined
+ ++boost/utility.hpp + +
+ +
Example
+ ++An example of using the tie() function with the +vertices() function, which returns a pair of +type std::pair<vertex_iterator,vertex_iterator>. The +pair of iterators is assigned to the iterator variables i and +end. + +
+
+ graph_traits< adjacency_list<> >::vertex_iterator i, end; + for(tie(i,end) = vertices(G); i != end; ++i) + // ... ++ +
+Here is another example that uses tie() for handling +operaitons with std::set. + +
+
+#include <set>
+#include <algorithm>
+#include <iostream>
+#include <boost/utility.hpp>
+
+int
+main(int, char*[])
+{
+ {
+ typedef std::set<int> SetT;
+ SetT::iterator i, end;
+ bool inserted;
+
+ int vals[5] = { 5, 2, 4, 9, 1 };
+ SetT s(vals, vals + 5);
+
+ // Using tie() with a return value of pair<iterator,bool>
+
+ int new_vals[2] = { 3, 9 };
+
+ for (int k = 0; k < 2; ++k) {
+ boost::tie(i,inserted) = s.insert(new_vals[k]);
+ if (!inserted)
+ std::cout << *i << " was already in the set." << std::endl;
+ else
+ std::cout << *i << " successfully inserted." << std::endl;
+ }
+ }
+ {
+ int* i, *end;
+ int vals[6] = { 5, 2, 4, 4, 9, 1 };
+ std::sort(vals, vals + 6);
+
+ // Using tie() with a return value of pair<iterator,iterator>
+
+ boost::tie(i,end) = std::equal_range(vals, vals + 6, 4);
+ std::cout << "There were " << std::distance(i,end)
+ << " occurances of " << *i << "." << std::endl;
+ // Footnote: of course one would normally just use std::count()
+ // to get this information, but that would spoil the example :)
+ }
+ return 0;
+}
+
+The output is:
++ 3 successfully inserted. + 9 was already in the set. + There were 2 occurances of 4. ++ +
+
+
| Copyright © 2000 |
+Jeremy Siek,
+Univ.of Notre Dame (jsiek@lsc.nd.edu) +Lie-Quan Lee, Univ.of Notre Dame (llee1@lsc.nd.edu) +Andrew Lumsdaine, +Univ.of Notre Dame (lums@lsc.nd.edu) + |