Crate lazy_rc

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lazy_rc provides implementations of Rc<T> and Arc<T> with lazy initialization.

In other words, the “inner” value of an LazyRc<T> or LazyArc<T> instance is created when it is accessed for the first time, using the supplied initialization function. Initialization may fail, in which case the error is passed through.

Thread Safety

LazyRc<T> is single-threaded, because so is Rc<T>. Therefore, an LazyRc<T> instance can not be shared by multiple threads, and you can not use LazyRc<T> for static variables. However, it can be used for thread_local! variables.

LazyArc<T> is thread-safe, because so is Arc<T>. Therefore, an LazyArc<T> instance can be shared by multiple threads, and you can even use LazyArc<T> for global static variables.

Const Warning

Do not use LazyRc<T> or LazyArc<T> as a const value! That is because, in Rust, const values are “inlined”, effectively creating a new instance at every place where the const value is used. This obviously breaks “lazy” initialization 😨


use lazy_rc::{LazyRc, LazyArc};
static GLOBAL_INSTANCE: LazyArc<MyStruct> = LazyArc::empty();
thread_local! {
    static THREAD_INSTANCE: LazyRc<MyStruct>  = LazyRc::empty();
struct MyStruct {
   /* ... */
impl MyStruct {
    fn new() -> Result<Self> {
        /* ... */
    /// Returns a thread-local instance that will be created on first access.
    /// If the initialization function fails, then an Error will be returned.
    pub fn instance() -> Result<Rc<Self>> {
        THREAD_INSTANCE.with(|lazy| lazy.or_try_init_with(Self::new))


A thread-safe reference-counting pointer, akin to Arc<T>, but with lazy initialization
A single-threaded reference-counting pointer, akin to Rc<T>, but with lazy initialization