Re: [Boost-users] [Boost.Functional.OverloadedFunction][C++03] Number of overloads in boost::overload_function<...>

Subject: Re: [Boost-users] [Boost.Functional.OverloadedFunction][C++03] Number of overloads in boost::overload_function<...>
From: Felipe Magno de Almeida (felipe.m.almeida_at_[hidden])
Date: 2012-10-17 07:24:22

On Wed, Oct 17, 2012 at 2:49 AM, Lorenzo Caminiti <lorcaminiti_at_[hidden]> wrote:
> On Tue, Oct 16, 2012 at 5:40 PM, Felipe Magno de Almeida
> <felipe.m.almeida_at_[hidden]> wrote:
>> Is there a way to know how many overloads are there in a
>> overloaded_function
>
> I'm not sure I understand your question... for example, in the code below:
>
> boost::overloaded_function<
>       const std::string& (const std::string&)
>     , int (int)
>     , double (double)
>> identity(identity_s, identity_i, identity_d);
>
> You want a way to query the type of identity at compile-time to get 3

I want to query boost::overloaded_function<x, y, z> to get 3 at compile-time.

> and/or the identity object at run-time to get 3? If so, why?
>
> Don't you
> know already know that such a number is 3 given that you are
> constructing the overloaded_function type passing 3 function
> signatures and/or constructing the identity object passing 3
> functors... what's your actual use case for this feature?

I don't know because I get the type as template parameter and I must
create an instance of that type with that many functors and return it.

>> without resorting to preprocessor metaprogramming
>
> The pp will only tell you the max number of possible overloads
> BOOST_FUNCTIONAL_OVERLOADED_FUNCTION_CONFIG_OVERLOAD_MAX but not the
> actual number of overloads that an overloaded_function type/object has
> at compile/run time. For example, in the code above
> BOOST_FUNCTIONAL_OVERLOADED_FUNCTION_CONFIG_OVERLOAD_MAX is always 5
> (unless you #define it differently) even if the number of identity
> overloads is 3.

Yeah, I would have to resort on knowing that the default template
parameter is void, or maybe
check if the type boost::is_function.

>> in C++03? It doesn't seem to be a MPL sequence, for example.
>
> No, it's not. The overloaded_function accepts a number of type
> template parameters from a min of 1 to a max of
> BOOST_FUNCTIONAL_OVERLOADED_FUNCTION_CONFIG_OVERLOAD_MAX.

That's a bit restricting. I could really use a overloaded_function
that received a MPL or Fusion
sequence as well.

> --Lorenzo

Regards,
-- 
Felipe Magno de Almeida