[section:zip Zip Iterator]

The zip iterator provides the ability to parallel-iterate
over several controlled sequences simultaneously. A zip 
iterator is constructed from a tuple of iterators. Moving
the zip iterator moves all the iterators in parallel.
Dereferencing the zip iterator returns a tuple that contains
the results of dereferencing the individual iterators. 

[section:zip_example Example]

There are two main types of applications of the `zip_iterator`. The first
one concerns runtime efficiency: If one has several controlled sequences
of the same length that must be somehow processed, e.g., with the 
`for_each` algorithm, then it is more efficient to perform just
one parallel-iteration rather than several individual iterations. For an 
example, assume that `vect_of_doubles` and `vect_of_ints`
are two vectors of equal length containing doubles and ints, respectively,
and consider the following two iterations:

    std::vector<double>::const_iterator beg1 = vect_of_doubles.begin();
    std::vector<double>::const_iterator end1 = vect_of_doubles.end();
    std::vector<int>::const_iterator beg2 = vect_of_ints.begin();
    std::vector<int>::const_iterator end2 = vect_of_ints.end();

    std::for_each(beg1, end1, func_0());
    std::for_each(beg2, end2, func_1());

These two iterations can now be replaced with a single one as follows:


    std::for_each(
      boost::make_zip_iterator(
        boost::make_tuple(beg1, beg2)
        ),
      boost::make_zip_iterator(
        boost::make_tuple(end1, end2)
        ),
      zip_func()
      );

A non-generic implementation of `zip_func` could look as follows:


      struct zip_func : 
        public std::unary_function<const boost::tuple<const double&, const int&>&, void>
      {
        void operator()(const boost::tuple<const double&, const int&>& t) const
        {
          m_f0(t.get<0>());
          m_f1(t.get<1>());
        }

      private:
        func_0 m_f0;
        func_1 m_f1;
      };

The second important application of the `zip_iterator` is as a building block
to make combining iterators. A combining iterator is an iterator
that parallel-iterates over several controlled sequences and, upon
dereferencing, returns the result of applying a functor to the values of the
sequences at the respective positions. This can now be achieved by using the
`zip_iterator` in conjunction with the `transform_iterator`. 

Suppose, for example, that you have two vectors of doubles, say 
`vect_1` and `vect_2`, and you need to expose to a client
a controlled sequence containing the products of the elements of 
`vect_1` and `vect_2`. Rather than placing these products
in a third vector, you can use a combining iterator that calculates the
products on the fly. Let us assume that `tuple_multiplies` is a
functor that works like `std::multiplies`, except that it takes
its two arguments packaged in a tuple. Then the two iterators 
`it_begin` and `it_end` defined below delimit a controlled
sequence containing the products of the elements of `vect_1` and
`vect_2`:

    typedef boost::tuple<
      std::vector<double>::const_iterator,
      std::vector<double>::const_iterator
      > the_iterator_tuple;

    typedef boost::zip_iterator<
      the_iterator_tuple
      > the_zip_iterator;

    typedef boost::transform_iterator<
      tuple_multiplies<double>,
      the_zip_iterator
      > the_transform_iterator;

    the_transform_iterator it_begin(
      the_zip_iterator(
        the_iterator_tuple(
          vect_1.begin(),
          vect_2.begin()
          )
        ),
      tuple_multiplies<double>()
      );

    the_transform_iterator it_end(
      the_zip_iterator(
        the_iterator_tuple(
          vect_1.end(),
          vect_2.end()
          )
        ),
      tuple_multiplies<double>()
      );

[endsect]

[section:zip_reference Reference]

[h2 Synopsis]

  template<typename IteratorTuple>
  class zip_iterator
  {  

  public:
    typedef /* see below */ reference;
    typedef reference value_type;
    typedef value_type* pointer;
    typedef /* see below */ difference_type;
    typedef /* see below */ iterator_category;

    zip_iterator();
    zip_iterator(IteratorTuple iterator_tuple);

    template<typename OtherIteratorTuple>
    zip_iterator(
          const zip_iterator<OtherIteratorTuple>& other
        , typename enable_if_convertible<
                OtherIteratorTuple
              , IteratorTuple>::type* = 0     // exposition only
    );

    const IteratorTuple& get_iterator_tuple() const;

  private:
    IteratorTuple m_iterator_tuple;     // exposition only
  };

  template<typename IteratorTuple> 
  zip_iterator<IteratorTuple> 
  make_zip_iterator(IteratorTuple t);

The `reference` member of `zip_iterator` is the type of the tuple
made of the reference types of the iterator types in the `IteratorTuple`
argument.

The `difference_type` member of `zip_iterator` is the `difference_type`
of the first of the iterator types in the `IteratorTuple` argument.

The `iterator_category` member of `zip_iterator` is convertible to the
minimum of the traversal categories of the iterator types in the `IteratorTuple`
argument. For example, if the `zip_iterator` holds only vector
iterators, then `iterator_category` is convertible to 
`boost::random_access_traversal_tag`. If you add a list iterator, then
`iterator_category` will be convertible to `boost::bidirectional_traversal_tag`,
but no longer to `boost::random_access_traversal_tag`.

[h2 Requirements]

All iterator types in the argument `IteratorTuple` shall model Readable Iterator.  

[h2 Concepts]

The resulting `zip_iterator` models Readable Iterator.

The fact that the `zip_iterator` models only Readable Iterator does not 
prevent you from modifying the values that the individual iterators point
to. The tuple returned by the `zip_iterator`'s `operator*` is a tuple 
constructed from the reference types of the individual iterators, not 
their value types. For example, if `zip_it` is a `zip_iterator` whose
first member iterator is an `std::vector<double>::iterator`, then the
following line will modify the value which the first member iterator of
`zip_it` currently points to:

    zip_it->get<0>() = 42.0;


Consider the set of standard traversal concepts obtained by taking
the most refined standard traversal concept modeled by each individual
iterator type in the `IteratorTuple` argument.The `zip_iterator` 
models the least refined standard traversal concept in this set.

`zip_iterator<IteratorTuple1>` is interoperable with
`zip_iterator<IteratorTuple2>` if and only if `IteratorTuple1`
is interoperable with `IteratorTuple2`.

[h2 Operations]

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

  zip_iterator();

[*Returns:] An instance of `zip_iterator` with `m_iterator_tuple`
  default constructed.


  zip_iterator(IteratorTuple iterator_tuple);

[*Returns:] An instance of `zip_iterator` with `m_iterator_tuple`
  initialized to `iterator_tuple`.


    template<typename OtherIteratorTuple>
    zip_iterator(
          const zip_iterator<OtherIteratorTuple>& other
        , typename enable_if_convertible<
                OtherIteratorTuple
              , IteratorTuple>::type* = 0     // exposition only
    );

[*Returns:] An instance of `zip_iterator` that is a copy of `other`.\n
[*Requires:] `OtherIteratorTuple` is implicitly convertible to `IteratorTuple`.


  const IteratorTuple& get_iterator_tuple() const;

[*Returns:] `m_iterator_tuple`


  reference operator*() const;

[*Returns:] A tuple consisting of the results of dereferencing all iterators in
  `m_iterator_tuple`.


  zip_iterator& operator++();

[*Effects:] Increments each iterator in `m_iterator_tuple`.\n
[*Returns:] `*this`


  zip_iterator& operator--();

[*Effects:] Decrements each iterator in `m_iterator_tuple`.\n
[*Returns:] `*this`

    template<typename IteratorTuple> 
    zip_iterator<IteratorTuple> 
    make_zip_iterator(IteratorTuple t);

[*Returns:] An instance of `zip_iterator<IteratorTuple>` with `m_iterator_tuple`
  initialized to `t`.

[endsect]

[endsect]