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Boost Iterator Adaptor Library


The Iterator Adaptor library allows you transform an arbitrary ``base'' type into a standard-conforming iterator with the behaviors you choose. Doing so is especially easy if the ``base'' type is itself an iterator. The library also supplies several example adaptors which apply specific useful behaviors to arbitrary base iterators.

Backward Compatibility Note

The library's interface has changed since it was first released, breaking backward compatibility:

  1. Policies classes now operate on instances of the whole iterator_adaptor object, rather than just operating on the Base object. This change not only gives the policies class access to both members of a pair of interacting iterators, but also eliminates the need for the ugly type<Reference> and type<Difference> parameters to various policy functions.
  2. The Named Template Parameter interface has been made simpler, easier to use, and compatible with more compilers.

Other Documentation

``Policy Adaptors and the Boost Iterator Adaptor Library'' is a technical paper describing this library and the powerful design pattern on which it is based. It was presented at the C++ Template Workshop at OOPSLA 2001; the slides from the talk are available here. Please note that while the slides incorporate the minor interface changes described in the previous section, the paper does not.

Table of Contents

Dave Abrahams started the library, applying policy class technique and handling const/non-const iterator interactions. He also contributed the indirect_ and reverse_ iterator generators, and expanded counting_iterator_generator to cover all incrementable types. He edited most of the documentation, sometimes heavily.
Jeremy Siek contributed the transform iterator adaptor, the integer-only version of counting_iterator_generator, the function output iterator adaptor, and most of the documentation.
John Potter contributed the projection_ and filter_ iterator generators and made some simplifications to the main iterator_adaptor template.
Jens Maurer contributed the generator iterator adaptor.
Toon Knapen contributed the permutation iterator adaptor.
Ronald Garcia contributed the shared container iterator adaptor.

Class template iterator_adaptor

Implementing standard conforming iterators is a non-trivial task. There are some fine points such as the interactions between an iterator and its corresponding const_iterator, and there are myriad operators that should be implemented but are easily forgotten or mishandled, such as operator->(). Using iterator_adaptor, you can easily implement an iterator class, and even more easily extend and adapt existing iterator types. Moreover, it is easy to make a pair of interoperable const and non-const iterators.

iterator_adaptor is declared like this:

template <class Base, class Policies, 
    class ValueOrNamedParam = typename std::iterator_traits<Base>::value_type,
    class ReferenceOrNamedParam = ...(see below),
    class PointerOrNamedParam = ...(see below),
    class CategoryOrNamedParam = typename std::iterator_traits<Base>::iterator_category,
    class DistanceOrNamedParam = typename std::iterator_traits<Base>::difference_type>
struct iterator_adaptor;

Template Parameters

Although iterator_adaptor takes seven template parameters, defaults have been carefully chosen to minimize the number of parameters you must supply in most cases, especially if Base is an iterator.
Parameter Description Requirements
Base The data type on which the resulting iterator is based. Do not be misled by the name "Base": this is not a base class. Assignable, Default Constructible
Policies A policy class that supplies core functionality to the resulting iterator. See table below.
Value The value_type of the resulting iterator, unless const. If Value is const X the value_type will be (non-const) X[1]. If the value_type you wish to use is an abstract base class see note [5].
Default: std::iterator_traits<Base>::value_type [2]
Reference The reference type of the resulting iterator, and in particular, the result type of operator*().
Default: If Value is supplied, Value& is used. Otherwise std::iterator_traits<Base>::reference is used. [7]
ForwardIterators, BidirectionalIterators, and RandomAccessIterators require that Reference is a true reference type (e.g. not a proxy).
Pointer The pointer type of the resulting iterator, and in particular, the result type of operator->().
Default: If Value was not supplied, std::iterator_traits<Base>::pointer. [7] Otherwise, if iterator_category is input_iterator, then a class yielding Value* when operator->() is applied. Otherwise, Value*.
value_type* or a class which yields value_type* when operator->() is applied.
Category The iterator_category type for the resulting iterator.
Default: std::iterator_traits<Base>::iterator_category
One of std::input_iterator_tag, std::output_iterator_tag, std::forward_iterator_tag, std::bidirectional_iterator_tag, or std::random_access_iterator_tag.
Distance The difference_type for the resulting iterator.
Default: std::iterator_traits<Base>::difference_type
A signed integral type
NamedParam A named template parameter (see below).

Named Template Parameters

With seven template parameters, providing arguments for iterator_adaptor in the correct order can be challenging. Also, often times one would like to specify the sixth or seventh template parameter, but use the defaults for the third through fifth. As a solution to these problems we provide a mechanism for naming the last five template parameters, and providing them in any order through a set of named template parameters. The following classes are provided for specifying the parameters. Any of these classes can be used for any of the last five template parameters of iterator_adaptor.
template <class Value> struct value_type_is;
template <class Reference> struct reference_is;
template <class Pointer> struct pointer_is;
template <class Distance> struct difference_type_is;
template <class Category> struct iterator_category_is;
For example, the following adapts foo_iterator to create an InputIterator with reference type foo, and whose other traits are determined according to the defaults described above.
typedef iterator_adaptor<foo_iterator, foo_policies,
  reference_is<foo>, iterator_category_is<std::input_iterator_tag>
  > MyIterator;

The Policies Class

The main task in using iterator_adaptor is creating an appropriate Policies class. The Policies class will become the functional heart of the resulting iterator, supplying the core operations that determine its behavior. The iterator_adaptor template defines all of the operators required of a Random Access Iterator by dispatching to a Policies object. Your Policies class must implement a subset of the core iterator operations below corresponding to the iterator categories you want it to support.

Policies Class Requirements
T: adapted iterator type; x, y: objects of type T; p: T::policies_type d: T::difference_type; i1, i2: T::base_type
Expression Effects Implements Operations Required for Iterator Categories
p.initialize(b) optionally modify base iterator during iterator construction constructors Input/ Output/ Forward/ Bidirectional/ Random Access
p.dereference(x) returns an element of the iterator's reference type *x, x[d]
p.equal(x, y) tests the iterator for equality i1 == i2, i1 != i2
p.increment(x) increments the iterator ++x, x++
p.decrement(x) decrements the iterator --x, x-- Bidirectional/ Random Access
p.distance(x, y) measures the distance between iterators y - x, x < y Random Access
p.advance(x, n) adds an integer offset to iterators x + d, d + x,
x += d, x - d,
x -= d

The library also supplies a "trivial" policy class, default_iterator_policies, which implements all seven of the core operations in the usual way. If you wish to create an iterator adaptor that only changes a few of the base type's behaviors, then you can derive your new policy class from default_iterator_policies to avoid retyping the usual behaviors. You should also look at default_iterator_policies as the ``boilerplate'' for your own policy classes, defining functions with the same interface. This is the definition of default_iterator_policies:

struct default_iterator_policies
    // Some of these members were defined static, but Borland got confused
    // and thought they were non-const. Also, Sun C++ does not like static
    // function templates.

    template <class Base>
    void initialize(Base&)
        { }

    template <class IteratorAdaptor>
    typename IteratorAdaptor::reference dereference(const IteratorAdaptor& x) const
        { return *x.base(); }

    template <class IteratorAdaptor>
    void increment(IteratorAdaptor& x)
        { ++x.base(); }

    template <class IteratorAdaptor>
    void decrement(IteratorAdaptor& x)
        { --x.base(); }

    template <class IteratorAdaptor, class DifferenceType>
    void advance(IteratorAdaptor& x, DifferenceType n)
        { x.base() += n; }

    template <class IteratorAdaptor1, class IteratorAdaptor2>
    typename IteratorAdaptor1::difference_type
    distance(const IteratorAdaptor1& x, const IteratorAdaptor2& y) const
        { return y.base() - x.base(); }

    template <class IteratorAdaptor1, class IteratorAdaptor2>
    bool equal(const IteratorAdaptor1& x, const IteratorAdaptor2& y) const
        { return x.base() == y.base(); }

Template member functions are used throughout default_iterator_policies so that it can be employed with a wide range of iterators. If we had used concrete types above, we'd have tied the usefulness of default_iterator_policies to a particular range of adapted iterators. If you follow the same pattern with your Policies classes, you can use them to generate more specialized adaptors along the lines of those supplied by this library.

Additional Members

In addition to all of the member functions required of a Random Access Iterator, the iterator_adaptor class template defines the following members.

explicit iterator_adaptor(const Base&, const Policies& = Policies())

Construct an adapted iterator from a base object and a policies object. As this constructor is explicit, it does not provide for implicit conversions from the Base type to the iterator adaptor.
template <class B, class V, class R, class P>
iterator_adaptor(const iterator_adaptor<B,Policies,V,R,P,Category,Distance>&)

This constructor allows for conversion from mutable to constant adapted iterators. See below for more details.
Requires: B is convertible to Base.
const base_type& base() const;

Return a const reference to the base object.
base_type& base();

Return a reference to the base object. This is to give the policies object access to the base object. See above for policies iterator_adaptor interaction.[8]
const Policies& policies() const;

Return a const reference to the policies object.
Policies& policies();

Return a reference to the policies object.


It is often useful to automatically apply some function to the value returned by dereferencing an iterator. The transform iterator makes it easy to create an iterator adaptor which does just that. Here we will show how easy it is to implement the transform iterator using the iterator_adaptor template.

We want to be able to adapt a range of iterators and functions, so the policies class will have a template parameter for the function type and it will have a data member of that type. We know that the function takes one argument and that we'll need to be able to deduce the result_type of the function so we can use it for the adapted iterator's value_type. AdaptableUnaryFunction is the Concept that fulfills those requirements.

To implement a transform iterator we will only change one of the base iterator's behaviors, so the transform_iterator_policies class can inherit the rest from default_iterator_policies. We will define the dereference() member function, which is used to implement operator*() of the adapted iterator. The implementation will dereference the base iterator and apply the function object. The complete code for transform_iterator_policies is:

template <class AdaptableUnaryFunction>
struct transform_iterator_policies : public default_iterator_policies
    transform_iterator_policies() { }

    transform_iterator_policies(const AdaptableUnaryFunction& f)
        : m_f(f) { }
    template <class IteratorAdaptor>
    typename IteratorAdaptor::reference
    dereference(const IteratorAdaptor& iter) const
        { return m_f(*iter.base()); }

    AdaptableUnaryFunction m_f;

The next step is to use the iterator_adaptor template to construct the transform iterator type. The nicest way to package the construction of the transform iterator is to create a type generator. The first template parameter to the generator will be the type of the function object and the second will be the base iterator type. We use iterator_adaptor to define the transform iterator type as a nested typedef inside the transform_iterator_generator class. Because the function may return by-value, we must limit the iterator_category to Input Iterator, and the iterator's reference type cannot be a true reference (the standard allows this for input iterators), so in this case we can use few of iterator_adaptor's default template arguments.

template <class AdaptableUnaryFunction, class Iterator>
struct transform_iterator_generator
    typedef typename AdaptableUnaryFunction::result_type value_type;
    typedef iterator_adaptor<Iterator, 
        value_type, value_type, value_type*, std::input_iterator_tag>

As a finishing touch, we will create an object generator for the transform iterator. Our object generator makes it more convenient to create a transform iterator.

template <class AdaptableUnaryFunction, class Iterator>
typename transform_iterator_generator<AdaptableUnaryFunction,Iterator>::type
make_transform_iterator(Iterator base,
                        const AdaptableUnaryFunction& f = AdaptableUnaryFunction())
    typedef typename transform_iterator_generator<AdaptableUnaryFunction,
      Iterator>::type result_t;
    return result_t(base, f);

Here is an example that shows how to use a transform iterator to iterate through a range of numbers, multiplying each of them by 2 and printing the result to standard output.

#include <functional>
#include <algorithm>
#include <iostream>
#include <boost/iterator_adaptors.hpp>

int main(int, char*[])
  int x[] = { 1, 2, 3, 4, 5, 6, 7, 8 };
  const int N = sizeof(x)/sizeof(int);
  std::cout << "multiplying the array by 2:" << std::endl;
  std::copy(boost::make_transform_iterator(x, std::bind1st(std::multiplies<int>(), 2)),
      boost::make_transform_iterator(x + N, std::bind1st(std::multiplies<int>(), 2)),
      std::ostream_iterator<int>(std::cout, " "));
  std::cout << std::endl;
  return 0;
This output is:
2 4 6 8 10 12 14 16

Iterator Interactions

C++ allows const and non-const pointers to interact in the following intuitive ways:

Getting user-defined iterators to work together that way is nontrivial (see here for an example of where the C++ standard got it wrong), but iterator_adaptor can make it easy. The rules are as follows:


The Projection Iterator adaptor is similar to the transform iterator adaptor in that its operator*() applies some function to the result of dereferencing the base iterator and then returns the result. The difference is that the function must return a reference to some existing object (for example, a data member within the value_type of the base iterator).

The projection_iterator_pair_generator template is a special two-type generator for mutable and constant versions of a projection iterator. It is defined as follows:

template <class AdaptableUnaryFunction, class Iterator, class ConstIterator>
struct projection_iterator_pair_generator {
    typedef typename AdaptableUnaryFunction::result_type value_type;
    typedef projection_iterator_policies<AdaptableUnaryFunction> policies;
    typedef iterator_adaptor<Iterator,policies,value_type> iterator;
    typedef iterator_adaptor<ConstIterator,policies,value_type,
        const value_type&,const value_type*> const_iterator;

It is assumed that the Iterator and ConstIterator arguments are corresponding mutable and constant iterators.


There is an unlimited number of ways the iterator_adaptors class can be used to create iterators. One interesting exercise would be to re-implement the iterators of std::list and slist using iterator_adaptors, where the adapted Iterator types would be node pointers.

Concept Model

Depending on the Base and Policies template parameters, an iterator_adaptor can be a Input Iterator, Forward Iterator, Bidirectional Iterator, or Random Access Iterator.

Declaration Synopsis

template <class Base, class Policies, 
    class Value = typename std::iterator_traits<Base>::value_type,
    class Reference = ...(see below),
    class Pointer = ...(see below),
    class Category = typename std::iterator_traits<Base>::iterator_category,
    class Distance = typename std::iterator_traits<Base>::difference_type
struct iterator_adaptor
    typedef Distance difference_type;
    typedef typename boost::remove_const<Value>::type value_type;
    typedef Pointer pointer;
    typedef Reference reference;
    typedef Category iterator_category;
    typedef Base base_type;
    typedef Policies policies_type;

    explicit iterator_adaptor(const Base&, const Policies& = Policies());

    base_type&       base();
    const base_type& base() const;

    template <class B, class V, class R, class P>
        const iterator_adaptor<B,Policies,V,R,P,Category,Distance>&);

    reference operator*() const; [6]
    operator_arrow_result_type operator->() const; [3]
    value_type operator[](difference_type n) const; [4], [6]

    iterator_adaptor& operator++();
    iterator_adaptor& operator++(int);
    iterator_adaptor& operator--();
    iterator_adaptor& operator--(int);

    iterator_adaptor& operator+=(difference_type n);
    iterator_adaptor& operator-=(difference_type n);

    iterator_adaptor& operator-(Distance x) const;

template <class B, class P, class V, class R, class Ptr, 
    class C, class D1, class D2>
operator+(iterator_adaptor<B,P,V,R,Ptr,C,D1>, D2);

template <class B, class P, class V, class R, class Ptr,
    class C, class D1, class D2>
operator+(D2, iterator_adaptor<B,P,V,R,Ptr,C,D1> p);

template <class B1, class B2, class P, class V1, class V2,
    class R1, class R2, class P1, class P2, class C, class D>
Distance operator-(const iterator_adaptor<B1,P,V1,R1,P1,C,D>&, 
                   const iterator_adaptor<B2,P,V2,R2,P2,C,D>&);

template <class B1, class B2, class P, class V1, class V2,
    class R1, class R2, class P1, class P2, class C, class D>
bool operator==(const iterator_adaptor<B1,P,V1,R1,P1,C,D>&, 
                const iterator_adaptor<B2,P,V2,R2,P2,C,D>&);

// and similarly for operators !=, <, <=, >=, >


Generally, the iterator adaptors library can be compiled with all compilers supporting iterator traits and type traits.

Microsoft VC++ is not able to handle iterator adaptors based on a vector::iterator without specifying all template paramters explicitly. In case not all template parameters are specified explicitly, the iterator adaptors library will deduce these types using iterator_traits. But since in VC++ a vector::iterator is a T*, VC++ can't handle using iterator_traits due to the lack of partial template specialization.


[1] The standard specifies that the value_type of const iterators to T (e.g. const T*) is non-const T, while the pointer and reference types for all Forward Iterators are const T* and const T&, respectively. Stripping the const-ness of Value allows you to easily make a constant iterator by supplying a const type for Value, and allowing the defaults for the Pointer and Reference parameters to take effect. Although compilers that don't support partial specialization won't strip const for you, having a const value_type is often harmless in practice.

[2] If your compiler does not support partial specialization and the base iterator is a builtin pointer type, you will not be able to use the default for Value and will have to specify this type explicitly.

[3] The result type for the operator->() depends on the category and value type of the iterator and is somewhat complicated to describe. But be assured, it works in a stardard conforming fashion, providing access to members of the objects pointed to by the iterator.

[4] The result type of operator[]() is value_type instead of reference as might be expected. There are two reasons for this choice. First, the C++ standard only requires that the return type of an arbitrary Random Access Iterator's operator[]be ``convertible to T'' (Table 76), so when adapting an arbitrary base iterator we may not have a reference to return. Second, and more importantly, for certain kinds of iterators, returning a reference could cause serious memory problems due to the reference being bound to a temporary object whose lifetime ends inside of the operator[].

[5] The value_type of an iterator may not be an abstract base class, however many common uses of iterators never need the value_type, only the reference type. If you wish to create such an iterator adaptor, use a dummy type such as char for the Value parameter, and use a reference to your abstract base class for the Reference parameter. Note that such an iterator does not fulfill the C++ standards requirements for a Forward Iterator, so you will need to use a less restrictive iterator category such as std::input_iterator_tag.

[6] There is a common misconception that an iterator should have two versions of operator* and of operator[], one version that is a const member function and one version that is non-const. Perhaps the source of this misconception is that containers typically have const and non-const versions of many of their member functions. Iterators, however, are different. A particular iterator type can be either mutable or constant (but not both). One can assign to and change the object pointed to by a mutable iterator whereas a constant iterator returns constant objects when dereferenced. Whether the iterator object itself is const has nothing to do with whether the iterator is mutable or constant. This is analogous to the way built-in pointer types behave. For example, one can modify objects pointed to by a const pointer

    int* const x = new int;
    int i = 3;
    *x = i;
but one cannot modify objects pointed to by a pointer to const
    int const* x = new int;
    int i = 3;
    *x = i;

[7] If you are using a compiler that does not have a version of std::iterator_traits that works for pointers (i.e., if your compiler does not support partial specialization) then if the Base type is a const pointer, then the correct defaults for the reference and pointer types can not be deduced. You must specify these types explicitly.

[8] Exposing the base object might be considered as being dangerous. A possible fix would require compiler support for template friends. As this is not widely available today, the base object remains exposed for now.

Revised 30 Nov 2001

© Copyright Dave Abrahams and Jeremy Siek 2001. 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.