boost_1_45_0/libs/fusion/doc/quick_start.qbk

[/==============================================================================
    Copyright (C) 2001-2007 Joel de Guzman, Dan Marsden, Tobias Schwinger

    Use, modification and distribution is subject to the Boost Software
    License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
    http://www.boost.org/LICENSE_1_0.txt)
===============================================================================/]
[section Quick Start]

I assume the reader is already familiar with tuples (__tuple__) and its
ancestor `std::pair`. The tuple is a generalization of `std::pair` for
multiple heterogeneous elements (triples, quadruples, etc.). The tuple is
more or less a synonym for fusion's `__vector__`.

For starters, we shall include all of Fusion's __sequence__(s) [footnote There
are finer grained header files available if you wish to have more control
over which components to include (see section __organization__ for
details).]:

    #include <boost/fusion/sequence.hpp>
    #include <boost/fusion/include/sequence.hpp>

Let's begin with a `__vector__` [footnote Unless otherwise noted, components are
in namespace `boost::fusion`. For the sake of simplicity, code in this
quick start implies `using` directives for the fusion components we will be
using.]:

    __vector__<int, char, std::string> stuff(1, 'x', "howdy");
    int i = __at_c__<0>(stuff);
    char ch = __at_c__<1>(stuff);
    std::string s = __at_c__<2>(stuff);

Just replace `tuple` for `__vector__` and `get` for `__at_c__` and this is exactly
like __tuple__. Actually, either names can be used interchangeably. Yet,
the similarity ends there. You can do a lot more with Fusion `__vector__` or
`tuple`. Let's see some examples.

[heading Print the vector as XML]

First, let's include the algorithms:

    #include <boost/fusion/algorithm.hpp>
    #include <boost/fusion/include/algorithm.hpp>

Now, let's write a function object that prints XML of the form <type>data</type>
for each member in the tuple.

    struct print_xml
    {
        template <typename T>
        void operator()(T const& x) const
        {
            std::cout
                << '<' << typeid(x).name() << '>'
                << x
                << "</" << typeid(x).name() << '>'
                ;
        }
    };

Now, finally:

    __for_each__(stuff, print_xml());

That's it! `__for_each__` is a fusion algorithm. It is a generic algorithm
similar to __stl__'s. It iterates over the sequence and calls a user
supplied function. In our case, it calls `print_xml`'s `operator()` for
each element in `stuff`.

[caution The result of `typeid(x).name()` is platform specific. The code
here is just for exposition. Of course you already know that :-)]

`__for_each__` is generic. With `print_xml`, you can use it to print just about
any Fusion __sequence__.

[heading Print only pointers]

Let's get a little cleverer. Say we wish to write a /generic/ function
that takes in an arbitrary sequence and XML prints only those elements
which are pointers. Ah, easy. First, let's include the `is_pointer` boost
type trait:

    #include <boost/type_traits/is_pointer.hpp>

Then, simply:

    template <typename Sequence>
    void xml_print_pointers(Sequence const& seq)
    {
        __for_each__(__filter_if__<boost::is_pointer<_> >(seq), print_xml());
    }

`__filter_if__` is another Fusion algorithm. It returns a __filter_view__,
a conforming Fusion sequence. This view reflects only those elements that
pass the given predicate. In this case, the predicate is
`boost::is_pointer<_>`. This "filtered view" is then passed to the
__for_each__ algorithm, which then prints the "filtered view" as XML.

Easy, right?

[heading Associative tuples]

Ok, moving on...

Apart from `__vector__`, fusion has a couple of other sequence types to choose
from. Each sequence has its own characteristics. We have `__list__`, `__set__`,
`__map__`, plus a multitude of `views` that provide various ways to present the
sequences.

Fusion's `__map__` associate types with elements. It can be used as a cleverer
replacement of the `struct`. Example:

    namespace fields
    {
        struct name;
        struct age;
    }

    typedef __map__<
        __fusion_pair__<fields::name, std::string>
      , __fusion_pair__<fields::age, int> >
    person;

`__map__` is an associative sequence. Its elements are Fusion pairs which differ
somewhat from `std::pair`. Fusion pairs only contain one member, with the type of
their second template parameter. The first type parameter of the pair is used as an
index to the associated element in the sequence. For example, given a `a_person`
of type, `person`, you can do:

    using namespace fields;
    std::string person_name = __at_key__<name>(a_person);
    int person_age = __at_key__<age>(a_person);

Why go through all this trouble, you say? Well, for one, unlike the
`struct`, we are dealing with a generic data structure. There are a
multitude of facilities available at your disposal provided out of the box
with fusion or written by others. With these facilities, introspection
comes for free, for example. We can write one serialization function (well,
two, if you consider loading and saving) that will work for all your fusion
`__map__`s. Example:

    struct saver
    {
        template <typename Pair>
        void operator()(Pair const& data) const
        {
            some_archive << data.second;
        }
    };

    template <typename Stuff>
    void save(Stuff const& stuff)
    {
        __for_each__(stuff, saver());
    }

The `save` function is generic and will work for all types of `stuff`
regardless if it is a `person`, a `dog` or a whole `alternate_universe`.

[heading Tip of the Iceberg]

And... we've barely scratched the surface! You can compose and expand the
data structures, remove elements from the structures, find specific data
types, query the elements, filter out types for inspection, transform data
structures, etc. What you've seen is just the tip of the iceberg.

[endsect]