Crate generic_singleton

generic_singleton

Rust does NOT monomorphize it’s static generic items. This means you cannot use a generic static item in a generic function. You’ll get the following error:

error[E0401]: can't use generic parameters from outer function

That’s pretty frustrating when you want to write a singleton pattern that rely’s on a generic parameter. This crate allows for this pattern with minimal runtime overhead.

generic_singleton uses anymap behind the scenes to store a map of each generic type. The first time you hit the get_or_init macro we initialize the singleton in thread local storage. Subsequent calls to get_or_init will retrieve the singleton from the map.

Example

use std::{cell::RefCell, ops::AddAssign};

use num_traits::{One, Zero};

fn generic_call_counter<T: Zero + One + Copy + AddAssign + 'static>() -> T {
    let mut count = generic_singleton::get_or_init!(|| RefCell::new(T::zero())).borrow_mut();
    *count += T::one();
    *count
}

fn main() {
    // Works with usize
    assert_eq!(generic_call_counter::<usize>(), 1);
    assert_eq!(generic_call_counter::<usize>(), 2);
    assert_eq!(generic_call_counter::<usize>(), 3);

    // Works with i32
    assert_eq!(generic_call_counter::<i32>(), 1);
    assert_eq!(generic_call_counter::<i32>(), 2);
    assert_eq!(generic_call_counter::<i32>(), 3);

    // Works with f32
    assert_eq!(generic_call_counter::<f32>(), 1.0);
    assert_eq!(generic_call_counter::<f32>(), 2.0);
    assert_eq!(generic_call_counter::<f32>(), 3.0);
}

Current limitations:

• Each thread has a different map. I may add get_or_init_local and get_or_init_global if I can figure out how to implement concurrent access and the required trait bound to make it safe.

Re-exports

pub extern crate anymap;

Macros

get_or_init

Get a static reference to a generic singleton or initialize it if it doesn’t exist.

Functions

get_from_map

Used by the get_or_init macro to access the generic item from the map. Do not use directly. Use the get_or_init macro instead.