//@ `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<u64>,
+ pub data: Vec<u64>, // least significant digit first, no trailing zeros
}
// Now that we fixed the data representation, we can start implementing methods on it.
//@ 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] } /*@*/
}
}
// 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<u64>) -> 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.
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.
//@ `#[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 just borrow `self`.
// ## 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
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,
//@ 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](https://www.ralfj.de/git/rust-101.git/blob_plain/HEAD:/workspace/src/part05.rs) | [next](part06.html)
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