[Interest] QFlags: Typesafe way to check "any flag set"

Mon Feb 23 11:52:18 CET 2026

Hi,

thanks for your elaboration!
I'll make myself a reminder to file to bugs (if the new Jira allows me to do that...), one for the QT_TYPESAFE_ENUM and one for the missing convertion to int.

My usecase is like this example https://godbolt.org/z/Y7Yaefvor
I want to know whether a specific flag is set, and return the result as 'bool' from a function, like so:

bool isAskingWhen(Question q)
{
    return {q & QuestionFlag::When};
}

As you can see in the CE link, this produces a "narrowing" warning, which I want to avoid.

I guess the proper solution would be

return ((q & QuestionFlag::When) != 0);

...or is there some better option?

Have a great day!

Robert

> -----Original Message-----
> From: Interest <interest-bounces at qt-project.org> On Behalf Of Giuseppe
> D'Angelo via Interest
> Sent: Sunday, 22 February 2026 22:18
> To: interest at qt-project.org
> Subject: Re: [Interest] QFlags: Typesafe way to check "any flag set"
>
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> Hi,
>
> Il 19/02/26 08:31, Schimkowitsch Robert ha scritto:
> > Hi Guiseppe,
> >
> > thanks for the reply!
> >
> > QT_TYPESAFE_FLAGS features prominently in the Qt documentation, see
> > e.g
> > https://doc/.
> > qt.io%2Fqt-6%2Fqflags.html%23operator-and-
> eq&data=05%7C02%7Crobert.sch
> >
> imkowitsch%40andritz.com%7Ca09bda655ebc4cffb8dc08de72581984%7C678
> 5298f
> >
> e857464b9b4b807178402632%7C1%7C0%7C639073919876105010%7CUnkno
> wn%7CTWFp
> >
> bGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4z
> MiIsIk
> >
> FOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=NBL6HbErIfvoP
> VY7Vxq%
> > 2BOLxnDEczqQBHD9YHKXm9rVI%3D&reserved=0
>
> I think that's a mistake, as QT_TYPESAFE_FLAGS itself is never documented...
>
> > What I found confusing is that operator!() is documented, but operator
> bool() is not. Also, the implicit conversion to int is not documented (at least I
> could not find it in the above page).
>
> That's also a mistake. Would you mind filing a QTBUG for documentation?
>
> > There is "toInt()", but then I'd have to explicitly write
> >
> > if (flags.toInt())
> >     // something
> >
> > I noticed that I can apparently static_cast flags to bool, but if I want to return
> a bool from a function, I have to use the explicit cast (which would be fine, if
> that is the way it's supposed to work).
>
> Going back to:
>
> > #ifdef QT_TYPESAFE_FLAGS
> >     constexpr inline explicit operator Int() const noexcept { return i; }
> >     constexpr inline explicit operator bool() const noexcept { return
> > i; } #else
> >     constexpr inline Q_IMPLICIT operator Int() const noexcept { return i; }
> >     constexpr inline bool operator!() const noexcept { return !i; }
> > #endif
>
> if QT_TYPESAFE_FLAGS is not defined (the default), then a QFlags object will
> implicitly convert to bool through the operator Int(), including on function
> return:
>
> https://gcc.god/
> bolt.org%2Fz%2FoPjhM661G&data=05%7C02%7Crobert.schimkowitsch%40an
> dritz.com%7Ca09bda655ebc4cffb8dc08de72581984%7C6785298fe857464b9b
> 4b807178402632%7C1%7C0%7C639073919876130763%7CUnknown%7CTWF
> pbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4
> zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=as1f8Xa
> 5IHoi1Jveknr0IUn0IvismBvsOSjbvBT5VR4%3D&reserved=0
>
>
> If instead it is defined (not recommended), then you can still test them using
> `if (flags)`, but you need an explicit cast when using `return` or similar forms
> of copy initializations, because the operator towards bool becomes `explicit`.
> This is a somewhat annoying quirk of C++ that may cause source
> incompatibilities (cf.
> https://codere/
> view.qt-
> project.org%2Fc%2Fqt%2Fqtbase%2F%2B%2F311100&data=05%7C02%7Crob
> ert.schimkowitsch%40andritz.com%7Ca09bda655ebc4cffb8dc08de72581984
> %7C6785298fe857464b9b4b807178402632%7C1%7C0%7C6390739198761487
> 05%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjA
> uMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C
> %7C%7C&sdata=sy3z4wL12FyIzfNsthVEpc5x%2Bx7KtqlNdztN7QErfGc%3D&re
> served=0 )
>
> For example,
>
> // under QT_TYPESAFE_FLAGS
> QFlags<E> flags;
>
> bool f() {
>    bool ok = flags;   // ERROR
>    bool ok { flags }; // OK
>    if (flags) {}      // OK
>    return flags;      // ERROR
>    return bool(flags); // OK ; or (bool)flags, static_cast<bool>(flags), etc.
> }
>
>
>
> HTH,
>
>
> --
> Giuseppe D'Angelo | giuseppe.dangelo at kdab.com | Senior Software
> Engineer KDAB (France) S.A.S., a KDAB Group company Tel. France +33 (0)4 90
> 84 08 53,
> http://www.kd/
> ab.com%2F&data=05%7C02%7Crobert.schimkowitsch%40andritz.com%7Ca0
> 9bda655ebc4cffb8dc08de72581984%7C6785298fe857464b9b4b80717840263
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