[Development] More Q_SLOTS! (was: Re: Proposal: adding overloads to pre-existing member functions is ok as long as not a slot)

[Volker Hilsheimer]

> On 28 Jul 2026, at 11:52, Marc Mutz via Development <development at qt-project.org> wrote:
>>> On 28 Jul 2026, at 02:13, Thiago Macieira <thiago.macieira at intel.com> wrote:
>>> 
>>> The connection mechanism is slightly more efficient when you pass a function 
>>> pointer, member or not, because it can tell what arguments you meant to 
>>> receive. When you pass a functor/lambda, the receiver may itself have 
>>> overloads or may be a template, so we can't do that. That means we can't look 
>>> at the receiver's arguments to do interesting things; instead, we must try 
>>> things and see if they compile.
>> 
>> 
>> Also, to the compiler, a lambda is always a unique type (even if the operator() has the same prototype, the captures might be different etc), so using lambdas in connect statements requires the compiler to general several specialisations of QtPrivate::QSlotObject that cannot be reused. Making several connections to the same kind of function pointer (i.e. same prototype) on the other hand gives the compiler more options to reuse an existing QSlotObject specialisation.
> 
> 
> I optimized QSlotObjectBase for a large number of instantiations, but Thiago's comment shows its a concern above the code bloat aspect.
> 
> I guess we should just start marking slots as Q_SLOTS again. I have observed a marked decline in Q_SLOTS in API reviews. Stuff (like setters on QObject subclasses) that would have been marked as Q_SLOTS in Qt 4 aren't, anymore, because "you can connect to non-slots" these days. We were also thinking about routing PMF-syntax calls addressing actual slots through qt_metacall() again, to avoid them accessing partially-deleted objects.
> 
> So IMHO, that's the discussion to be had: should we do more Q_SLOTS again?
> 
> I'm biased, obviously, but, to me, the answer is clear.


Agree, a function that might be connected to a signal should be marked as a slot. That’s not just a question of technicality, but also of communicating intent.

Volker