Re: [Boost-users] [fusion] sequence conversion

From: Istvan Buki (buki.istvan_at_[hidden])
Date: 2008-07-23 03:49:53

On Tue, Jul 22, 2008 at 10:38 AM, Joel de Guzman
<joel_at_[hidden]>wrote:

> Istvan Buki wrote:
>
>     Why do you have to use metl::result_of?  Wouldn't it be simpler to
>>    have a
>>    result typedef inside the filter? ('typedef filter_t::result
>>    filter_result_t')
>>
>>
>> Again, this  started as a fusion learning experiment and I decided to use
>> the same convention as fusion to construct the result types.
>> I admit it is not clear to me what reason decided the fusion developers to
>> use the result_of namespace instead of a typedef inside the algorithm
>> classes.
>>
>
> Various reasons:
> 1) A typedef is not good enough[x]. It has to be a template because
>   the result-type is potentially dependent on the argument types.
>   IOTW, the actual result type can change depending on what you
>   pass.
>
> 2) The result_of namespace contains all the metafunction counterparts
>   of all the functions. They mimic, as much as possible, what you have
>   in MPL. So, if you want to do pure type computations, you can
>   use the result_of namespace.
>
> [x]: Take a simple function:
>
>       template <typename T>
>       T square (T val) { return val * val; }
>
> - What is the result type of square? Can it be a typedef?
> - How can you write a C++ utility that will give you this type?
>  E.g. I want to call this function with an argument of int,
>  but I want to know the result type beforehand so I can put
>  the result in a vector.
>
> It's obvious what the result is with our example above. But say
> I have another function with a more complicated result type.
> How do you automate this task for the user?
>

Thanks for explaining. It makes perfect sense now.