# Transform Iterator

Author: | David Abrahams, Jeremy Siek, Thomas Witt
Contact: | dave@boost-consulting.com, jsiek@osl.iu.edu, witt@ive.uni-hannover.de
Organization: | Boost Consulting, Indiana University Open Systems Lab, University of Hanover Institute for Transport Railway Operation and Construction
Date: | 2006-09-11
Copyright: | Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003.

abstract: | The transform iterator adapts an iterator by modifying the `operator*` to apply a function object to the result of dereferencing the iterator and returning the result.

Table of Contents

- `transform_iterator` synopsis
- `transform_iterator` requirements
- `transform_iterator` models
- `transform_iterator` operations
- Example

# `transform_iterator` synopsis

    template <class UnaryFunction,
              class Iterator,
              class Reference = use_default,
              class Value = use_default>
    class transform_iterator
    {
    public:
      typedef /* see below */ value_type;
      typedef /* see below */ reference;
      typedef /* see below */ pointer;
      typedef iterator_traits<Iterator>::difference_type difference_type;
      typedef /* see below */ iterator_category;

      transform_iterator();
      transform_iterator(Iterator const& x, UnaryFunction f);

      template<class F2, class I2, class R2, class V2>
      transform_iterator(
            transform_iterator<F2, I2, R2, V2> const& t
          , typename enable_if_convertible<I2, Iterator>::type* = 0      // exposition only
          , typename enable_if_convertible<F2, UnaryFunction>::type* = 0 // exposition only
      );
      UnaryFunction functor() const;
      Iterator const& base() const;
      reference operator*() const;
      transform_iterator& operator++();
      transform_iterator& operator--();
    private:
      Iterator m_iterator; // exposition only
      UnaryFunction m_f;   // exposition only
    };

If `Reference` is `use_default` then the `reference` member of `transform_iterator` is `result_of<UnaryFunction(iterator_traits<Iterator>::reference)>::type`. Otherwise, `reference` is `Reference`.

If `Value` is `use_default` then the `value_type` member is `remove_cv<remove_reference<reference> >::type`. Otherwise, `value_type` is `Value`.

If `Iterator` models Readable Lvalue Iterator and if `Iterator` models Random Access Traversal Iterator, then `iterator_category` is convertible to `random_access_iterator_tag`. Otherwise, if `Iterator` models Bidirectional Traversal Iterator, then `iterator_category` is convertible to `bidirectional_iterator_tag`. Otherwise `iterator_category` is convertible to `forward_iterator_tag`. If `Iterator` does not model Readable Lvalue Iterator then `iterator_category` is convertible to `input_iterator_tag`.

# `transform_iterator` requirements

The type `UnaryFunction` must be Assignable, Copy Constructible, and the expression `f(*i)` must be valid where `f` is an object of type `UnaryFunction`, `i` is an object of type `Iterator`, and where the type of `f(*i)` must be `result_of<UnaryFunction(iterator_traits<Iterator>::reference)>::type`.

The argument `Iterator` shall model Readable Iterator.

# `transform_iterator` models

The resulting `transform_iterator` models the most refined of the following that is also modeled by `Iterator`.

> - Writable Lvalue Iterator if `transform_iterator::reference` is a non-const reference.
> - Readable Lvalue Iterator if `transform_iterator::reference` is a const reference.
> - Readable Iterator otherwise.

The `transform_iterator` models the most refined standard traversal concept that is modeled by the `Iterator` argument.

If `transform_iterator` is a model of Readable Lvalue Iterator then it models the following original iterator concepts depending on what the `Iterator` argument models.

If `Iterator` models | then `transform_iterator` models
--- | ---
Single Pass Iterator | Input Iterator
Forward Traversal Iterator | Forward Iterator
Bidirectional Traversal Iterator | Bidirectional Iterator
Random Access Traversal Iterator | Random Access Iterator

If `transform_iterator` models Writable Lvalue Iterator then it is a mutable iterator (as defined in the old iterator requirements).

`transform_iterator<F1, X, R1, V1>` is interoperable with `transform_iterator<F2, Y, R2, V2>` if and only if `X` is interoperable with `Y`.

# `transform_iterator` operations

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

`transform_iterator();`

Returns: | An instance of `transform_iterator` with `m_f` and `m_iterator` default constructed.

`transform_iterator(Iterator const& x, UnaryFunction f);`

Returns: | An instance of `transform_iterator` with `m_f` initialized to `f` and `m_iterator` initialized to `x`.

    template<class F2, class I2, class R2, class V2>
    transform_iterator(
          transform_iterator<F2, I2, R2, V2> const& t
        , typename enable_if_convertible<I2, Iterator>::type* = 0      // exposition only
        , typename enable_if_convertible<F2, UnaryFunction>::type* = 0 // exposition only
    );

Returns: | An instance of `transform_iterator` with `m_f` initialized to `t.functor()` and `m_iterator` initialized to `t.base()`.
Requires: | `OtherIterator` is implicitly convertible to `Iterator`.

`UnaryFunction functor() const;`

Returns: | `m_f`

`Iterator const& base() const;`

Returns: | `m_iterator`

`reference operator*() const;`

Returns: | `m_f(*m_iterator)`

`transform_iterator& operator++();`

Effects: | `++m_iterator`
Returns: | `*this`

`transform_iterator& operator--();`

Effects: | `--m_iterator`
Returns: | `*this`

    template <class UnaryFunction, class Iterator>
    transform_iterator<UnaryFunction, Iterator>
    make_transform_iterator(Iterator it, UnaryFunction fun);

Returns: | An instance of `transform_iterator<UnaryFunction, Iterator>` with `m_f` initialized to `f` and `m_iterator` initialized to `x`.

    template <class UnaryFunction, class Iterator>
    transform_iterator<UnaryFunction, Iterator>
    make_transform_iterator(Iterator it);

Returns: | An instance of `transform_iterator<UnaryFunction, Iterator>` with `m_f` default constructed and `m_iterator` initialized to `x`.

# Example

This is a simple example of using the transform_iterators class to generate iterators that multiply (or add to) the value returned by dereferencing the iterator. It would be cooler to use lambda library in this example.

    int x[] = { 1, 2, 3, 4, 5, 6, 7, 8 };
    const int N = sizeof(x)/sizeof(int);

    typedef boost::binder1st< std::multiplies<int> > Function;
    typedef boost::transform_iterator<Function, int*> doubling_iterator;

    doubling_iterator i(x, boost::bind1st(std::multiplies<int>(), 2)),
      i_end(x + N, boost::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;

    std::cout << "adding 4 to each element in the array:" << std::endl;
    std::copy(boost::make_transform_iterator(x, boost::bind1st(std::plus<int>(), 4)),
              boost::make_transform_iterator(x + N, boost::bind1st(std::plus<int>(), 4)),
              std::ostream_iterator<int>(std::cout, " "));
    std::cout << std::endl;

The output is:

    multiplying the array by 2:
    2 4 6 8 10 12 14 16
    adding 4 to each element in the array:
    5 6 7 8 9 10 11 12

The source code for this example can be found here.