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 : (+33)1 69 15 66 35