-----Original Message----- From: boost-bounces@lists.boost.org [mailto:boost-bounces@lists.boost.org] On Behalf Of Matthias Schabel Sent: Thursday, March 29, 2007 2:50 PM To: boost@lists.boost.org Subject: Re: [boost] units: review
You're starting to raise more questions than you're answering. ;)
Hmm. I still don't understand the rationale for the extra multiplication operation. How is this any different from:
quantity<SI::meter> q(2);
It's a faux operation - the multiplication of a scalar times a unit (a class with no data members at all) decorates the scalar to produce a quantity of the appropriate unit and value type. The problem with using a raw value type for construction can be demonstrated here :
It's not a faux operation: the unit is already plainly specified as part of the type and the value of the quantity is plainly specified as the constructor argument. Pretty straight forward if you ask me.
using namespace SI;
/// this is two meters quantity<length> q(2);
First of all, that code should fail to compile. Ignoring the comment, it is (or should be) impossible to construct a quantity with a length of 2. The unit must be specified to the compiler somehow and comments don't count. That is why I distinquished a quantity from a measure in my preview of last year: a quantity is an abstract value that is qualified with one or more dimensions and cannot be constructed directly from arithmetic value. A measure on the other hand accepts a list of units and therefore can be constructed directly from an arithmetic value. A measure can also be converted to a compatible quantity and vice versa. Eric.