Sometimes you need several references to an immutable value at the same time, this is also called shared ownership. The pointer type Box<T> can't help us out here, because this type has a single owner by definition. For this, Rust provides the generic reference counted box Rc<T>, where multiple references can share the same resource. The std::rc module provides a way to share ownership of the same value between different Rc pointers; the value remains alive as long as there is least one pointer referencing it.
In the following example, we have Aliens that have a number of tentacles. Each Tentacle is also a struct instance and has to indicate to which Alien it belongs; besides that it has other properties like a degree of poison. A first attempt at this could be the following code which however does not compile:
// see Chapter 7/code/refcount_not_good.rs):
struct Alien {
name: String,
n_tentacles: u8
}
struct Tentacle {
poison: u8,
owner...