X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/63b086dfbd9a5e90700f595c2e6ef7ee10522559..7b5af93d756464638204b0b703485712d88d3656:/src/part05.rs?ds=sidebyside diff --git a/src/part05.rs b/src/part05.rs index f031f09..72c787d 100644 --- a/src/part05.rs +++ b/src/part05.rs @@ -1,214 +1,148 @@ -// Rust-101, Part 05: Clone, Copy -// ============================== +// Rust-101, Part 05: Clone +// ======================== -use std::cmp; -use std::ops; +// ## 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. -// In the course of the next few parts, we are going to build a data-structure for -// computations with *bug* 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 of "small" numbers, which we will then consider the "digits" -// of the big number. This is like "1337" being a vector of 4 small numbers (1, 3, 3, 7), -// except that we will use `u64` as type of our base numbers. 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 collection of a bunch of named fields. Every field can be private to the current module -// (which is the default), or public (which would be indicated by a `pub` in front of the name). -// For the sake of the tutorial, we make `dat` 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. +//@ 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. + //@ 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] } + 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. + //@ 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 + 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`. In this case, we need to make that annotation to be able to call `pop` - // on `v`. + // 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 { - while v.len() > 0 && v[v.len()-1] == 0 { - v.pop(); - } - BigInt { data: v } + unimplemented!() } } -// 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. 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. +// ## 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 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. +//@ 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() } + 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! +//@ 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. +// 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 just borrow `v` for the duration of the match - // arm. That's good enough for cloning it. - Something(ref v) => Something(v.clone()), - } + 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`. +//@ Again, Rust will generate this implementation automatically if you add +//@ `#[derive(Clone)]` right before the definition of `SomethingOrNothing`. -// With `BigInt` being about numbers, we should be able to write a version of `vec_min` -// that computes the minimum of a list of `BigInt`. We start by writing `min` for -// `BigInt`. Now our assumption of having no trailing zeros comes in handy! -impl BigInt { - fn min(self, other: Self) -> Self { - // Just to be sure, we first check that both operands actually satisfy our invariant. - // `debug_assert!` is a macro that checks that its argument (must be of type `bool`) - // is `true`, and panics otherwise. It gets removed in release builds, which you do with - // `cargo build --release`. - // - // If you carefully check the type of `BigInt::test_invariant`, you may be surprised that - // we can call the function this way. Doesn't it take `self` in borrowed form? Indeed, - // the explicit way to do that would be to call `(&other).test_invariant()`. However, the - // `self` argument of a method is treated specially by Rust, and borrowing happens automatically here. - debug_assert!(self.test_invariant() && other.test_invariant()); - // If the lengths of the two numbers differ, we already know which is larger. - if self.data.len() < other.data.len() { - self - } else if self.data.len() > other.data.len() { - other - } else { - // **Exercise 05.1**: Fill in this code. - panic!("Not yet implemented."); - } - } -} +// **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? -// Now we can write `vec_min`. In order to make it type-check, we have to write it as follows. -fn vec_min(v: &Vec) -> Option { - let mut min: Option = None; - for e in v { - min = Some(match min { - None => e.clone(), - Some(n) => e.clone().min(n) - }); +// ## 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, } - min + /* var = Variant::Text(text); */ + *ptr = 1337; } -// Now, what's happening here? Why do we have to write `clone()`, and why did we not -// have to write that in our previous version? -// -// The answer is already hidden in the type of `vec_min`: `v` is just borrowed, but -// the Option that it returns is *owned*. We can't just return one -// of the elements of `v`, as that would mean that it is no longer in the vector! -// In our code, this comes up when we update the intermediate variable `min`, which -// also has type `Option`. If you replace `e.clone()` in the `None` arm -// with `*e`, Rust will complain "Cannot move out of borrowed content". That's because -// `e` is a `&BigInt`. Assigning `min` to `*e` works just like a function call: -// Ownership of the underlying data (in this case, the digits) is transferred from -// the vector to `min`. But that's not allowed, since we must retain the vector -// in its existing state. After cloning `e`, we own the copy that was created, -// and hence we can store it in `min`.
-// Of course, making such a full copy is expensive, so we'd like to avoid it. -// That's going to happen in the next part. -// -// But before we go there, I should answer the second question I brought up above: -// Why did our old `vec_min` work? We stored the minimal `i32` locally without -// cloning, and Rust did not complain. That's because there isn't really much -// of an "ownership" when it comes to types like `i32` or `bool`: If you move -// the value from one place to another, then both instance are independent -// and complete instances of their type. This is in stark contrast to types -// like `Vec`, where merely moving the value results in both the old -// and the new vector to point to the same underlying buffer. -// -// Rust calls types like `i32` that can be freely duplicated `Copy` types. -// `Copy` is another trait, and it is implemented for the basic types of -// the language. Remember how we defined the trait `Minimum` by writing -// `trait Minimum : Copy { ...`? This tells Rust that every type that -// implements `Minimum` must also implement `Copy`, and that's why Rust -// accepted our generic `vec_min` in part 02. -// -// Curiously, `Copy` is a trait that does not require any method to -// be implemented. Implementing `Copy` is merely a semantic statement, -// saying that the idea of ownership does not really apply to this type. -// Traits without methods are called *marker traits*. We will see some -// more examples of such traits later. -// -// If you try to implement `Copy` for `BigInt`, you will notice that Rust -// does not let you do that. A type can only be `Copy` if all its elements -// are `Copy`, and that's not the case for `BigInt`. However, we can make -// `SomethingOrNothing` copy if `T` is `Copy`. -impl Copy for SomethingOrNothing{} -// Again, Rust can generate implementations of `Copy` automatically. If -// you add `#[derive(Copy,Clone)]` right before the definition of `SomethingOrNothing`, -// both `Copy` and `Clone` will automatically be implemented. +//@ 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. -// In closing this part, I'd like to give you another perspective on the -// move semantics (i.e., ownership passing) that Rust applies, and how -// `Copy` and `Clone` fit.
-// When Rust code is executed, passing a value (like `i32` or `Vec`) -// to a function will always result in a shallow copy being performed: Rust -// just copies the bytes representing that value, and considers itself done. -// That's just like the default copy constructor in C++. Rust, however, will -// consider this a destructive operation: After copying the bytes elsewhere, -// the original value must no longer be used. After all, the two could not -// share a pointer! If, however, you mark a type `Copy`, then Rust will *not* -// consider a move destructive, and just like in C++, the old and new value -// can happily coexist. Now, Rust does not allow to to overload the copy -// constructor. This means that passing a value around will always be a fast -// operation, no allocation or copying of large data of the heap will happen. -// In the situations where you would write a copy constructor in C++ (and hence -// incur a hidden cost on every copy of this type), you'd have the type *not* -// implement `Copy`, but only `Clone`. This makes the cost explicit. +//@ [index](main.html) | [previous](part04.html) | [next](part06.html)