Re: [Boost-users] [units] Creating a velocity unit

Subject: Re: [Boost-users] [units] Creating a velocity unit
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2013-02-21 13:41:10

AMDG

On 02/21/2013 10:25 AM, Ryan wrote:
> Thanks for the link it was helpful.  I tried both methods and they both
> compiled.  It seems we can use the simpler form of decltype because it
> provides perfect forwarding.  Is this a correct interpretation?
> 

Yes.  The two are equivalent.  decltype is a
bit nicer, as long as you know that you'll
only be using a compiler that supports it.

> Neither of these two typedef seems to behave as I expected.  When creating
> a quantity with these typedefs I had to call them as a method to get the
> code to compile.
> 
> quantity<si::velocity> a(2.3 * nautical_miles_per_hour1());
> quantity<si::velocity> b(2.4 * nautical_miles_per_hour2());
> quantity<si::velocity> c(2.5 * si::meters_per_second);
> 
> Is there a way to get my type of nautical_miles_per_hour to behave like
> meters_per_second?
> 

The difference is that meters_per_second is an object,
not a type:

static const si::velocity meters_per_second;

At this point, I prefer to use the type, because
I don't like having to come up with two different
names for the type and the global constant.  I suppose
that it would also be possible to use only the
object, and use decltype whenever you need a type.

> 
> //Provided for the Discussion
> using namespace boost::units;
> typedef metric::nautical_mile_base_unit::unit_type  nautical_mile_unit;
> typedef metric::hour_base_unit::unit_type  hour_unit;
> 
> typedef decltype(nautical_mile_unit() / hour_unit())
>  nautical_miles_per_hour1;
> typedef divide_typeof_helper<nautical_mile_unit, hour_unit>::type
>  nautical_miles_per_hour2;
> 

In Christ,
Steven Watanabe