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

From: Stjepan Rajko (stipe_at_[hidden])
Date: 2008-08-13 16:41:05

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, 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.

>
> 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.

Kind regards,

Stjepan