Re: [boost] [fusion][parameter] a solution for named parameters / parameter packs using fusion maps

From: David Abrahams (dave_at_[hidden])
Date: 2008-08-13 17:03:20

on Wed Aug 13 2008, "Stjepan Rajko" <stipe-AT-asu.edu> wrote:

> On Wed, Aug 13, 2008 at 11:54 AM, David Abrahams <dave_at_[hidden]> wrote:
>>>
>>> Testing is a good suggestion, but I don't have the time right now to
>>> do that :-) I'm not necessarily interested in replicating all
>>> Boost.Parameter features.  I'm quite happy with the fusion-based
>>> solution as a simple one.
>>
>> Okay, so what are you trying to accomplish?
>>
>
> Being able to have named parameters and not using Boost.Parameter.

Well, then I'm afraid that Boost.Parameter can never help you :-)

>> Well, at the time we redesigned the library, I encouraged Daniel Wallin
>> to look into whether we could gain anything by using fusion, because the
>> fundamental data structure we use is like a fusion map.  He said we
>> couldn't gain anything significant, and at the time I took his word for
>> it.  The entire code corresponding to fusion::map is contained in
>> boost/parameter/aux_/arg_list.hpp.  The rest of the library consists
>> primarily of functionality you're not providing.
>>
>
>>From what you're saying, it could be that our approaches are similar
> but using a different map type.  Using the fusion-based approach, I
> can do this:
>
> class named_constructable
> {
> public:
>     // a macro could extend the number of parameters if desired
>     template<typename FM0, typename FM1>
>     named_constructable(const FM0 &a0, const FM1 &a1)
>     {
>         // this could be done lazily...
>         map<field::label, field::size>::type args = a0 & a1;
>
>         label = args.at<field::label>();
>         size = args.at<field::size>();
>     }
>     std::string label;
>     int size;
> };
>
> BOOST_AUTO_TEST_CASE( test_multi_param_construction )
> {
>     named_constructable constructed(label_ = "hello", size_ = 1);
>     BOOST_CHECK_EQUAL(constructed.label, "hello");
>     BOOST_CHECK_EQUAL(constructed.size, 1);
> }
>
> If our approaches are similar, that would suggest Boost.Parameter
> could do the same.

With Boost.Parameter it would look like:

  class named_constructable
  {
  public:
      // a macro could extend the number of parameters if desired
      template<typename FM0, typename FM1>
      named_constructable(const FM0 &a0, const FM1 &a1)
      {
        label = (a0,a1)[_label];
        size = (a0,a1)[_size];
      }
      std::string label;
      int size;
  };

But if you use the recommended method of building constructors,
Boost.Parameter also supports:

  named_constructable x0("hello", 1);
  named_constructable x0("hello", _size=1);
  named_constructable x0(_label="hello", _size=1);
  named_constructable x0(_size=1, _label="hello");

and in this case (using deduced parameters), even

  named_constructable x0(1, "hello");

>> I'm just saying, if you're going to make comparisons of the two
>> libraries, they should be informed comparisons and not speculation.
>
> Well, I tried to make sure that my speculations were qualified as such
> (and I consider speculation to be a valid component of discussion).
> If my speculations came across as assertions, I apologize.

They didn't, but I did find your presumption that we hadn't already gone
over this ground in designing the library rather... premature.

-- 
Dave Abrahams
BoostPro Computing
http://www.boostpro.com