Re: [boost] [chrono] type_traits/common_type and integer/ratio

Subject: Re: [boost] [chrono] type_traits/common_type and integer/ratio
From: joel falcou (joel.falcou_at_[hidden])
Date: 2009-12-02 12:46:26

Michael Fawcett wrote:
> template <typename X, typename Y>
> BOOST_TYPEOF_TPL(X() * X() + Y() * Y()) magnitude_squared(const
> vector2<X, Y> &v) {
>    return v.x * v.x + v.y * v.y;
> }
>
> Typeof works, but I'm wondering if it's overkill and that perhaps
> common_type is much more lightweight and appropriate.  promote_args
> doesn't do quite what I need here, but thank you for pointing that
> facility out to me.
>
> Regardless, the original post was about how to proceed, and I
> definitely see reason to break those components out of Boost.Chrono.
> I didn't mean to hijack the thread  ;)
>   
Why not having a proper polymorphic function object that performs the 
computation and
hide the potential uglyness of the type computation ?

struct magnitude_squared_
{
  template<class Sig> struct result;
  template<class This,class X, class Y> struct result<This(vector2<X,Y>)>
  {
    typedef BOOST_TYPEOF_TPL(X() * X() + Y() * Y()) type;
  };

  template<class X,class Y>
  typename result<magnitude_squared_(vector2<X,Y>)>::type
  operator()( vector2<X,Y> const& v )
  {
      return v.x*v.x + v.y*v.y;
  }
};

magnitude_squared_ const magnitude_square = {};

or

template<class X,class Y>
typename boost::result_of<magnitude_squared_(vector2<X,Y>)>::type
magnitude_squared( vector2<X,Y> const& v )
{
   magnitude_squared_ callee;
   return callee(v);
}

Bonus is that you can ask the return type of an arbitrary 
magnitude_squared call in other template
function if neede using boost::result_of.

-- 
___________________________________________
Joel Falcou - Assistant Professor
PARALL Team - LRI - Universite Paris Sud XI
Tel : [phone]