[Development] QOptional

[Julien Blanc]

On 21/08/2014 00:07, Thiago Macieira wrote:
> On Wednesday 20 August 2014 20:49:03 Marc Mutz wrote:
>> I don't find the QVariant::isNull behaviour any useful or intuitive. It's
>> too  smart. You can always use v.value<T>().isNull() because value() will
>> return a default-constructed T if invalid.
> I agree it's not intuitive, but it's there and this feature is used by QtSql.
>
> When getting a nullable column, a null entry is signalled by a typed QVariant
> for which isNull() == true.
>
> I'd have preferred that QtSql use optionals to indicate such columns:
>
> 	varchar		=> QOptional<QString>
> 	varchar not null	=> QString

QString may not be the best example, as it already supports the 
nullability concept, which makes QOptional<QString> quite useless...

> The question I have is: how can we prepare a transition from current state
> (both a QString) in such a way that doesn't break existing code?
>
> Should it be a connection-time option for QSqlDatabase? Maybe a query-time
> option?

Do you mean that you would change QSqlRecord::value to return, instead 
of a QVariant that can be null, a QVariant holding either an optional or 
a non-optional object ? I don’t see any advantages in doing so. That 
would make sense if you could get rid of the whole QVariant stuff, but 
that would imho need a complete redesign of QtSql. Otherwise, it’ll just 
add an extra layer, not providing any usefull feature (QVariant already 
does the job quite well in providing the nullability feature).

Regards,

Julien