:(įurther tests show that the & is used for the slot, but in an unusual fashion. Each connected slot gets a new copy of the object. I can confirm, from testing, that the const & part is, as alluded to, completely ignored in queued connections. The second version appears just to register the signal in the global namespace for easier use. I have the feeling that the simple shared_ptr version is all that is needed and it is the boost namespace that is interfering with the signals. Having some sort of guarantee that this is supposed to work as such would be nice however. It appears that the const reference type in a signal/slot will just be marshalled as a normal shared_ptr, which is totally okay here. The documentation here isn't clear on what actually happens. I'm trying to verify now whether this truly works correctly. QRegisterMetaType >('std::shared_ptrconst&') So far I have found that I can simply do this: Q_DECLARE_METATYPE(shared_ptr) If the shared_ptr does in fact just use an atomic op this cost is also trivial, the general question about const reference in signals remains however.* As multiple threads use the object this introduces a noticeable slow-down/bottleneck.

**The biggest problem for performance is not the copying time, but the amount of mutex locking/unlocking as the object is copied to every receiver - there are lots of them. Many of the standard APIs have QString const & so I assume it is fundamentally possible and I just can't figure out the syntax. The same syntax for reference type isn't working: Q_DECLARE_METATYPE(shared_ptr const &)

I know how to do this without a constant reference, that is simply the type shared_ptr but for efficiency* reasons I'd like to avoid the copying.

Is it possible, and if so, how can I create a signal/slot in Qt that is a const reference to a shared_ptr? I want a signal that looks like this:

Qt visual studio signal slot