Re: [Boost-users] [units] unit conversion on construction in relation to argument passing

25 Mar 2011

      On 3/25/2011 10:40 AM, alfC wrote:
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On Mar 25, 9:46 am, Matthias Schabel  wrote:
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nice! But for some reason I believe this would work for argument
functions
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double f(quantitysi::length    v){ return 1.;}
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  quantitycgs::lengh    A = 4.*cgs::centimeter
  f(A);   // doesn't work! not matching function
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why is this? isn't the argument of the call a sort of construction
argument for the function argument, or is more like a plain
assignment. Is there a way to force the automatic conversion of the
function call.
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Or I am forced to use this other long call?
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f(quantitysi::length(A));
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Yes.  The constructor is explicit.
thanks for the clarification
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Or you can write it as a template function that takes any argument that is a quantity of length :
[...code...]
Thank you for the suggestion, the problem is that a nice looking
function such as
void f(
  quantity<volume>  v,
  quantity<temperature>  t,
  quantity<length>  l
){
  ... use v, l, t  in a given controlled set of units
}
transfroms into this mess,
template<
   class VolumeUnit,
   class TemperatureUnit,
   class LengthUnit
...
void f(
  quantity<VolumeUnit>  v_,
  quantity<TemperatureUnit>  t_,
  quantity<LengthUnit>  l_
){
  quantity<volume>  v(v);
  quantity<temperature>  t(t_);
  quantity<length>  l(l_);
  ... use v, l, t  in a given controlled set of unit
}
just to obtain the same effect. and that without even static asserting
that XXXUnit is_unit.
Not usually.  If your equations are dimensionally coherent, and not 
empirical, you can generally do without a lot of what you just showed. 
For example, one might be tempted to write the head/pressure conversion as:

quantitysi::pressure head_to_press( quantitysi::length head
                                     , quantitysi::mass_density den )
{
   quantitysi::acceleration g = 9.80665 * si::meters_per_second_squared
   return head * den * g;
}

All you really need to do though is:

template < typename System, typename T >
quantity< unit
head_to_press( quantity< unit, T> head
              , quantity< unit, T> den )
{
   quantity< unit, T> g(9.8066 * 
si::meters_per_second_squared);

   return head * den * g;
}

The only part you have to do explicit conversions on is your constants.

Of course, when dealing with empirical equations you've got a different 
thing going on.  More often than not you have to break out of the units 
anyway and use the "unsafe" operations like ::from_value.

Re: [Boost-users] [units] unit conversion on construction in relation to argument passing

Noah Roberts