Re: Re: Conversion policies

At 15:19 2003-08-08, Daniel Wallin wrote:
>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.
><snip>

Ok I've given this some thought. The issue is that we need to keep the 
converter
object instances on the stack of the invoker function, to be able to have
storage between check() and to_cxx(). This is important for lvalue-conversions
which can perform the entire conversion in the checking part, and stores the
pointer. It's likely important for other more advanced policies as well, which
could need intermediate storage.

---
Daniel Wallin



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