How can I make a structure with internal references?

Question

I\'m trying to make a graph with adjacency lists, but I can\'t figure out how to specify an appropriate lifetime for the references in the adjacency list.

What I\'m

Answer 1

It is not possible to represent this concept in Rust with just references due to Rust’s memory safety—such an object could not be constructed without already existing. As long as nodes and adjacencies are stored separately, it’s OK, but as soon as you try to join them inside the same structure, it can’t be made to work thus.

The alternatives are using reference counting (Rc<T> if immutable is OK or Rc<RefCell<T>> with inner mutability) or using unsafe pointers (*const T or *mut T).

Answer 2

I think there is an issue here. If we transpose this to C++:

template <typename T>
struct Graph {
    std::vector<T> nodes;
    std::vector<std::vector<T*>> adjacencies;
};

where adjacencies points into nodes, then one realizes that there is an issue: an operation on nodes that invalidates references (such as reallocation) will leave dangling pointers into adjacencies.

I see at least two ways to fix this:

• Use indexes in adjacencies, as those are stable
• Use a level of indirection in nodes, so that the memory is pinned

In Rust, this gives:

struct Graph<T> {
    nodes: Vec<T>,
    adjacencies: Vec<Vec<uint>>,
};

struct Graph<T> {
    nodes: Vec<Rc<RefCell<T>>>,
    adjacencies: Vec<Vec<Rc<RefCell<T>>>>,
};

Note: the RefCell is to allow mutating T despite it being aliased, by introducing a runtime check.