Re: [Boost-users] Dynamically creating new units

Subject: Re: [Boost-users] Dynamically creating new units
From: OvermindDL1 (overminddl1_at_[hidden])
Date: 2009-12-26 00:58:11

On Fri, Dec 25, 2009 at 10:49 PM, Brandon Olivares
<programmer2188_at_[hidden]> wrote:
> On 2009-12-25, OvermindDL1 wrote:
>> On Fri, Dec 25, 2009 at 11:33 AM, Brandon Olivares
>> <programmer2188_at_[hidden]> wrote:
>>> On 2009-12-25, OvermindDL1 wrote:
>>>> Â As for dynamically creating types, you can do that all you wish, but
>>>> you have no type information then (unless you use variant or any or
>>>> something, but that has needless overhead, well, maybe not variant).
>>>> That is the point of Boost.Units, to enforce that you are doing things
>>>> correctly, at compile time, it optimizes completely out at
>> run-time.
>>> Â OK thanks, perhaps you are right. I'm just really trying to figure out
>>> a way to make compilation a bit faster. My concern is that since these
>>> are in header files, well any source file that needs units will require
>>> them all to be processed again, and that is really
>> inconvenient.
>>
>> Yep, that is the major hazard of C++. Â If you learn how to structure
>> things differently and use forward declarations and less class member
>> functions (use external functions instead, ala C), that can speed up
>> compilation tremendously.
>
> OK, well perhaps there's another method. The primary bulk of the units I was defining were velocities. I just realized though that you can do l/t though, where l is some length and t is some time, and it doesn't have to define an existing unit.
>
> But if I do that, I'm not sure what type to actually define the variable as.
>
> sometype v = l/t;
>
> What is the type in this case?
>
> So for instance if l is light_year and t is hour, just as an example, I don't want to have to define light_year_per_hour as I was before. It's obvious that this is allowed:
>
> quantity<astronomical::light_year_unit> l(3.0 * astronomical::light_years);
> quantity<metric::hour_unit> t(8.0 * metric::hours);
> quantity<?> v(l/t);

You could just auto it.  :)
  BOOST_AUTO(v, l/t);
Then you do not need to bother figuring out what type it is, although
it is your light_year_per_hour, although maybe a little more efficient
representation.

If you just want the type of an expression like l/t if you want to
typedef it or something, then you can use BOOST_TYPEOF.  It is all in
the documentation.  :)