Re: [boost] Boost Units library preview

From: Matthias Troyer (troyer_at_[hidden])
Date: 2006-08-19 03:55:00

On Aug 18, 2006, at 7:43 PM, Andy Little wrote:

>
> "Eric Lemings" <lemings_at_[hidden]> wrote in message
>>
>> I think Thorsten's question is a sympotom of a larger issue.
>>
>> Why does Quan use such convoluted terminology?  Fixed quantity?
>
> fixed_quantity is  the name of the class template implemented so  
> far ( one of 3
> or possibly 4 different classes). It has gone through various  
> names. Once just
> quantity, then ct_quantity, then t1_quantity, now fixed_quantity.  
> It is a
> quantity where the Unit is fixed at runtime. All suggestions for  
> alternate names
> are welcome.
>
>> Abstract quantity?
>
> An abstract quantity is a quantity without a numeric value or a  
> conversion
> factor. It still conforms to dimensional analysis and dimensional  
> math.
> In fact the abstract quantity concept (small c) has now been  
> gobbled up by the
> all encompassing Static Unit concept.

I am confused. What is a quantity without a numeric value? Just a unit?
I don't see the reason for a quantity without a value. Just the name  
quantity implies a value.
>
>> Named quantity?
>
> A named quantity has a name. In practise in Quan-0_2_0 this means  
> that, where U
> is a model of StaticUnit and is_named_quantity<U>::value is true, that
> get_named_quantity_traits<U>::type , returns a traits class, which  
> provides
> useful information, such as the name of the quantity and other  
> information
> mainly useful for I/O purposes.

I assume that this mean that the unit has a special name, and  
additional information?

>> Concrete quantity?
>
> A concrete quantity is a quantity which has a unit and a numeric  
> value.

Same confusion as with abstract quantity. Isn't abstract quantity  
just the unit? I

>> Anonymous quantity?
>
> An anonymous quantity doesnt have a name, kind of the opposite of a  
> named
> quantity. is_named_quantity<U>::value is false for anonymous  
> quantities in the
> next version of Quan, and so their  Named Quantity Traits class is  
> effectively
> void.

Does this mean that the name will be derived, like in m/s^2 ? Or that  
there  is no unit symbol at all that can be printed? Again this seems  
to be a property of the unit of the quantity. Wouldn't unnamed unit  
be a better name?

>
>> Static unit?
>
> This is a proper C++ Concept. Effectively its a compile time database
> which can be queried about various aspects of a particular  
> fixed_quantity. It is
> a template parameter to fixed_quantity:
>
> fixed_quantity<StaticUnit,NumericType>
>
>> United value?
>
> It is a detail of the implementation. Has it leaked into the docs  
> somewhere? If
> so I will remove it as it may change without notice.
>
>> A
>> general-purpose library should use simple terms and a bare minimum
>> of names.

If I understand the above concepts correctly then we only need three  
terms:

- named unit
- unnamed unit
- quantity (value + unit)

Matthias