Converting from a std::unique_ptr into a std::shared_ptr

In this recipe, we will learn how to convert from a std::unique_ptr into a std::shared_ptr. This recipe is important as it is often convenient to define an API as accepting std::unique_ptr when the API itself really needs std::shared_ptr for internal use. A good example of this is when creating a GUI API. You might pass a widget to the API to store and own, without knowing if, later on down the road, the implementation of your GUI might need to add threads, in which case std::shared_pointer might be a better option. This recipe will provide you with the skills to convert a std::unique_ptr into a std::shared_ptr if needed, without having to modify the API itself. 

Getting ready

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