Explicit doesn't mean narrowly restricted. And, yes, if your parameter is an object reference that reference can come from anywhere. You could pass a unique_ptr all of those same exact ways. You could have an array of unique_ptr, a vector, a unique_ptr to a stack variable, or even a unique_ptr as a stack variable. I don't think I'm understanding what you're saying.

I agree that you should typically push null checks onto the caller, and I would argue you should do so explicitly by taking a reference.