-// ***Remember to enable/add this part in `main.rs`!***
-
// Rust-101, Part 06: Copy, Lifetimes
// ==================================
// 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.
}
}
-// 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()`.
+// Now we can write `vec_min`.
fn vec_min(v: &Vec<BigInt>) -> Option<BigInt> {
let mut min: Option<BigInt> = None;
+ // If `v` is a shared reference to a vector, then the default for iterating over it is to call `iter`, the iterator that borrows the elements.
for e in v {
let e = e.clone();
- min = Some(match min {
- None => e,
- Some(n) => e.min_try1(n)
- });
+ unimplemented!()
}
min
}
-// 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 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
-// the copy that was created, and hence we can store it in `min`.<br/>
-// Of course, making such a full copy is expensive, so we'd like to avoid it. We'll some to that in the next part.
// ## `Copy` types
-// 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 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 proper duplication.
-//
-// 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
-// any methods, but makes a promise about the behavior of the type - in this case, being duplicable.
-// 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<T>` copy if `T` is `Copy`.
use part02::{SomethingOrNothing,Something,Nothing};
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 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 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 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
-// `Clone`. This makes the cost explicit.
// ## Lifetimes
-// 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*.
-// 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> {
if v.len() > 0 {
- Some(&v[0])
+ unimplemented!()
} else {
None
}
return *first;
}
*/
-// 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.)
fn rust_foo(mut v: Vec<i32>) -> i32 {
let first: Option<&i32> = head(&v);
/* v.push(42); */
*first.unwrap()
}
-// 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 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, 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
-// argument and return value together, this means that `&v` also has to borrow `v` for the entire duration of
-// the function. So when we try to borrow `v` mutable for `push`, Rust complains that the two borrows (the one
-// for `head`, and the one for `push`) overlap. Lucky us! Rust caught our mistake and made sure we don't crash the program.
-//
-// So, to sum this up: Lifetimes enable Rust to reason about *how long* a pointer has been borrowed. We can thus
-// safely write functions like `head`, that return pointers into data they got as argument, and make sure they
-// are used correctly, *while looking only at the function type*. At no point in our analysis of `rust_foo` did
-// we have to look *into* `head`. That's, of course, crucial if we want to separate library code from application code.
-// Most of the time, we don't have to explicitly add lifetimes to function types. This is thanks to *lifetimes elision*,
-// where Rust will automatically insert lifetimes we did not specify, following some [simple, well-documented rules](http://doc.rust-lang.org/stable/book/lifetimes.html#lifetime-elision).
-// [index](main.html) | [previous](part05.html) | [next](part07.html)
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