On 1/15/07, Matthias Schabel <boost@schabel-family.org> wrote:
And quantity conversion should require explicit casting:
quantity< double, SI::centigrade > q4; quantity< double, SI::kelvin > q5; q5 = quantity_cast< SI::kelvin >(q4);
Of course, the centigrade<->kelvin issue opens a new can of worms since that conversion is affine, not linear. At present, this conversion is not implemented in the library. My preference would be to define a value_type that models a unit with offset that could be converted to linear value_types... Right now, the library allows implicit unit system conversions (as described above), and, with quantity_cast, would allow casting as well.
That difference between an affine space and a linear (vector) space is an important one. The Boost date & time library makes the distinction: http://www.boost.org/doc/html/date_time.html#date_time.domain_concepts If this library made the vector/point distinction, temperature conversion would be a non-issue. Something like: dimensioned<int, SI::kelvin> k1(100*SI::kelvin), k2(200*SI::kelvin); quantity<int, SI::kelvin> kdiff = k2 - k1; quantity<int, SI::celsius> cdiff = difference; // Celsius and Kelvin differences are really the same thing. —Ben