[Development] API style guide: scoped enum or not?

Marc Mutz marc.mutz at qt.io
Thu May 4 17:27:21 CEST 2023

On 04.05.23 15:51, Sune Vuorela wrote:
> On 2023-05-04, Marc Mutz via Development <development at qt-project.org> wrote:
>> that keeps unported code running. The main issue isn't the scoping, the
>> main issue will be the missing implicit conversion to underlying_type.
> In few cases the implicit conversion to underlying_type is kind of important.
> Especially in the cases where the api has int and is mostly used with
> enums or is somehow user extendable.
> Qt::ItemDataRole is one of them that comes to mind.
> switch(role) {
>     case Qt::UserRole+1:
>     ....
> QEvent::Type is another one where one registerEventType, recieves an int
> and then force-cast it to QEvent::Type and still compares back to that
> int in other places.

The int is the problem, not the enum. With C++17, scoped enums have a 
kind of explicit ctor from underlying_type, so at least that direction 
is reasonably convenient. For the other direction, there's 

So, enum-backed APIs taking int will have to be ported to take the enum 
instead. On the plus side, you get type-richer and safer APIs.


Marc Mutz <marc.mutz at qt.io>
Principal Software Engineer

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