Re: Conversion policies

Daniel Wallin <[email protected]> writes:

> At 17:20 2003-08-22, David Abrahams wrote:
>
>>2. If this is just an issue of user-customization, I am learning to
>>    prefer not distorting interfaces for broken compilers.  I'd rather
>>    ask those users to apply BOOST_TT_BROKEN_COMPILER_SPEC in order to
>>    produce the neccessary specializations for us to work in one
>>    uniform way.  I know that doesn't *directly* address partial
>>    ordering, but I think you can usually transform all partial
>>    ordering problems into partial specialization problems, can't you?
>
> Yes, in this particular case we could just use a template parameter
> to the converter to deduce the type, given that we don't care about
> non conforming compilers.
>
> template<int N>
> struct my_policy : converter_generator_tag<N>
> {
>     template<class T>
>     struct generate_converter

This should be called "apply" so that my_policy<N> is a metafunction
class.

>     {
>        typedef my_converter<T> type;
>     };
> };
>
> One important thing here is that we can't have any additional layers in
> conversion process; the result from the converter must go directly to
> the wrapped function. If it isn't obvious this is because we want to be
> able to return temporaries when the function expects a const&.
> This is also why it can be useful to deduce the T from const T&
> in the converter function.

Well, there *is* another way: you can use in-place construction in the
converter object and always return a T const& instead of a T.  I
believe that's exactly what happens in Boost.Python.

> How would we use the BOOST_TT_BROKEN_COMPILER_SPEC macro
> to provide specializations?

We'd tell the user to write 

     BOOST_TT_BROKEN_COMPILER_SPEC(X)

for any type X for which we need to be able to get
remove_reference<X>::type

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com



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