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

Subject: Re: [boost] [chrono] type_traits/common_type and integer/ratio
From: Michael Fawcett (michael.fawcett_at_[hidden])
Date: 2009-12-02 12:53:13

On Wed, Dec 2, 2009 at 12:46 PM, joel falcou <joel.falcou_at_[hidden]> wrote:
> Michael Fawcett wrote:
>>
>> template <typename X, typename Y>
>> BOOST_TYPEOF_TPL(X() * X() + Y() * Y()) magnitude_squared(const
>> vector2<X, Y> &v) {
>> Â  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. Â 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_
> {
> Â template<class Sig> struct result;
> Â template<class This,class X, class Y> struct result<This(vector2<X,Y>)>
> Â {
> Â  typedef BOOST_TYPEOF_TPL(X() * X() + Y() * Y()) type;
> Â };
>
> Â template<class X,class Y>
> Â typename result<magnitude_squared_(vector2<X,Y>)>::type
> Â operator()( vector2<X,Y> const& v )
> Â {
> Â  Â  return v.x*v.x + v.y*v.y;
> Â }
> };
>
> 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 )
> {
> Â magnitude_squared_ callee;
> Â 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.

I'll look into this.  It certainly sounds attractive!

I went down a similar path once, but I didn't like how I had to do
this for every free function - dot_product, cross_product, magnitude,
normalize, etc.  Maybe it's just a one time cost that pays off in the
end with usability.

Thanks,

--Michael Fawcett