use part05::BigInt;
// 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!
+// that computes the minimum of a list of `BigInt`. First, we have to write `min` for `BigInt`.
impl BigInt {
fn min_try1(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`.
debug_assert!(self.test_invariant() && other.test_invariant());
+ // Now our assumption of having no trailing zeros comes in handy:
// If the lengths of the two numbers differ, we already know which is larger.
if self.data.len() < other.data.len() {
self
}
}
-// Now we can write `vec_min`. In order to make it type-check, we have to write it as follows.
+// Now we can write `vec_min`. However, in order to make it type-check, we have to make a full (deep) copy of e
+// by calling `clone()`.
fn vec_min(v: &Vec<BigInt>) -> Option<BigInt> {
let mut min: Option<BigInt> = None;
for e in v {
+ let e = e.clone();
min = Some(match min {
- None => e.clone(),
- Some(n) => e.clone().min_try1(n)
+ None => e,
+ Some(n) => e.min_try1(n)
});
}
min
}
-// 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?
+// Now, what's happening here? Why do we have to clone `e`, and why did we not
+// have to do that in our previous version?
//
// The answer is already hidden in the type of `vec_min`: `v` is just borrowed, but
// the Option<BigInt> 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<BigInt>`. If you replace `e.clone()`
-// in the `None` arm with `*e`, Rust will complain "Cannot move out of borrowed content". That's because
+// the intermediate variable `min`, which also has type `Option<BigInt>`. If you replace get rid of the
+// `e.clone()`, Rust will complain "Cannot move out of borrowed content". That's because
// `e` is a `&BigInt`. Assigning `min = Some(*e)` works just like a function call: Ownership of the
// underlying data is transferred from where `e` borrows from to `min`. But that's not allowed, since
// we just borrowed `e`, so we cannot empty it! We can, however, call `clone()` on it. Then we own
// 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 "complete". We also say the value has been *duplicated*. This is in
+// place to another, then both instances are "complete". We also say the value has been *duplicated*. This is in
// stark contrast to types like `Vec<i32>`, where moving the value results in both the old and the new vector to
-// point to the same underlying buffer. We don't have two vectors, there's no duplication.
+// point to the same underlying buffer. We don't have two vectors, there's no proper duplication.
//
-// Rust calls types that can be freely duplicated `Copy` types. `Copy` is another trait, and it is implemented for
+// Rust calls types that can be easily duplicated `Copy` types. `Copy` is another trait, and it is implemented for
// types like `i32` and `bool`. 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 the compiler
// accepted our generic `vec_min` in part 02. `Copy` is the first *marker trait* that we encounter: It does not provide
// are `Copy`, and that's not the case for `BigInt`. However, we can make
// `SomethingOrNothing<T>` copy if `T` is `Copy`.
use part02::{SomethingOrNothing,Something,Nothing};
-impl<T: Copy> Copy for SomethingOrNothing<T>{}
+impl<T: Copy> Copy for SomethingOrNothing<T> {}
// 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.
// ## An operational perspective
// Instead of looking at what happens "at the surface" (i.e., visible in Rust), one can also explain
-// ownership passing and how `Copy` and `Clone` fit by looking at what happens on the machine.<br/>
+// ownership passing and how `Copy` and `Clone` fit in by looking at what happens on the machine.<br/>
// When Rust code is executed, passing a value (like `i32` or `Vec<i32>`) 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`,
+// no longer be used. After all, the two could now 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
+// can happily coexist. Now, Rust does not allow you to overload the copy constructor. This means that
// passing a value around will always be a fast operation, no allocation or any other kind of heap access
// 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
// To fix the performance problems of `vec_min`, we need to avoid using `clone()`. We'd like
// the return value to not be owned (remember that this was the source of our need for cloning), but *borrowed*.
-// This is demonstrated by the function `head` that borrows the first element of a vector if it is non-empty.
+// The function `head` demonstrates how that could work: It borrows the first element of a vector if it is non-empty.
// The type of the function says that it will either return nothing, or it will return a borrowed `T`.
// We can then borrow the first element of `v` and use it to construct the return value.
fn head<T>(v: &Vec<T>) -> Option<&T> {
None
}
}
-
-// Now, coming back to `head` - here, we are returning a pointer to the first element. But doesn't
-// that mean that callers have to be careful? Imagine `head` would be a C++ function, and we would
-// write the following code.
+// Technically, we are returning a pointer to the first element. But doesn't that mean that callers have to be
+// careful? Imagine `head` would be a C++ function, and we would write the following code.
/*
int foo(std::vector<int> v) {
int *first = head(v);
return *first;
}
*/
-// This is very much like our very first motivating example for ownership, at the beginning of part 04.
-// But this time, the bug is hidden behind the call to `head`. How does Rust solve this? If we translate
+// This is very much like our very first motivating example for ownership, at the beginning of part 04:
+// `push_back` could reallocate the buffer, making `first` an invalid pointer. Again, we have aliasing (of `first`
+// and `v`) and mutation. But this time, the bug is hidden behind the call to `head`. How does Rust solve this? If we translate
// the code above to Rust, it doesn't compile, so clearly we are good - but how and why?
// (Notice that have to explicitly assert using `unwrap` that `first` is not `None`, whereas the C++ code
// above would silently dereference a `NULL`-pointer. But that's another point.)
// To give the answer to this question, we have to talk about the *lifetime* of a borrow. The point is, saying that
// you borrowed your friend a `Vec<i32>`, or a book, is not good enough, unless you also agree on *how long*
-// your friend can borrow. After all, you need to know when you can rely on owning your data (or book) again.
+// your friend can borrow it. After all, you need to know when you can rely on owning your data (or book) again.
//
-// Every borrow in Rust has an associated lifetime. The full type of `head` reads as follows:
-// `fn<'a, T>(&'a Vec<T>) -> Option<&'a T>`. Here, `'a` is a *lifetime variable*, which represents how long the vector has
-// been borrowed. The function type expresses that argument and return value have *the same lifetime*.
+// Every borrow in Rust has an associated lifetime, written `&'a T` for a borrow of type `T` with lifetime `'a`. The full
+// type of `head` reads as follows: `fn<'a, T>(&'a Vec<T>) -> Option<&'a T>`. Here, `'a` is a *lifetime variable*, which
+// represents how long the vector has been borrowed. The function type expresses that argument and return value have *the same lifetime*.
//
// When analyzing the code of `rust_foo`, Rust has to assign a lifetime to `first`. It will choose the scope
// where `first` is valid, which is the entire rest of the function. Because `head` ties the lifetime of its
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