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// Copyright 2014-2015 The Rust Project Developers. See the COPYRIGHT // Original work Copyright (c) 2014 The Rust Project Developers // Modified work Copyright (c) 2016 Nikita Pekin // See the README.md file at the top-level directory of this distribution. // // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your // option. This file may not be copied, modified, or distributed // except according to those terms. //! This crate provides a LazyCell struct which acts as a lazily filled Cell, //! but with frozen contents. //! //! With a `RefCell`, the inner contents cannot be borrowed for the lifetime of //! the entire object, but only of the borrows returned. A `LazyCell` is a //! variation on `RefCell` which allows borrows tied to the lifetime of the //! outer object. //! //! The limitation of a `LazyCell` is that after initialized, it can never be //! modified. #![deny(missing_docs)] #![cfg_attr(feature = "nightly", feature(plugin))] #![cfg_attr(feature = "clippy", plugin(clippy))] use std::cell::RefCell; use std::default::Default; use std::mem; /// A lazily filled Cell, with frozen contents. pub struct LazyCell<T> { inner: RefCell<Option<T>>, } impl<T> LazyCell<T> { /// Creates a new empty lazy cell. pub fn new() -> LazyCell<T> { LazyCell { inner: RefCell::new(None) } } /// Put a value into this cell. /// /// This function will fail if the cell has already been filled. pub fn fill(&self, t: T) { let mut slot = self.inner.borrow_mut(); if slot.is_some() { panic!("lazy cell is already filled") } *slot = Some(t); } /// Test whether this cell has been previously filled. pub fn filled(&self) -> bool { self.inner.borrow().is_some() } /// Borrows the contents of this lazy cell for the duration of the cell /// itself. /// /// This function will return `Some` if the cell has been previously /// initialized, and `None` if it has not yet been initialized. pub fn borrow(&self) -> Option<&T> { match *self.inner.borrow() { Some(ref inner) => unsafe { Some(mem::transmute(inner)) }, None => None, } } /// Consumes this `LazyCell`, returning the underlying value. pub fn into_inner(self) -> Option<T> { self.inner.into_inner() } } impl<T> Default for LazyCell<T> { #[inline] fn default() -> LazyCell<T> { Self::new() } }