C++ Design: Passing pointer/reference to ref-counted object

Question

I have a directed acyclic graph, composed of Node objects. Each node has a list of std::shared_ptrs to other nodes, which are its children in the graph. I have lots of useful methods I need, such as inserting/emplacing/reparenting nodes, testing if a node is an ancestor of another, etc. Some are standard STL-like methods, and some are specific to directed acyclic graphs and specific to my needs.

The question is, When such a method takes a node as a parameter, should it take a reference? of a weak_ptr? or a shared_ptr? I tried to examine use cases but it's hard to tell. What's the best design here? I'm new to smart pointers and I'm not sure what's the best choice. Should I treat shared_ptr<Node> as "the representation" of node objects? Or maybe the way to choose is more sophisticated?

Thanks in advance

Answer 1

Only pass a shared_ptr (by value) or copy it when the set of owners is meaningfully extended. It's safe, and preferred, to pass pointers when dealing with nodes as pure information.

Note the std::enable_shared_from_this facility to retrieve the correct std::shared_ptr from any graph object. With that base class, a valid naked pointer and a shared pointer are essentially equivalent. I'm not sure how much, if any, overhead it adds. (It definitely ensures that there will be no additional heap fragmentation, which std::make_shared also does.)

Passing shared_ptr anywhere is just an optimization of functionality elegantly provided by shared_from_this. But when you do, pass them by const reference, since they are just providing information without actively arbitrating ownership.