[Boost-users] [Boost.Units] quantity<unit> problem with template specialisation algorithm?

Subject: [Boost-users] [Boost.Units] quantity<unit> problem with template specialisation algorithm?
From: Thomas Shorrock (Thomas.Shorrock_at_[hidden])
Date: 2010-05-13 10:14:43

Hello,

many thankyous to the developers of Boost.Units and Boost libraries generally, 
I have been using them for a few years now and this is my first difficulty.

I have been having some problems when using template specialisation (g++ 4.4.3) with Boost.Units (Boost 1_42_0). 
When calling a specialised template function I have found that I need to be very careful with the argument,

In short,
specialised_templated_function(1.0 * boost::units::si::metre)   works fine
specialised_templated_function(1    * boost::units::si::metre)   specialised function is not found (default tried)

This caught me out for a while and so I thought I would report it in case it is a small bug.
If this is not a Boost issue then my appologies.

In detail, here is the code to reproduce:

#include <iostream>
#include <boost/units/systems/si.hpp>

namespace bu = boost::units;

/** Convert arbitrary units into seconds
 * When a distance is provided, the  speed parameter is used to calculate the time.
 * Otherwise the speed variable is ignored.
 */
template<class unit>
bu::quantity<bu::si::time>
any2second(const unit& t, 
  const bu::quantity<bu::si::velocity>& speed
    =1*bu::si::meters_per_second)
{
  return bu::quantity<bu::si::time>(t);
}

/* Specialisations for quantity<length> */
template<>
bu::quantity<bu::si::time>
any2second(const bu::quantity<bu::si::length>& d,
           const bu::quantity<bu::si::velocity>& speed)
{
  return bu::quantity<bu::si::time>(d / speed);
}

int main()
{
  //The following use the default function and work fine
  std::cout<<"any2second(1.0*s) = "<<any2second(1.0*bu::si::second)<<std::endl; 
  std::cout<<"any2second(1*s)   = "<<any2second(1*bu::si::second)<<std::endl;

  //The following uses the specialised function and works fine
  std::cout<<"any2second(1.0*m) = "<<any2second(1.0*bu::si::metre, 1*bu::si::meters_per_second)<<std::endl;
  
  //The following tries to use the defualt function rather than the specialisation.
  //It does not compile because (correctly) a metre cannot be converted into a second.
  //std::cout<<"any2second(1*m)   = "<<any2second(1*bu::si::metre, 1*bu::si::meters_per_second)<<std::endl;
  
  return 0;
}

Many thanks,

Tom

The Institute of Cancer Research: Royal Cancer Hospital, a charitable Company Limited by Guarantee, Registered in England under Company No. 534147 with its Registered Office at 123 Old Brompton Road, London SW7 3RP.

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