Re: [boost] [Review] Quantitative units - compile time vs runtime

From: Noah Roberts (roberts.noah_at_[hidden])
Date: 2007-03-27 17:33:25

Michael Fawcett wrote:
> On 3/26/07, Martin Schulz <Martin.Schulz_at_[hidden]> wrote:
>> B) The presentation layer consists of everthing that is visible to the
>> user, namely the GUI or the like. People in academics can hardly imagine
>> what expectations engineers might have towards the units of their data.
>> (Before entering industry, I was no exception to that rule :-) )  This
>> is partly due to historical reasons; the choice of units in the
>> presentation layer may vary by corporate guidelines, personal
>> preference, professional background of the user or geographical
>> location. If you want to get around with a single piece of code then,
>> you cannot live with a fixed units for the quantities in your program.
>> (The presentation layer, that is -- computation kernels are another
>> story.)
> 
> I'm a little unclear on why some people think run-time support is
> required for robust interaction with GUIs.  It could very well be a
> misunderstanding on my part, but with a user interface that allows
> arbitrary units, wouldn't you still need something like this:
> 
> // Assume quantity<abstract> provides run-time support
> quantity<abstract> on_dialog_ok()
> {
> 	int selection = get_combobox_selection(ID_COMBO_DISTANCE);
> 	double value = get_editbox_value<double>(ID_EDIT_DISTANCE);
> 	quantity<abstract> distance;
> 
> 	// This must be kept in sync with the dialog resource
> 	switch (selection)
> 	{
> 		case Meters:
> 			distance.reset(distance * meters);
> 			break;
> 		case Feet:
> 			distance.reset(distance * feet);
> 			break;
> 		// etc
> 	}
> 	return distance;
> }

Nope.  Your dialog manager would keep track of the unit and set 
appropriately upon initialization and/or user interaction.  Your reader 
would then be as simple as setting the value directly to the quantity. 
Alternatively you might have something like this:

quantity<abstract> on_dialog_ok()
{
   quantity<abstract> dist;
   dist.set_unit(combo.selected_item().data());
   dist.set_value(entry.value().as_double());
}

No more switch statement smell.