X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/09a36e34a7b4f163c25fb971771bc4c7edd63e2b..3192415c398766f2dba869c6a3c6dad69f300cd7:/src/part07.rs diff --git a/src/part07.rs b/src/part07.rs index c12081e..85fe071 100644 --- a/src/part07.rs +++ b/src/part07.rs @@ -1,9 +1,9 @@ -// Rust-101, Part 07: Operator Overloading, Tests, Output -// ====================================================== +// Rust-101, Part 07: Operator Overloading, Tests, Formatting +// ========================================================== pub use part05::BigInt; -// With our new knowledge on Lifetimes, we are now able to write down the desired type +// With our new knowledge of lifetimes, we are now able to write down the desired type // of `min`: We want the function to take two borrows *of the same lifetime*, and then // return a borrow of that lifetime. If the two input lifetimes would be different, we // would not know which lifetime to use for the result. @@ -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!() @@ -45,13 +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 -// tests. However, before we go there, we need to have a way of checking whether the results are +// 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`. Once a type implements that trait, one can use the `==` operator on it. +// 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] @@ -63,19 +62,27 @@ impl PartialEq for BigInt { // Since implementing `PartialEq` is a fairly mechanical business, you can let Rust automate this // by adding the attribute `derive(PartialEq)` to the type definition. In case you wonder about // 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). Again, `Eq` can be automatically derived. +// 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! 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 // function that's generic in the trait. For example, instead of overloading a function for all -// the ways a string can be represented, one write a generic functions over [ToString](http://doc.rust-lang.org/std/string/trait.ToString.html). +// the ways a string can be represented, one writes a generic functions over [ToString](http://doc.rust-lang.org/std/string/trait.ToString.html). // Usually, there is a trait like this that fits the purpose - and if there is, this has the great // advantage that any type *you* write, that can convert to a string, just has to implement // 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); @@ -83,7 +90,7 @@ fn compare_big_ints() { } // ## Testing -// With our equality test written, we are now ready to write out first testcase. It doesn't get much +// With our equality test written, we are now ready to write our first testcase. It doesn't get much // simpler: You just write a function (with no arguments or return value), and give it the `test` attribute. // `assert!` is like `debug_assert!`, but does not get compiled away in a release build. #[test] @@ -105,7 +112,7 @@ fn test_min() { // that users can understand, while `Debug` is meant to show the internal state of data and targeted at // the programmer. The latter is what we want for `assert_eq!`, so let's get started. -// Al formating is handled by [`std::fmt`](http://doc.rust-lang.org/std/fmt/index.html). I won't explain +// All formating is handled by [`std::fmt`](http://doc.rust-lang.org/std/fmt/index.html). I won't explain // all the details, and refer you to the documentation instead. use std::fmt; @@ -118,32 +125,25 @@ impl fmt::Debug for BigInt { } // `Debug` implementations can be automatically generated using the `derive(Debug)` attribute. -// Now we are ready to use `assert_eq!` to test `vec_min`. While we are at it, let's also follow the usual -// Rust style of putting tests into a *submodule*, to avoid polluting the namespace. The attribute `cfg(test)` -// at the submodule means that it will only be compiled when building the tests. -#[cfg(test)] -mod tests { - use super::*; - - #[test] - fn test_vec_min() { - let b1 = BigInt::new(1); - let b2 = BigInt::new(42); - let b3 = BigInt::from_vec(vec![0, 1]); - - let v1 = vec![b2.clone(), b1.clone(), b3.clone()]; - let v2 = vec![b2.clone(), b3.clone()]; - assert_eq!(vec_min(&v1), Some(&b1)); - assert_eq!(vec_min(&v2), Some(&b2)); - } +// Now we are ready to use `assert_eq!` to test `vec_min`. +#[test] +fn test_vec_min() { + let b1 = BigInt::new(1); + let b2 = BigInt::new(42); + let b3 = BigInt::from_vec(vec![0, 1]); + + let v1 = vec![b2.clone(), b1.clone(), b3.clone()]; + let v2 = vec![b2.clone(), b3.clone()]; + assert_eq!(vec_min(&v1), Some(&b1)); + assert_eq!(vec_min(&v2), Some(&b2)); } // **Exercise 07.1**: Add some more testcases. In particular, make sure you test the behavior of // `vec_min` on an empty vector. Also add tests for `BigInt::from_vec` (in particular, removing -// trailing zeros) and the functions you wrote for exercise 05.1. Finally, break one of your -// functions in a subtle way and watch the test fail. +// 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)