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### 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.

The tuple of iterators is now implemented in terms of a Boost fusion sequence. Because of this the 'tuple' may be any Boost fusion sequence and, for backwards compatibility through a Boost fusion sequence adapter, a Boost tuple. Because the 'tuple' may be any boost::fusion sequence the 'tuple' may also be any type for which a Boost fusion adapter exists. This includes, among others, a std::tuple and a std::pair. Just remember to include the appropriate Boost fusion adapter header files for these other Boost fusion adapters. The zip_iterator header file already includes the Boost fusion adapter header file for Boost tuple, so you need not include it yourself to use a Boost tuple as your 'tuple'.

#### 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>()
      );