The problem with the "multiplier" usage is it uses a unit name (plural, singular, whatever) for both type information and rvalue expressions.
quantity<second[s]> t1 (2.0 * second[s]);
This usage may be preferable to scientists and engineers but not C++ programmers, I'll gar-on-tee.
What you wrote doesn't make sense, since seconds is an object, not a type. The point is that the actual syntax is: quantity<SI::time> t1 = 2.0 * SI::seconds; I think this is an important abstraction, that there is a fundamental difference between the physical dimension (time), and the particular units used to measure it (seconds). The syntax makes this clear. The user may be aware that SI::time is represented in seconds in the library internals, but they don't *need* to know this to write dimensionally correct code. (And they will find out eg when they output the value). The fact that SI::seconds is just a static object of type SI::time is not relevant to the fact that "time" and "seconds" have very different meanings. This becomes more clear when you do something like quantity<SI::time> t1 = 2.0 * SI::seconds; quantity<SI::time> t2 = 2.0 * English::fortnights; cout << t1 - t2 << endl; Here, the user doesn't need to worry that t1 and t2 were specified in different units, because they both are quantities of type quantity<SI::time>, so it makes sense to subtract them. The multiplicative syntax seems to me like the most natural choice to handle this usage. As far as I can tell, you seem to be arguing that the usage of "SI::seconds" in the definition of t1 is redundant, because SI::time already implies that the time is measured in seconds, but that's not really the spirit of what a quantity<> is. A quantity<> doesn't represent something in a particular set of units (such as seconds), but rather something that measures a particular physical dimension (such as time). The fact that the quantity<> internally has to choose a unit to represent that dimension is not as important as the fact that the quantity<> does fundamentally represent a physical dimension. Anyway, that's just my interpretation of how this library design works, and IMHO I think it is a good, logical design. -Lewis