Re: [Boost-users] more Boost::Units questions

12 Jan 2009

      On Jan 12, 2009, at 3:48 PM, Steven Watanabe wrote:
...
AMDG
Mark Volkmann wrote:
...
On Jan 9, 2009, at 5:33 PM, Matthias Schabel wrote:
...
...
My biggest issue is that my recent software development  
experience has involved using Java much more than C++ and I'm  
trying to learn how to use a C++ library that makes pretty heavy  
use of templates. Is there a tutorial somewhere that shows  
examples of doing basic things with this library? The kind of  
things
The examples directory has simple, functional demonstrations of  
just about every type of functionality that anyone has wanted to  
date.
Okay, I've looked at the code related to conversions.  
conversion_factor.cpp sets up a bunch of double values that I  
assume are used to convert values in one unit to values in another.  
However, they aren't used in any code.
conversion_factor isn't used for conversions.  It just returns the  
value that
would be used for a conversion.
I thought that was the case. However, I haven't yet worked out how to  
create a conversion_factor for miles to kilometers. That should be  
just a few lines I think. Here's what I tried:

#include 
...
#typedef bu::us::mile_base_unit::unit_type mile;
...
double kilometer = 1000 * si::meter;
double mile_to_km = conversion_factor(mile, kilometer);

That last line doesn't compile. Do you see what I'm doing wrong?
...
...
I don't see any example that actually performs a conversion using  
conversion_factor. Can you show me how to use that to convert miles  
to kilometers? It looks like kilometers isn't defined in any of  
the .hpp files. I can just multiply meters by 1000 myself, but if  
there's something in the library that hides this detail, I'd like  
to know about it.
http://www.boost.org/doc/html/boost_units/Examples.html#boost_units.Examples...
The example uses SI and CGS, but converting between any units is  
similar.
Thanks. I've read that page several times already and still don't get  
it. I suspect my lack of experience with C++ templates is the issue. I  
see this:

   This example demonstrates the various allowed conversions between  
SI and CGS units.

   Defining some quantities
   quantitysi::length     L1 =  
quantitysi::length,int(int(2.5)*si::meters);
   quantitysi::length,int  
L2(quantitysi::length,double(2.5*si::meters));

     *  explicit casting of a quantity  to a different value_type  :

       quantitysi::length,int L3 =  
static_castsi::length,int >(L1);

     * and explicit casting of a quantity to a different unit :

       quantitycgs::length    L4 = static_castcgs::length
...
(L1);
However, I don't know what to specify in place of cgs::length in order  
to use miles instead of centimeters.

I wish that page contained an example of converting from some U.S.  
unit (such as miles) to some SI unit (such as meters).

---
Mark Volkmann