X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/229b86d07e94cd3ec175051a44b3f3cb45b40b65..ab7f9b241429bd675b437d2437799de75d2f409b:/workspace/src/part05.rs diff --git a/workspace/src/part05.rs b/workspace/src/part05.rs deleted file mode 100644 index 538bf5f..0000000 --- a/workspace/src/part05.rs +++ /dev/null @@ -1,150 +0,0 @@ -// ***Remember to enable/add this part in `main.rs`!*** - -// Rust-101, Part 05: Clone -// ======================== - -// ## Big Numbers -// In the course of the next few parts, we are going to build a data-structure for computations with -// *big* numbers. We would like to not have an upper bound to how large these numbers can get, with -// the memory of the machine being the only limit. -// -// We start by deciding how to represent such big numbers. One possibility here is -// to use a vector "digits" of the number. This is like "1337" being a vector of four digits (1, 3, 3, 7), -// except that we will use `u64` as type of our digits, meaning we have 2^64 individual digits. Now we just -// have to decide the order in which we store numbers. I decided that we will store the least significant -// digit first. This means that "1337" would actually become (7, 3, 3, 1).
-// Finally, we declare that there must not be any trailing zeros (corresponding to -// useless leading zeros in our usual way of writing numbers). This is to ensure that -// the same number can only be stored in one way. - -// To write this down in Rust, we use a `struct`, which is a lot like structs in C: -// Just a bunch of named fields. Every field can be private to the current module (which is the default), -// or public (which is indicated by a `pub` in front of the name). For the sake of the tutorial, we make -// `data` public - otherwise, the next parts of this course could not work on `BigInt`s. Of course, in a -// real program, one would make the field private to ensure that the invariant (no trailing zeros) is maintained. -pub struct BigInt { - pub data: Vec, -} - -// Now that we fixed the data representation, we can start implementing methods on it. -impl BigInt { - // Let's start with a constructor, creating a `BigInt` from an ordinary integer. - // To create an instance of a struct, we write its name followed by a list of - // fields and initial values assigned to them. - pub fn new(x: u64) -> Self { - if x == 0 { - BigInt { data: vec![] } - } else { - BigInt { data: vec![x] } - } - } - - // It can often be useful to encode the invariant of a data-structure in code, so here - // is a check that detects useless trailing zeros. - pub fn test_invariant(&self) -> bool { - if self.data.len() == 0 { - true - } else { - self.data[self.data.len() - 1] != 0 - } - } - - // We can convert any vector of digits into a number, by removing trailing zeros. The `mut` - // declaration for `v` here is just like the one in `let mut ...`, it says that we will locally - // change the vector `v`. - // - // **Exercise 05.1**: Implement this function. - // - // *Hint*: You can use `pop()` to remove the last element of a vector. - pub fn from_vec(mut v: Vec) -> Self { - unimplemented!() - } -} - -// ## Cloning -// If you have a close look at the type of `BigInt::from_vec`, you will notice that it -// consumes the vector `v`. The caller hence loses access to its vector. There is however something -// we can do if we don't want that to happen: We can explicitly `clone` the vector, -// which means that a full (or *deep*) copy will be performed. Technically, -// `clone` takes a borrowed vector, and returns a fully owned one. -fn clone_demo() { - let v = vec![0,1 << 16]; - let b1 = BigInt::from_vec((&v).clone()); - let b2 = BigInt::from_vec(v); -} -// Rust has special treatment for methods that borrow its `self` argument (like `clone`, or -// like `test_invariant` above): It is not necessary to explicitly borrow the receiver of the -// method. Hence you could replace `(&v).clone()` by `v.clone()` above. Just try it! - -// To be clonable is a property of a type, and as such, naturally expressed with a trait. -// In fact, Rust already comes with a trait `Clone` for exactly this purpose. We can hence -// make our `BigInt` clonable as well. -impl Clone for BigInt { - fn clone(&self) -> Self { - BigInt { data: self.data.clone() } - } -} -// Making a type clonable is such a common exercise that Rust can even help you doing it: -// If you add `#[derive(Clone)]` right in front of the definition of `BigInt`, Rust will -// generate an implementation of `Clone` that simply clones all the fields. Try it! -// These `#[...]` annotations at types (and functions, modules, crates) are called *attributes*. -// We will see some more examples of attributes later. - -// We can also make the type `SomethingOrNothing` implement `Clone`. However, that -// can only work if `T` is `Clone`! So we have to add this bound to `T` when we introduce -// the type variable. -use part02::{SomethingOrNothing,Something,Nothing}; -impl Clone for SomethingOrNothing { - fn clone(&self) -> Self { - match *self { - Nothing => Nothing, - // In the second arm of the match, we need to talk about the value `v` - // that's stored in `self`. However, if we would write the pattern as - // `Something(v)`, that would indicate that we *own* `v` in the code - // after the arrow. That can't work though, we have to leave `v` owned by - // whoever called us - after all, we don't even own `self`, we just borrowed it. - // By writing `Something(ref v)`, we borrow `v` for the duration of the match - // arm. That's good enough for cloning it. - Something(ref v) => Something(v.clone()), - } - } -} -// Again, Rust will generate this implementation automatically if you add -// `#[derive(Clone)]` right before the definition of `SomethingOrNothing`. - -// **Exercise 05.2**: Write some more functions on `BigInt`. What about a function that returns the number of -// digits? The number of non-zero digits? The smallest/largest digit? - -// ## Mutation + aliasing considered harmful (part 2) -// Now that we know how to borrow a part of an `enum` (like `v` above), there's another example for why we -// have to rule out mutation in the presence of aliasing. First, we define an `enum` that can hold either -// a number, or a string. -enum Variant { - Number(i32), - Text(String), -} -// Now consider the following piece of code. Like above, `n` will be a borrow of a part of `var`, -// and since we wrote `ref mut`, the borrow will be mutable. In other words, right after the match, `ptr` -// points to the number that's stored in `var`, where `var` is a `Number`. Remember that `_` means -// "we don't care". -fn work_on_variant(mut var: Variant, text: String) { - let mut ptr: &mut i32; - match var { - Variant::Number(ref mut n) => ptr = n, - Variant::Text(_) => return, - } - /* var = Variant::Text(text); */ - *ptr = 1337; -} -// Now, imagine what would happen if we were permitted to also mutate `var`. We could, for example, -// make it a `Text`. However, `ptr` still points to the old location! Hence `ptr` now points somewhere -// into the representation of a `String`. By changing `ptr`, we manipulate the string in completely -// unpredictable ways, and anything could happen if we were to use it again! (Technically, the first field -// of a `String` is a pointer to its character data, so by overwriting that pointer with an integer, -// we make it a completely invalid address. When the destructor of `var` runs, it would try to deallocate -// that address, and Rust would eat your laundry - or whatever.) -// -// I hope this example clarifies why Rust has to rule out mutation in the presence of aliasing *in general*, -// not just for the specific case of a buffer being reallocated, and old pointers becoming hence invalid. - -// [index](main.html) | [previous](part04.html) | [next](part06.html)