[boost] [named_params] Metafunction for extracting parameter types.

From: Cromwell Enage (sponage_at_[hidden])
Date: 2004-11-26 15:22:36

In the course of refactoring my multi-state mazes
library to use Boost.NamedParams, I discovered that I
couldn't create a default predecessor map without the
proper vertex type, and I couldn't get that type
without knowing the type of the input graph.  So I
went ahead and created a metafunction called
find_named_param that helped me to accomplish this
task.

For example, you have the following tag types for your
key words:

    struct input_graph_t;
    struct input_index_map_t;
    ...

Then in your function, you can extract your types like
so:

    typedef typename find_named_param<
                       Params
                     , input_graph_t
                     >::type
            InputGraph;
    typedef typename find_named_param<
                       Params
                     , input_index_map_t
                     , property_map<
                         InputGraph
                       , vertex_index_t
                       >
                     >::type
            InputIndexMap;

Thus, given the following keywords:

    keyword<input_graph_t> input_graph;
    keyword<input_index_map_t> input_index_map;

Assignment becomes less tedious:

    InputGraph&
      in_g = p[input_graph];
    InputIndexMap
      index_map = p[
                    input_index_map
                  | get(vertex_index_t(), in_g)
                  ];

The third template parameter in find_named_param is
the type whose member typedef ::type will be returned
if the named parameter is unspecified.  Of course,
leaving this template parameter unspecified will
result in a compile-time error if the function
requires the named parameter.  The corresponding
argument type should be the same as (or
assignment-compatible with) the type of the default
value of the named parameter.

The changes made to <boost/named_params.hpp>, in
addition to effectively removing the dependency on
BOOST_MPL_LIMIT_METAFUNCTION_ARITY, now include moving

    typedef H head_type;
    typedef T tail_type;

out of the conditional macro body while remaining in
detail::list, and adding the necessary functionality
to make find_named_param work.

I wouldn't call it sufficiently tested; however, the
attached program works under MinGW (GCC 3.2) and MSVC
7.1.

                              Happy Thanksgiving!
                              Cromwell Enage

                
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