Shared References

A reference provides a way to access another value without taking responsibility for 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;
    println!("r: {}", *r);
    r = &b;
    println!("r: {}", *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.

Rust will statically forbid dangling references:

fn x_axis(x: i32) -> &(i32, i32) {
    let point = (x, 0);
    return &point;
}
This slide should take about 10 minutes.

  • 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.

  • Rust does not automatically create references for you - the & is always required.

  • 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. x_axis would return a reference to point, but point will be deallocated when the function returns, so this will not compile.

  • We will talk more about borrowing when we get to ownership.