-// Rust-101, Part 07: Operator Overloading, Tests, Output
-// ======================================================
+// Rust-101, Part 07: Operator Overloading, Tests, Formatting
+// ==========================================================
pub use part05::BigInt;
// Now we can implement a generic function `vec_min` that works on above trait.
// The code is pretty much straight-forward, and Rust checks that all the
-// lifetimes actually work out.
+// lifetimes actually work out. Observe that we don't have to make any copies!
pub fn vec_min<T: Minimum>(v: &Vec<T>) -> Option<&T> {
let mut min: Option<&T> = None;
for e in v {
// Notice that the return type `Option<&T>` is technically (leaving the borrowing story aside) a
// pointer to a `T`, that could optionally be invalid. In other words, it's just like a pointer in
// C(++) or Java that can be `NULL`! However, thanks to `Option` being an `enum`, we cannot forget
-// to check the pointer for validity, avoiding the safety issues of C(++). At the same time, when we
-// have a borrow like `v` above that's not an `Option`, we *know* that is has to be a valid
-// pointer, so we don't even need to do the `NULL`-check that Java does all the time.<br/>
+// to check the pointer for validity, avoiding the safety issues of C(++).<br/>
// Also, if you are worried about wasting space, notice that Rust knows that `&T` can never be
// `NULL`, and hence optimizes `Option<&T>` to be no larger than `&T`. The `None` case is represented
// as `NULL`. This is another great example of a zero-cost abstraction: `Option<&T>` is exactly like
// a pointer in C(++), if you look at what happens during execution - but it's much safer to use.
-// For our `vec_min` to be usable with `BigInt`, we need to provide an implementation of
-// `Minimum`. You should be able to pretty much copy the code you wrote for exercise 06.1.
+// **Exercise 07.1**: For our `vec_min` to be usable with `BigInt`, you will have to provide an implementation of
+// `Minimum`. You should be able to pretty much copy the code you wrote for exercise 06.1. You should *not*
+// make any copies!
impl Minimum for BigInt {
fn min<'a>(&'a self, other: &'a Self) -> &'a Self {
unimplemented!()
// ## Operator Overloading
// How can we know that our `min` function actually does what we want it to do? One possibility
-// here is to do *testing*. Rust comes with nice build-in support for both unit tests and integration
+// here is to do *testing*. Rust comes with nice built-in support for both unit tests and integration
// tests. However, before we go there, we need to have a way of checking whether the results of function calls are
// correct. In other words, we need to define how to test equality of `BigInt`. Being able to
// test equality is a property of a type, that - you guessed it - Rust expresses as a trait: `PartialEq`.
-// Doing this for `BigInt` is fairly easy, thanks to our requirement that there be no trailing zeros.
+// Doing this for `BigInt` is fairly easy, thanks to our requirement that there be no trailing zeros. We simply
+// re-use the equality test on vectors, which compares all the elements individually.
// The `inline` attribute tells Rust that we will typically want this function to be inlined.
impl PartialEq for BigInt {
#[inline]
// the "partial", I suggest you check out the documentation of [`PartialEq`](http://doc.rust-lang.org/std/cmp/trait.PartialEq.html)
// and [`Eq`](http://doc.rust-lang.org/std/cmp/trait.Eq.html). `Eq` can be automatically derived as well.
-// Now we can compare `BigInt`s using `==`! Speaking in C++ terms, we just overloaded the `==` operator
+// Now we can compare `BigInt`s. Rust treats `PratialEq` special in that it is wired to the operator `==`:
+// That operator can not be used on our numbers! Speaking in C++ terms, we just overloaded the `==` operator
// for `BigInt`. Rust does not have function overloading (i.e., it will not dispatch to different
// functions depending on the type of the argument). Instead, one typically finds (or defines) a
// trait that catches the core characteristic common to all the overloads, and writes a single
// that trait to be immediately usable with all the functions out there that generalize over `ToString`.
// Compare that to C++ or Java, where the only chance to add a new overloading variant is to
// edit the class of the receiver.
+//
+// Why can we also use `!=`, even though we just overloaded `==`? The answer lies in what's called a *default implementation*.
+// If you check out the documentation of `PartialEq` I linked above, you will see that the trait actually provides
+// two methods: `eq` to test equality, and `ne` to test inequality. As you may have guessed, `!=` is wired to `ne`.
+// The trait *definition* also provides a default implementation of `ne` to be the negation of `eq`. Hence you can just
+// provide `eq`, and `!=` will work fine. Or, if you have a more efficient way of deciding inequality, you can provide
+// `ne` for your type yourself.
fn compare_big_ints() {
let b1 = BigInt::new(13);
let b2 = BigInt::new(37);
// trailing zeros). Finally, break one of your functions in a subtle way and watch the test fail.
//
// **Exercise 07.2**: Go back to your good ol' `SomethingOrNothing`, and implement `Display` for it. (This will,
-// of course, need a `Display` bound on `T`.) Then you should be able to use them with `println!` just like you do with numbers.
+// of course, need a `Display` bound on `T`.) Then you should be able to use them with `println!` just like you do
+// with numbers, and get rid of the inherent functions to print `SomethingOrNothing<i32>` and `SomethingOrNothing<f32>`.
// [index](main.html) | [previous](part06.html) | [next](main.html)
projects / rust-101.git / blobdiff