Re: [Boost-users] [units] Creating specific unit quantities

Subject: Re: [Boost-users] [units] Creating specific unit quantities
From: Ryan (mccorywork_at_[hidden])
Date: 2011-03-08 09:16:42

On Mon, Mar 7, 2011 at 5:35 PM, Matthias Schabel
<boost_at_[hidden]>wrote:

>      // takes any unit of length
>     template<class System>
>     void setX(quantity<unit<length_dimension,System> > const& x) {
>       // explicit conversion to si::length = meters
>       m_X = quantity<si::length>(x);
>      }
>

Is the following set of statements correct.  si::length has a
length_dimension and is part of the si::system.  boost::units::si::
meter_base_unit::unit_type has a length_dimension and is part of the
si::system.  boost::units::us::foot_base_unit::unit_type has a
length_dimension and is part of the us system.

Since an explicit conversion is needed I definitely don't understand the
differences between the following three statements.  I've included examples
to show my limited understanding.

 implicit conversion of quantity<Unit,Y> to quantity<Unit,Z> is allowed if Yand
Z are implicitly convertible.

quantity<si::length, int> to quantity<si::length, double>

 explicit conversion between quantity<Unit1,Y> and quantity<Unit2,Z> is
allowed if Unit1 and Unit2 have the same dimensions and if Y and Z are
implicitly convertible.

quantity<us::foot_base_unit::unit_type, int> to quantity<si::length, double>
Both have the same length dimension.

 implicit conversion between quantity<Unit1,Y> and quantity<Unit2,Z> is
allowed if Unit1 reduces to exactly the same combination of base units as
Unit2 and if Y and Z are convertible.

I'm not sure on the example for this one.

Ryan