X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/f5a79e8aaa07e56cf2dd8fe8438d96624dd84fbc..d9fc9a4d89d8e4950f43308318962d9179bd1177:/src/part07.rs diff --git a/src/part07.rs b/src/part07.rs index 8fad5a8..85fe071 100644 --- a/src/part07.rs +++ b/src/part07.rs @@ -1,5 +1,5 @@ -// Rust-101, Part 07: Operator Overloading, Tests, Output -// ====================================================== +// Rust-101, Part 07: Operator Overloading, Tests, Formatting +// ========================================================== pub use part05::BigInt; @@ -13,7 +13,7 @@ pub trait Minimum { // 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(v: &Vec) -> Option<&T> { let mut min: Option<&T> = None; for e in v { @@ -27,16 +27,15 @@ pub fn vec_min(v: &Vec) -> Option<&T> { // 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.
+// to check the pointer for validity, avoiding the safety issues of C(++).
// 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!() @@ -45,12 +44,13 @@ impl Minimum for BigInt { // ## 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] @@ -64,7 +64,8 @@ impl PartialEq for BigInt { // 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 @@ -75,6 +76,13 @@ impl PartialEq for BigInt { // 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); @@ -135,6 +143,7 @@ fn test_vec_min() { // 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` and `SomethingOrNothing`. // [index](main.html) | [previous](part06.html) | [next](main.html)