X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/c25f3400060ea1a02f8fa9de69c39fd7b020e8a5..fb0c404837b2356067489ef5643ff6bc6e99b9ac:/src/part05.rs diff --git a/src/part05.rs b/src/part05.rs index 3992fb1..eaad980 100644 --- a/src/part05.rs +++ b/src/part05.rs @@ -21,7 +21,7 @@ //@ `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 +31,7 @@ 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] } /*@*/ } @@ -48,29 +48,31 @@ 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`. + // 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 +//@ 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, and returns a fully owned one. +//@ `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! @@ -97,7 +99,7 @@ impl Clone for SomethingOrNothing { 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. @@ -111,18 +113,18 @@ impl Clone for SomethingOrNothing { //@ `#[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? +// 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 +//@ 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` +//@ 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) { @@ -131,7 +133,7 @@ fn work_on_variant(mut var: Variant, text: String) { 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, @@ -145,4 +147,4 @@ fn work_on_variant(mut var: Variant, text: String) { //@ 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)