CODEWITHSUNDEEP
rustlifetime
Helmut Januschka
Helmut Januschka

Reputation: 1636

Lifetime with shared state

I want to have:

Below is the idea I came up with, but I am not able to get the lifetime markers done correctly. 🤯

struct IdService<'a> {
    name: String,
    state: &'a AppState,
}
struct AppServices<'a> {
    id: Option<&'a IdService>,
}

struct AppState<'a> {
    services: &'a AppServices,
}

impl<'a> AppState<'a> {
    pub fn new() -> Self {
        AppState {
            services: AppServices { id: None },
        };
    }
}

fn main() {
    let mut state = AppState::new();
    let id_service = IdService {
        name: "test".to_string(),
        state: state,
    };
    let services = AppServices {
        id: Some(id_service),
    };
    state.services = services;
}

Compiler output:

error[E0106]: missing lifetime specifier
 --> src/main.rs:3:16
  |
3 |     state: &'a AppState,
  |                ^^^^^^^^ expected lifetime parameter

error[E0106]: missing lifetime specifier
 --> src/main.rs:6:20
  |
6 |     id: Option<&'a IdService>,
  |                    ^^^^^^^^^ expected lifetime parameter

error[E0106]: missing lifetime specifier
  --> src/main.rs:10:19
   |
10 |     services: &'a AppServices,
   |                   ^^^^^^^^^^^ expected lifetime parameter

Upvotes: 3

Views: 237

Answers (1)

Akiner Alkan
Akiner Alkan

Reputation: 6918

Since your first struct has a reference that has a reference of another struct in itself, you need to specify these children lifetimes too:

struct AppState<'a> {
    services: &'a AppServices<'a>,
}

Playground

This way you are saying to the compiler that AppState and AppServices lifetimes are bound, thus AppServices members have the lifetime 'a as well.

However, apart from lifetime issues, you have a cyclic data structure. If you want to share data between structs, there are some specific smart pointers like Rc and for multithreaded applications Arc.

Instead of sharing raw pointers of the struct, you can share Arc smart pointers and mutate data with the help of Mutex to guarantee single access for a thread.

A solution that uses an Arc to solve such a cyclic data structure problem.

Upvotes: 4

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