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The Full Extension Mechanism

The Fusion library is designed to be extensible, new sequences types can easily be added. In fact, the library support for std::pair, boost::array and MPL sequences is entirely provided using the extension mechanism.

The process for adding a new sequence type to Fusion is:

  1. Enable the tag dispatching mechanism used by Fusion for your sequence type
  2. Design an iterator type for the sequence
  3. Provide specialized behaviour for the intrinsic operations of the new Fusion sequence
Our example

In order to illustrate enabling a new sequence type for use with Fusion, we are going to use the type:

namespace example
{
    struct example_struct
    {
        std::string name;
        int age;
        example_struct(
            const std::string& n,
            int a)
            : name(n), age(a)
        {}
    };
}

We are going to pretend that this type has been provided by a 3rd party library, and therefore cannot be modified. We shall work through all the necessary steps to enable example_struct to serve as an Associative Sequence as described in the Quick Start guide.

Enabling Tag Dispatching

The Fusion extensibility mechanism uses tag dispatching to call the correct code for a given sequence type. In order to exploit the tag dispatching mechanism we must first declare a new tag type for the mechanism to use. For example:

namespace example {
    struct example_sequence_tag; // Only definition needed
}

Next we need to enable the traits::tag_of metafunction to return our newly chosen tag type for operations involving our sequence. This is done by specializing traits::tag_of for our sequence type.

#include <boost/fusion/support/tag_of_fwd.hpp>
#include <boost/fusion/include/tag_of_fwd.hpp>

namespace boost { namespace fusion { namespace traits {
    template<>
    struct tag_of<example_struct>
    {
        typedef example::example_sequence_tag type;
    };
}}}

traits::tag_of also has a second template argument, that can be used in conjuction with boost::enable_if to provide tag support for groups of related types. This feature is not necessary for our sequence, but for an example see the code in:

#include <boost/fusion/adapted/array/tag_of.hpp>
#include <boost/fusion/include/tag_of.hpp>
Designing a suitable iterator

We need an iterator to describe positions, and provide access to the data within our sequence. As it is straightforward to do, we are going to provide a random access iterator in our example.

We will use a simple design, in which the 2 members of example_struct are given numbered indices, 0 for name and 1 for age respectively.

template<typename Struct, int Pos>
struct example_struct_iterator
    : boost::fusion::iterator_base<example_struct_iterator<Struct, Pos> >
{
    BOOST_STATIC_ASSERT(Pos >=0 && Pos < 3);
    typedef Struct struct_type;
    typedef boost::mpl::int_<Pos> index;
    typedef boost::fusion::random_access_traversal_tag category;

    example_struct_iterator(Struct& str)
        : struct_(str) {}

    Struct& struct_;
};

A quick summary of the details of our iterator:

  1. The iterator is parameterized by the type it is iterating over, and the index of the current element.
  2. The typedefs struct_type and index provide convenient access to information we will need later in the implementation.
  3. The typedef category allows the traits::category_of metafunction to establish the traversal category of the iterator.
  4. The constructor stores a reference to the example_struct being iterated over.

We also need to enable tag dispatching for our iterator type, with another specialization of traits::tag_of.

In isolation, the iterator implementation is pretty dry. Things should become clearer as we add features to our implementation.

A first couple of instructive features

To start with, we will get the result_of::value_of metafunction working. To do this, we provide a specialization of the boost::fusion::extension::value_of_impl template for our iterator's tag type.

template<>
struct value_of_impl<example::example_struct_iterator_tag>
{
    template<typename Iterator>
    struct apply;

    template<typename Struct>
    struct apply<example::example_struct_iterator<Struct, 0> >
    {
        typedef std::string type;
    };

    template<typename Struct>
    struct apply<example::example_struct_iterator<Struct, 1> >
    {
        typedef int type;
    };
};

The implementation itself is pretty simple, it just uses 2 partial specializations to provide the type of the 2 different members of example_struct, based on the index of the iterator.

To understand how value_of_impl is used by the library we will look at the implementation of value_of:

template <typename Iterator>
struct value_of
    : extension::value_of_impl<typename detail::tag_of<Iterator>::type>::
        template apply<Iterator>
{};

So value_of uses tag dispatching to select an MPL Metafunction Class to provide its functionality. You will notice this pattern throughout the implementation of Fusion.

Ok, lets enable dereferencing of our iterator. In this case we must provide a suitable specialization of deref_impl.

template<>
struct deref_impl<example::example_struct_iterator_tag>
{
    template<typename Iterator>
    struct apply;

    template<typename Struct>
    struct apply<example::example_struct_iterator<Struct, 0> >
    {
        typedef typename mpl::if_<
            is_const<Struct>, std::string const&, std::string&>::type type;

        static type
        call(example::example_struct_iterator<Struct, 0> const& it)
        {
            return it.struct_.name;
        }
    };

    template<typename Struct>
    struct apply<example::example_struct_iterator<Struct, 1> >
    {
        typedef typename mpl::if_<
            is_const<Struct>, int const&, int&>::type type;

        static type
        call(example::example_struct_iterator<Struct, 1> const& it)
        {
                return it.struct_.age;
            }
        };
    };
}

The use of deref_impl is very similar to that of value_of_impl, but it also provides some runtime functionality this time via the call static member function. To see how deref_impl is used, lets have a look at the implementation of deref:

namespace result_of
{
    template <typename Iterator>
    struct deref
        : extension::deref_impl<typename detail::tag_of<Iterator>::type>::
            template apply<Iterator>
    {};
}

template <typename Iterator>
typename result_of::deref<Iterator>::type
deref(Iterator const& i)
{
    typedef result_of::deref<Iterator> deref_meta;
    return deref_meta::call(i);
}

So again result_of::deref uses tag dispatching in exactly the same way as the value_of implementation. The runtime functionality used by deref is provided by the call static function of the selected MPL Metafunction Class.

The actual implementation of deref_impl is slightly more complex than that of value_of_impl. We also need to implement the call function, which returns a reference to the appropriate member of the underlying sequence. We also require a little bit of metaprogramming to return const references if the underlying sequence is const.

[Note] Note

Although there is a fair amount of left to do to produce a fully fledged Fusion sequence, value_of and deref illustrate all the signficant concepts required. The remainder of the process is very repetitive, simply requiring implementation of a suitable xxxx_impl for each feature xxxx.

Implementing the remaining iterator functionality

Ok, now we have seen the way value_of and deref work, everything else will work in pretty much the same way. Lets start with forward iteration, by providing a next_impl:

template<>
struct next_impl<example::example_struct_iterator_tag>
{
    template<typename Iterator>
    struct apply
    {
        typedef typename Iterator::struct_type struct_type;
        typedef typename Iterator::index index;
        typedef example::example_struct_iterator<struct_type, index::value + 1> type;

        static type
        call(Iterator const& i)
        {
             return type(i.struct_);
        }
    };
};

This should be very familiar from our deref_impl implementation, we will be using this approach again and again now. Our design is simply to increment the index counter to move on to the next element. The various other iterator manipulations we need to perform will all just involve simple calculations with the index variables.

We also need to provide a suitable equal_to_impl so that iterators can be correctly compared. A Bidirectional Iterator will also need an implementation of prior_impl. For a Random Access Iterator distance_impl and advance_impl also need to be provided in order to satisfy the necessary complexity guarantees. As our iterator is a Random Access Iterator we will have to implement all of these functions.

Full implementations of prior_impl, advance_impl, distance_impl and equal_to_impl are provided in the example code.

Implementing the intrinsic functions of the sequence

In order that Fusion can correctly identify our sequence as a Fusion sequence, we need to enable is_sequence for our sequence type. As usual we just create an impl type specialized for our sequence tag:

template<>
struct is_sequence_impl<example::example_sequence_tag>
{
    template<typename T>
    struct apply : mpl::true_ {};
};

We've some similar formalities to complete, providing category_of_impl so Fusion can correctly identify our sequence type, and is_view_impl so Fusion can correctly identify our sequence as not being a View type. Implementations are provide in the example code.

Now we've completed some formalities, on to more interesting features. Lets get begin working so that we can get an iterator to start accessing the data in our sequence.

template<>
struct begin_impl<example::example_sequence_tag>
{
    template<typename Sequence>
    struct apply
    {
        typedef example::example_struct_iterator<Sequence, 0> type;

        static type
        call(Sequence& seq)
        {
            return type(seq);
        }
    };
};

The implementation uses the same ideas we have applied throughout, in this case we are just creating one of the iterators we developed earlier, pointing to the first element in the sequence. The implementation of end is very similar, and is provided in the example code.

For our Random Access Sequence we will also need to implement size_impl, value_at_impl and at_impl.

Enabling our type as an associative container

In order for example_struct to serve as an associative container, we need to enable 3 lookup features, at_key, value_at_key and has_key. We also need to provide an implementation of the is_associative trait so that our sequence can be correctly identified as an associative container.

To implement at_key_impl we need to associate the fields::age and fields::age types described in the Quick Start guide with the appropriate members of example_struct. Our implementation is as follows:

template<>
struct at_key_impl<example::example_sequence_tag>
{
    template<typename Sequence, typename Key>
    struct apply;

    template<typename Sequence>
    struct apply<Sequence, fields::name>
    {
        typedef typename mpl::if_<
            is_const<Sequence>,
            std::string const&,
            std::string&>::type type;

        static type
        call(Sequence& seq)
        {
            return seq.name;
        };
    };

    template<typename Sequence>
    struct apply<Sequence, fields::age>
    {
        typedef typename mpl::if_<
            is_const<Sequence>,
            int const&,
            int&>::type type;

        static type
        call(Sequence& seq)
        {
            return seq.age;
        };
    };
};

Its all very similar to the implementations we've seen previously, such as deref_impl and value_of_impl. Instead of identifying the members by index or position, we are now selecting them using the types fields::name and fields::age. The implementations of value_at_key_impl and has_key_impl are equally straightforward, and are provided in the example code, along with an implementation of is_associative_impl.

Summary

We've now worked through the entire process for adding a new random access sequence and we've also enabled our type to serve as an associative container. The implementation was slightly longwinded, but followed a simple repeating pattern.

The support for std::pair, MPL sequences, and boost::array all use the same approach, and provide additional examples of the approach for a variety of types.


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