Re: Re: Conversion policies

At 15:13 2003-08-08, Daniel Wallin wrote:
>At 14:09 2003-08-08, David Abrahams wrote:
>>Daniel Wallin <[email protected]> writes:
>> > OK. Sorry for being vague. I'll try with some short examples:
>> >
>> > This is the policy system which handles the converter generators and
>> > precall/postcall. Consider a typical invoke-function:
>> >
>> > template<class R, class A0, class A1, class Policies>
>> > ret_type invoke(R(*fn)(A0, A1), const Policies& policies)
>> > {
>> >     // find the converter generators for the given parameter index
>> >     typedef typename find_conversion_policy<1, Policies>::type 
>> conversion_gen1;
>> >     typedef typename find_conversion_policy<2, Policies>::type 
>> conversion_gen2;
>> >
>> >     // generate a converter for the associated type
>> >     typename conversion_gen1::template generate_converter<A0,
>> >     xxx_to_cpp>::type c1;
>> >     typename conversion_gen2::template generate_converter<A1,
>> >     xxx_to_cpp>::type c2;
>> >
>> >     // the converter objects are later applied to the parameter passed 
>> from the
>> >     // interpreter state to get the actual parameters.
>> > }
>>
>>OK, I understand now.
>>
>>here's what I was thinking might make for a more efficient
>>implementation.  Instead of using all of that TMP inside of every
>>wrapped function implementation, arrange the policies type so that it
>>can do all the work with overloading:
>>
>>template<class R, class A0, class A1, class Policies>
>>ret_type invoke(R(*fn)(A0, A1), const Policies& policies, ArgTuple t)
>>{
>>     if !(
>>          policies.check(_1, identity<A0>(), t[0])
>>        , policies.check(_2, identity<A1>(), t[1])
>>     )
>>     return kOverloadFailed;
>>
>>     return policies.from_cxx(
>>         fn(
>>              policies.to_cxx(_1, identity<A0>(), t[0])
>>            , policies.to_cxx(_2, identity<A1>(), t[1])
>>         )
>>     );
>>}
>>
>>Just a thought.
>
>That's pretty nice, but wouldn't the forwarding cause problems with the return
>type? What should to_xxx() return?

Never mind that, obviously it would just look like:

template<int N, class T>
T to_cxx(boost::arg<N>, identity<T>, ..);

---
Daniel Wallin



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