Question

What is the best way of returning a pointer from a factory ? Should it be a std::unique_ptr or std::shared_ptr or just a raw pointer?

Also, I was told, that, one should go for std::unique_ptr if there is containment and std::shared_ptr if there is aggregation. Is this the correct way ?

Answer 1

You should be considering raw pointers only in very special cases, such as passing the pointer across a DLL boundary.

Between shared_ptr and unique_ptr, my opinion is to prefer the latter. This leaves the interface more flexible for your users. They can always convert the returned unique_ptr to a shared_ptr if they desire to, but more importantly, they can also call unique_ptr::release and then manually manage the pointer (maybe not a good idea, but it leaves the option open).

If your factory needs to assign a custom deleter for the returned unique_ptr, a difference in behavior between unique_ptr and shared_ptr you should be aware of is that the former will not call the deleter if the managed pointer is nullptr, but the latter will. So, if your factory may return nullptr (maybe as a failure condition), and someone converts the unique_ptr to a shared_ptr, then make sure the deleter can handle being called with nullptr as its argument.