X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/c25f3400060ea1a02f8fa9de69c39fd7b020e8a5..8592458fd3494ebce80727341663c7b0036e3ede:/src/part05.rs?ds=sidebyside diff --git a/src/part05.rs b/src/part05.rs index 3992fb1..7ad8754 100644 --- a/src/part05.rs +++ b/src/part05.rs @@ -2,26 +2,28 @@ // ======================== // ## 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).
+ +//@ 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. +//@ 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, + pub data: Vec, // least significant digit first, no trailing zeros } // Now that we fixed the data representation, we can start implementing methods on it. @@ -31,7 +33,7 @@ impl BigInt { //@ fields and initial values assigned to them. pub fn new(x: u64) -> Self { if x == 0 { - BigInt { data: vec![] } + BigInt { data: vec![] } /*@*/ } else { BigInt { data: vec![x] } /*@*/ } @@ -47,30 +49,32 @@ impl BigInt { } } - // 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`. + // We can convert any little-endian vector of digits (i.e., least-significant digit first) into + // a number, by removing trailing zeros. The `mut` declaration for `v` here is just like the + // one in `let mut ...`: We completely own `v`, but Rust still asks us to make our intention of + // modifying it explicit. This `mut` is *not* part of the type of `from_vec` - the caller has + // to give up ownership of `v` anyway, so they don't care anymore what you do to it. // // **Exercise 05.1**: Implement this function. // - // *Hint*: You can use `pop()` to remove the last element of a vector. + // *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. +//@ If you take 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. However, there is 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 in the +//@ form of a shared reference, 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 +//@ Rust has special treatment for methods that borrow their `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! @@ -88,16 +92,16 @@ impl Clone for BigInt { //@ 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 + //@ that's stored in `self`. However, if we were to 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. @@ -110,39 +114,43 @@ impl Clone for SomethingOrNothing { //@ 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? +// **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? Of course, these +// should all take `self` as a shared reference (i.e., in borrowed form). // ## 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. +//@ Now that we know how to create references to contents of an `enum` (like `v` above), there's +//@ another example we can look at 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". +//@ Now consider the following piece of code. Like above, `n` will be a reference to a part of +//@ `var`, and since we wrote `ref mut`, the reference will be unique and 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); */ + /* var = Variant::Text(text); */ /* BAD! */ *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.) +//@ 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. +//@ 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) +//@ [index](main.html) | [previous](part04.html) | [raw source](workspace/src/part05.rs) | +//@ [next](part06.html)