Shared References

A reference provides a way to access another value without taking ownership of the value, and is also called “borrowing”. Shared references are read-only, and the referenced data cannot change.

fn main() {
    let a = 'A';
    let b = 'B';

    let mut r: &char = &a;
    dbg!(r);

    r = &b;
    dbg!(r);
}

A shared reference to a type T has type &T. A reference value is made with the & operator. The * operator “dereferences” a reference, yielding its value.

This slide should take about 7 minutes.

  • References can never be null in Rust, so null checking is not necessary.

  • A reference is said to “borrow” the value it refers to, and this is a good model for students not familiar with pointers: code can use the reference to access the value, but is still “owned” by the original variable. The course will get into more detail on ownership in day 3.

  • References are implemented as pointers, and a key advantage is that they can be much smaller than the thing they point to. Students familiar with C or C++ will recognize references as pointers. Later parts of the course will cover how Rust prevents the memory-safety bugs that come from using raw pointers.

  • Explicit referencing with & is usually required. However, Rust performs automatic referencing and dereferencing when invoking methods.

  • Rust will auto-dereference in some cases, in particular when invoking methods (try r.is_ascii()). There is no need for an -> operator like in C++.

  • In this example, r is mutable so that it can be reassigned (r = &b). Note that this re-binds r, so that it refers to something else. This is different from C++, where assignment to a reference changes the referenced value.

  • A shared reference does not allow modifying the value it refers to, even if that value was mutable. Try *r = 'X'.

  • Rust is tracking the lifetimes of all references to ensure they live long enough. Dangling references cannot occur in safe Rust.

  • We will talk more about borrowing and preventing dangling references when we get to ownership.