[boost] Re: Review Request: FC++

From: David Abrahams (dave_at_[hidden])
Date: 2003-10-09 12:14:46

Douglas Gregor <gregod_at_[hidden]> writes:

> On Thursday 09 October 2003 12:25 pm, Joel de Guzman wrote:
>> David Abrahams <dave_at_[hidden]> wrote:
>> > *I'm* not confused.  I have no problem with the idea of having a
>> > class
>> >
>> >   a. whose _instances_ can be used as a function which squares numbers
>> >
>> >   b. which is itself a metafunction class that computes the instance's
>> >      return type based on the arguments passed
>>
>> Snips...
>>
>> For the record, I am not against naming it "apply". However, I am not sure
>> how that will jive with Doug's uniform return type proposal. I'm sure
>> we do agree that there must be one and only one way to do it.
>
> We've discussed this before, although I can't find a reference to it because I 
> believe it was an e-mail conversation. The reason we did not name the 
> "result" member class template "apply" is that we envisioned function object 
> types that were both metafunction classes and function objects, and therefore 
> would have both "apply" (in the MPL sense) and "result" (in the result_of 
> sense) member class templates. For instance:
>
> struct find_
> {
>   // Metafunction  class
>   template<typename Sequence, typename MetaFunction>
>     struct apply
>     { typedef <MPL find algorithm result> type; }
>
>   // Function object
>   template<typename FunctionCall> struct result;
>
>   template<typename F, typename InputIterator, typename T>
>     struct result<F(InputIterator, InputIterator, T)>
>     { typedef InputIterator type; };
>
>   template<typename InputIterator, typename T>
>     inline InputIterator 
>     operator()(InputIterator first, InputIterator last, const T& x)
>     {  return <standard library's find() algorithm result>; }
> } find;
>
> Perhaps I'm sticking words in everyone's mouth, but I had thought that we 
> agreed on this. I believe "we" included (at least): Joel, Dave A. Jaakko, and 
> Aleksey.

Yeah, sounds familiar.

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