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// refcell1.rs
//
// Interior mutability is a design pattern in Rust that allows you to mutate
// data even when there are immutable references to that data;
// normally, this action is disallowed by the borrowing rules.
// The RefCell<T> type represents single ownership over the data it holds.
// Recall the borrowing rules in Rust:
// 1. At any given time, you can have either (but not both) one mutable
// reference or any number of immutable references.
// 2. References must always be valid.
// With references and Box<T>, the borrowing rules’ invariants are enforced at
// compile time. With RefCell<T>, these invariants are enforced at runtime.
// With references, if you break these rules, you’ll get a compiler error.
// With RefCell<T>, if you break these rules, your program will panic and exit.
// The RefCell<T> type is useful when you’re sure your code follows the
// borrowing rules but the compiler is unable to understand and guarantee that.
// I AM NOT DONE
use std::cell::RefCell;
#[derive(Debug)]
#[allow(dead_code)]
struct User {
name: RefCell<String>,
}
#[allow(dead_code)]
impl User {
fn name(&self) -> String {
self.name.borrow().to_string()
}
// Note: do not use &mut self!
fn set_name(&self, name: String) {
todo!()
}
}
fn main() {
let u = User {
name: RefCell::new("Alice".to_string()),
};
println!("My name is {}!", *u.name.borrow());
let new_name = "Bob".to_string();
u.set_name(new_name.clone());
println!("My name is {}!", *u.name.borrow());
}

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@ -1106,6 +1106,20 @@ Check out https://doc.rust-lang.org/std/borrow/enum.Cow.html for documentation
on the `Cow` type.
"""
[[exercises]]
name = "refcell1"
path = "exercises/19_smart_pointers/refcell1.rs"
mode = "compile"
hint = """
Remember that RefCell<T> allows for an immutable object to be modified.
Use the .borrow_mut() method on the RefCell to get a mutable reference to
the underlying data.
See https://doc.rust-lang.org/book/ch15-05-interior-mutability.html for more
information on RefCell.
"""
# THREADS
[[exercises]]