Re: [Boost-users] 'Hashable' concept for type erasure

23 Mar 2014

      AMDG

On 03/22/2014 05:13 PM, Samuel Christie wrote:
...
Hmm. Maybe it would be best if I make a custom wrapper class anyway, then.
All I really need is the ability to 1) record 'seen' objects in the
history, and 2) be able to tell whether one of the same objects is in the
history or not. Up until this point, I've been using unordered_sets, but I
was hoping to support more sophisticated memory models that would only
remember recent history, for instance.
Of those choices, I think (b) makes the most sense for my purposes, though
a or c would also be acceptable. I can't really see how I would achieve
that with just type erasure though. This could easily be a case of an
over-used hammer.
Do you actually need to erase the value_type?
If you can use has_count, then
everything will just work.
...
If you don't mind my asking, which of them would you recommend, which do
you think is achievable with type-erasure, and what other alternatives do
you think I should be considering?
If it were just count, I would go for (b).  However,
that isn't an option for insert, since it's impossible
to insert something that isn't an int into a container
that stores ints.  Whether consistency between the
functions is more important than the extra convenience,
only you can decide.  Now that I think about it, all these
options can all be implemented  in basically the same way
(you will need typeid_<_value> for this to work):

template<class T>
std::size_t generic_count(const any_set& c, const T& t)
{
  typedef any val_ref;
  // check that the type of t matches the value_type
  // of the container.
  if(typeid_of<_value>(binding_of(c)) == typeid(t)) {
    // capture the value in a way that's compatible
    // with the container.
    val_ref v(t, binding_of(c));
    return c.count(v);
  } else {
    return 0; // or throw or whatever
  }
}

You can put this directly in the concept_interface,
if you define it manually instead of using
BOOST_TYPE_ERASURE_MEMBER.

In Christ,
Steven Watanabe