Thread: ideas for a generalized unit-handling class

it would be wonderful if i could implement a generalized unit class, but as simple of a concept as it is, a good way to implement it eludes me...

here's what i have so far, obviously it's clumsy and woefully inadequate, but at least it's a starting point for some discussion.

Code:

class unit
{
        public:
        unit(size_t NumUnits) : numUnits(NumUnits){} 
        double exponent;
        double coeff;
        const  size_t numUnits;
        unit *units;
        double convert(unit u)
        {
                //returns a conversion factor if this and u are of compatible dimensions
        };
        unit multiply(unit u)
        {
                //adds exponents of this->units of this and u->units
        }
        unit divide(unit u)
        {
                //subtracts exponents of this->units of and u->units
        }
};

there has got to be a more elegant and robust way of doing this. any suggestions (including 3rd party wares) would be greatly appreciated.

You're talking about units of measurement, I assume? First question is how would clients of the class use it? And are you talking about all units (length, volume, pressure, force, mass, etc.)?

A class hierarchy seems to wrong way to approach this. I'd start with a set of conversion functions.

Also, you might want to use operator overloading.

Try something like this:

5dam + 100cm = 51m

Code:

unit m = unit(5).deca_meters() + unit(100).centi_meters();
// m.meters() == 51

Originally Posted by rudyman

Also, you might want to use operator overloading.

Try something like this:

5dam + 100cm = 51m

Code:

unit m = unit(5).deca_meters() + unit(100).centi_meters();
// m.meters() == 51

yes, that would be a good idea for unit prefixes d,m,c,k,etc; but otherwise, it is not particularly useful.

i'd have to create operators for hundreds of possible permutations of units.

Originally Posted by CornedBee

Boost's review queue contains two different candidates for unit libraries.

just dl'd 1_35.07z and it doesn't appear to have been released yet, tho they say they'd added it. :\

so, what i see as the major weakness in the above class is the sorting of the array unit::units

in order to check dimensional compatibility or perform an operation i would have to loop over the entire array numUnits times.

Probably not too expensive, but it is a rather ugly brute force implementation...

yes, i use "kms" units as my algorithmic standard. having an internal standard like that is required to do a conversion at all; otherwise how do you know how many miles are in a parsec, for example.

i often need to change the way that the quantities are displayed; e.g. displaying 1ppm instead of 0.0001 wt%.

also, our customers in europe aren't particularly fond of "psi", whereas our U.S. customers are quite happy with that and might not be familiar with kPa.

it would also be useful for my purposes in my calculations for checking or preventing unit-conversion errors.



anyway, if i'm interpreting you correctly here, you're suggesting that i make a few derived classes of unit to represent the principal quantities: e.g. mass, length, time, temperature, etc.; then create a multi-unit container class that handles the conversion.

i had considered something like this, but i am at a loss for an efficient way to perform the operations and dimensional consistency checking...