[Const-me]

Quite a few times, I've made C++ library APIs like this:

    struct iface
    {
        virtual ~iface(){ }
        virtual void doSomething() = 0;
        static std::unique_ptr<iface> create();
    }

I don't know good ways to replace unique_ptr here.

Requiring library users to #include the concrete implementation is not good: inflates compilation time, pollutes namespaces, pollutes IDE's autocompletion DB.

Can go C-style i.e. pass double pointer argument to the factory, or return raw pointer. But then user needs to remember to destroy the object.

[ncmncm]

Returning them by value is fine. *RVO avoids inefficiencies.

But it would often be better to make a move-only value type, and return that instead.