[Boost-users] [MPL] - for_each operators with decision branching

Subject: [Boost-users] [MPL] - for_each operators with decision branching
From: Noah Roberts (roberts.noah_at_[hidden])
Date: 2010-05-19 13:23:50

I have a project in which I use these things I call "records" to attach 
data to edges and vertexes in a boost::graph.  They are visitable 
(acyclic visitor described by Alexandrescu) and I have metafunctions 
that will take their most derived type and give me a list of "fields" 
that they contain.  I can then write generic processors to do things 
like build editors for them or perform operations that can effect any 
field of a particular type or concept.  Thus I often find myself using 
mpl::for_each and writing things like this:

struct operator
{
  template < typename MetaField >
  void operator()(MetaField const&, boost::enable_if< some_test >::type 
* = 0);
  template < typename MetaField >
  void operator()(MetaField const&, boost::disable_if<some_test>::type * 
= 0);
};

I imagine that this is common enough that there may already be a utility 
to write things like this but I don't see one in the docs.  Is it out 
there?  What about something that would make this type of thing easy:

struct operator
{
  template < typename MetaField >
  void operator()(MetaField const&, boost::enable_if< test_a >::type * = 
0);  template < typename MetaField >
  void operator()(MetaField const&, boost::enable_if< and_< not_
<test_a>, test_b>>::type * = 0)
  void operator()(MetaField const&, boost::enable_if<not_<or_<test_a, 
test_b>>>::type * = 0);
};
  
That's supposed to be an if, elseif, else construct.

-- 
http://crazyeddiecpp.blogspot.com/