...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
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:
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.
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>
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:
struct_type
and index
provide convenient
access to information we will need later in the implementation.
category
allows the traits::category_of
metafunction to establish the traversal category of the iterator.
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.
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 result_of::value_of
:
template <typename Iterator> struct value_of : extension::value_of_impl<typename detail::tag_of<Iterator>::type>:: template apply<Iterator> {};
So result_of::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> structderef
: extension::deref_impl<typename detail::tag_of<Iterator>::type>:: template apply<Iterator> {}; } template <typename Iterator> typename result_of::deref<Iterator>::typederef
(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 result_of::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 | |
---|---|
Although there is a fair amount of left to do to produce a fully fledged
Fusion sequence, |
Ok, now we have seen the way result_of::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.
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
.
In order for example_struct
to serve as an associative forward sequence, we need to adapt the traversal
category of our sequence and our iterator accordingly and enable 3 intrinsic
sequence lookup features, at_key
, __value_at_key__ and has_key
.
We also need to enable 3 iterator lookup features, result_of::key_of
, result_of::value_of_data
and deref_data
.
To implement at_key_impl
we need to associate the fields::name
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 the other functions are equally straightforward, and
are provided in the example code.
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 sequence. 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.