I'm confused by this syntax. The intent here is to assign a quantity of 2 meters to an "anonymous" quantity of length, correct? If so, wouldn't the following syntax make more sense?
quantity<SI::length> q = quantity<SI::meter>(2);
The "2*meters" syntax just seems rather obscure. Likewise, a "measure", a quantity with a specified unit, should be constructed without the need for qualification.
quantity<SI::meter> q (2);
That is, the "2*meters" should not be required when the unit is already explicitly specified as part of the type.
By the way, while meter is an SI unit and therefore belongs in the SI namespace, length is not an SI dimension hence the name "length" does not belong in the SI namespace. It should probably be elevated to the boost::units namespace. I don't believe there is such a thing as an "SI dimension"; i.e. other system of measurement (e.g. Imperial) have units of length as well.
Here's the reasoning for this syntax : quantity<SI::length> represents a quantity of length in the SI system, not a generic length. Because everything is specified at compile time, units and quantities must be completely defined, otherwise it would be impossible to achieve zero runtime overhead. In this usage, meter is, in fact, completely equivalent to SI::length() - it is just a static constant. Another possibility would have been something like this : quantity<SI::meter> q(2*SI::meter()); where SI::meter is now a typedef for the full type representing an SI unit of length (what SI::length is now). Unfortunately, as you add more units, the extraneous parentheses become cumbersome. For example, the definition of the ideal gas constant would go from 8.314*joules/kelvin/mole to 8.314*joules()/kelvin()/mole(). Still legible, but definitely more difficult to follow. Furthermore, in the current syntax, if you had a program that used a bunch of SI units and you wanted to change it to CGS, you could just search and replace SI->CGS and everything would work correctly. If we used the alternative syntax, then a search and replace SI::meter-
CGS::centimeter would do this
quantity<SI::meter> q(2*SI::meters); -> quantity<CGS::centimeter> q (2*CGS::centimeters); which would break the code. Matthias