Re: Re: [Implementation] Calling wrapped functions, converters, policies

David Abrahams wrote:

> Daniel Wallin <[email protected]> writes:
> 
> 
>>David Abrahams wrote:
>>
>>>Daniel Wallin <[email protected]> writes:
>>>
>>>
>>>>David Abrahams wrote:
>>>>
>>>>
>>>>>I would be strongly inclined to say that unless you _explicitly_ say
>>>>>otherwise, each C++ argument comes from the Py/Lu argument in the
>>>>>same position.  Your example uses implicit argument shifting, which
>>>>>makes me  nervous.  
>>>>
>>>>Why does it make you nervous? Is there any potential danger in doing
>>>>this that I'm missing?
>>>
>>>Well, suppose in a 5-arg function you write:
>>>      getC++.arg(2).from_Python_arg(1)
> 
> 
> This is a policy which maps Python argument 1 to C++ argument 2
> 
> 
>>>[Sorry, can't stop inventing syntax ;-)]
>>>The question is, what happens to to the other 4 arguments?  It's far
>>>from obvious to me.  Now suppose you add
>>>      , getC++.arg(1).from_Python_arg(4)
>>>Now it's even less obvious.
> 
> 
> This is an additional policy that maps Python argument 4 to C++
> argument 1

OK. So we would end up with:

   get(..., mpl::int_<2>)
   {
      return get(..., mpl::int_<1>());
   }

   get(..., mpl::int_<1>)
   {
      return get(..., mpl::int_<4>());
   }

   template<int N>
   get(..., mpl::int_<N> n)
   {
      return whatever_native_thing_we_have(n);
   }

This has nothing to do with (implicit) shifting though, but rather with
argument_package. So in this case, nothing would happen to the other
arguments. I'm missing what's not obvious here, sorry. What I
considered before was just converters that could merge several
(consecutive) Python/Lua arguments into one C++ argument, in which case
implicit shifting would be great. I'm sure that there might be cases
that I haven't thought about though..

-- 
Daniel Wallin



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