Node (weight) containing it's own NodeIndex

Question

I want to create Nodes (weights) which contain its own NodeIndex

struct Node {
  index: NodeIndex,
  ...
}

However, in order to get the NodeIndex, I need to call graph.add_node(node) which requires a fully constructed Node.

Is there a way I can accomplish this without having to use Option<NodeIndex>?

Answer 1

I managed to change my design such that this is no longer necessary, and probably for the better.

The point of using petgraph (at least in my case) is to create a graph like structure without having to deal with self-referential types (see this post for more details).

This only works if petgraph's Graph has (sole?) ownership of the nodes. Once you add your node to the graph via graph.add_node(node), it returns you the NodeIndex which will be your handle towards that node in the future (think of it as a pointer).

So storing the NodeIndex inside the Node makes as much sense as storing the pointer to the object inside the object itself. In my case, it was a symptom of a misuse and misunderstanding of petgraph.

Answer 2

I'm not saying it's a good idea, but NodeIndex implements Default, so you can fill it initially with the default value, and once you call add_node() and get the real index, index the graph with it and overwrite the node data.