Re: [boost] boost::any visitor of known types

From: Eric Friedman (ebf_at_[hidden])
Date: 2007-09-08 14:22:43

Christian,

On 9/7/07, Christian Holmquist <c.holmquist_at_[hidden]> wrote:
> Is there any visitor mechanism somewhere in boost that allows one to
> introspect the contained type of a boost::any?
> I've been using boost::variant frequently and I love the way its visitor
> works, but it's not always practical to know all possible types at compile
> time (library dependencies, compilation time etc..).
[snip]
> Does anyone else have a use for this, or have another solution at hand?

Some long time back I had added support into Boost.Any for visitation:

http://boost-sandbox.cvs.sourceforge.net/boost-sandbox/boost-sandbox/boost/any.hpp?view=markup

http://boost-sandbox.cvs.sourceforge.net/boost-sandbox/boost-sandbox/libs/any/any_test.cpp?view=markup

Basically, the interface is as seen in the `test_dynamic_visit` test case:

  class my_visitor
    : public boost::dynamic_visitor_base,
      public boost::dynamic_visitor_interface<T1>,
      ...,
      public boost::dynamic_visitor_interface<TN>
  {
    ...

  public:

     /*override*/ void visit(T1&);
     ...
     /*override*/ void visit(TN&);

  };

and then an instance of the visitor may be passed to a boost::any:

  boost::any a = ...;
  my_visitor v;
  boost::apply_visitor(v, a);

Of course this assumes (parts of) the Boost.Visitor library, also in
the sandbox.

In any case, something based on this idea might be another alternative
to what you're proposing.

Eric