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9
src/ch04-00-understanding-ownership.md
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@ -1,6 +1,7 @@
# Understanding Ownership
Ownership is important to understand: it's Rust's most unique feature, and
enables Rust to make memory safety guarantees without needing a garbage
collector. We’ll also talk about several related features: borrowing, slices,
and how Rust lays things out in memory.
Ownership is Rust's most unique feature, and enables Rust to make memory safety
guarantees without needing a garbage collector. It's therefore important to
understand how ownership works in Rust. In this chapter we'll talk about
ownership as well as several related features: borrowing, slices, and how Rust
lays things out in memory.

229
src/ch04-01-ownership.md
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@ -1,8 +1,7 @@
## Ownership
Rust’s central feature is called *ownership*. It is a feature that is
straightforward to explain, but has deep implications for the rest of the
language.
Rust’s central feature is *ownership*. It is a feature that is straightforward
to explain, but has deep implications for the rest of the language.
All programs have to manage the way they use a computer's memory while running.
Some languages have garbage collection, while in others, the programmer has to
@ -11,7 +10,7 @@ managed through a system of ownership with a set of rules that the compiler
checks at compile-time. You do not pay any run-time cost for any of these
features.
However, because ownership is a new concept for many programmers, it does take
Since ownership is a new concept for many programmers, it does take
some time to get used to. There is good news, though: the more experienced you
become with Rust and the rules of the ownership system, the more you'll be
able to naturally develop code that is both safe and efficient. Keep at it!
@ -76,23 +75,35 @@ ownership exists can help explain why it works the way it does.
PROD: END BOX
### Variable binding scope
### Ownership Rules
First, let's take a look at the rules. Keep these in mind as we go through the
examples that will illustrate the rules:
> 1. Each value in Rust has a variable binding that’s called its *owner*.
> 2. There can only be one owner at a time.
> 3. When the owner goes out of scope, the value will be `drop()`ped.
### Variable Binding Scope
We've walked through an example of a Rust program already in the tutorial
chapter. Now that we’re past basic syntax, we won’t include all of the `fn
main() {` stuff in examples, so if you’re following along, you will have to put
them inside of a `main()` function. This lets our examples be a bit more
concise, letting us focus on the actual details rather than boilerplate.
the following examples inside of a `main()` function yourself. This lets our
examples be a bit more concise, letting us focus on the actual details rather
than boilerplate.
Anyway, here is our first example:
As a first example of ownership, we'll look at the *scope* of some variable
bindings. A scope is the range within a program for which an item is valid.
Let's say we have a variable binding that looks like this:
```rust
let s = "hello";
```
This variable binding refers to a string literal, where the value of the string
is hard coded into the text of our program. The binding is valid from the point
at which it’s declared until the end of the current _scope_. That is:
The variable binding `s` refers to a string literal, where the value of the
string is hard coded into the text of our program. The binding is valid from
the point at which it’s declared until the end of the current _scope_. That is:
```rust
{ // s is not valid here, it’s not yet declared
@ -104,13 +115,13 @@ at which it’s declared until the end of the current _scope_. That is:
In other words, there are two important points in time here:
- When `s` comes "into scope", it is valid.
- It remains so until it "goes out of scope".
- When `s` comes *into scope*, it is valid.
- It remains so until it *goes out of scope*.
At this point, things are similar to other programming languages. Now let’s
build on top of this understanding by introducing the `String` type.
### Strings
### The `String` Type
In order to illustrate the rules of ownership, we need a data type that is more
complex than the ones we covered in Chapter 3. All of the data types we've
@ -144,7 +155,7 @@ particular `from()` function under the `String` type itself, rather than using
some sort of name like `string_from()`. We’ll discuss this syntax more in the
“Method Syntax” and “Modules” chapters.
This kind of string can be mutated:
This kind of string *can* be mutated:
```rust
let mut s = String::from("hello");
@ -162,8 +173,8 @@ In the case of a string literal, because we know the contents at compile time,
the text is hard-coded directly into the final executable and stored with the
code on the stack. This makes string literals quite fast and efficient. But
these properties only come from its immutability. Unfortunately, we can’t put a
blob of memory into the binary for each string whose size is unknown at compile
time and whose size might change over the course of running the program.
blob of memory into the binary for each piece of text whose size is unknown at
compile time and whose size might change over the course of running the program.
With the `String` type, in order to support a mutable, growable piece of text,
we need to allocate an amount of memory on the heap, unknown at compile time,
@ -178,18 +189,18 @@ implementation requests the memory it needs. This is pretty much universal in
programming languages.
The second case, however, is different. In languages with a *garbage collector*
(*GC*), the GC will keep track and clean up memory that isn't being used
anymore, and we, as the programmer, don’t need to think about it. Without GC,
it’s our responsibility to identify when memory is no longer being used and
(GC), the GC will keep track and clean up memory that isn't being used anymore,
and we, as the programmer, don’t need to think about it. Without GC, it’s the
programmer's responsibility to identify when memory is no longer being used and
call code to explicitly return it, just as we did to request it. Doing this
correctly has historically been a difficult problem. If we forget, we will
waste memory. If we do it too early, we will have an invalid variable. If we do
it twice, that’s a bug too. We need to pair exactly one `allocate()` with
exactly one `free()`.
correctly has historically been a difficult problem in programming. If we
forget, we will waste memory. If we do it too early, we will have an invalid
variable. If we do it twice, that’s a bug too. We need to pair exactly one
`allocate()` with exactly one `free()`.
Rust takes a different path. Remember our example? Here’s a version with
`String`:
Rust takes a different path: the memory is automatically returned once the
binding to it goes out of scope. Here’s a version of our scope example from
earlier using `String`:
```rust
{
@ -199,15 +210,15 @@ Rust takes a different path. Remember our example? Here’s a version with
} // this scope is now over, and s is no longer valid
```
We have a natural point at which we can return the memory our `String` needs
back to the operating system: when it goes out of scope. When a variable goes
out of scope, Rust calls a special function for us. This function is called
`drop()`, and it is where the author of `String` can put the code to return the
memory.
There is a natural point at which we can return the memory our `String` needs
back to the operating system: when `s` goes out of scope. When a variable
binding goes out of scope, Rust calls a special function for us. This function
is called `drop()`, and it is where the author of `String` can put the code to
return the memory. Rust calls `drop()` automatically at the closing `}`.
> Aside: This pattern is sometimes called “Resource Acquisition Is
> Note: This pattern is sometimes called “Resource Acquisition Is
> Initialization” in C++, or “RAII” for short. While they are very similar,
> Rust’s take on this concept has a number of differences, and so we don’t tend
> Rust’s take on this concept has a number of differences, so we don’t tend
> to use the same term. If you’re familiar with this idea, keep in mind that it
> is _roughly_ similar in Rust, but not identical.
@ -216,9 +227,10 @@ seem simple right now, but things can get tricky in more advanced situations
when we want to have multiple variable bindings use the data that we have
allocated on the heap. Let’s go over some of those situations now.
### Move
#### Ways Bindings and Data Interact: Move
What would you expect this code to do?
There are different ways that multiple bindings can interact with the same data
in Rust. Let's take an example using an integer:
```rust
let x = 5;
@ -231,7 +243,7 @@ bind it to `y`”. We now have two bindings, `x` and `y`, and both equal `5`.
This is indeed what is happening since integers are simple values with a known,
fixed size, and these two `5` values are pushed onto the stack.
Now let’s look at `String`. What would you expect this code to do?
Now let’s look at the `String` version:
```rust
let s1 = String::from("hello");
@ -252,13 +264,14 @@ length, and a capacity. This group of data is stored on the stack. On the right
is the memory that holds the contents, and this is on the heap.
<img alt="String in memory" src="img/trpl04-01.svg" class="center" style="width: 50%;" />
Figure 4-1: Representation in memory of a `String` holding the value "hello"
bound to `s1`
A `String` is made up of three parts: a pointer to the memory that holds the
contents of the string, a length, and a capacity. The length is how much memory
the `String` is currently using. The capacity is the total amount of memory the
The length is how much memory, in bytes, the contents of the `String` is
currently using. The capacity is the total amount of memory, in bytes, that the
`String` has gotten from the operating system. The difference between length
and capacity matters but not in this context, so don’t worry about it too much.
For right now, it's fine to ignore the capacity.
and capacity matters but not in this context, so for now, it's fine to ignore
the capacity.
When we assign `s1` to `s2`, the `String` data itself is copied, meaning we
copy the pointer, the length, and the capacity that are on the stack. We do not
@ -266,6 +279,8 @@ copy the data on the heap that the `String`'s pointer refers to. In other
words, it looks like figure 4-2.
<img alt="s1 and s2 pointing to the same value" src="img/trpl04-02.svg" class="center" style="width: 50%;" />
Figure 4-2: Representation in memory of the binding `s2` that has a copy of
`s1`'s pointer, length and capacity
And _not_ Figure 4-3, which is what memory would look like if Rust instead
copied the heap data as well. If Rust did this, the operation `s2 = s1` could
@ -311,35 +326,28 @@ println!("{}", s1);
If you have heard the terms "shallow copy" and "deep copy" while working with
other languages, the concept of copying the pointer, length, and capacity
without copying the data probably sounded like a shallow copy. Because Rust
without copying the data probably sounds like a shallow copy. But because Rust
also invalidates the first binding, instead of calling this a shallow copy,
it's called a _move_. Here we would read this by saying that `s1` was _moved_
into `s2`. So what actually happens looks like this:
it's known as a _move_. Here we would read this by saying that `s1` was _moved_
into `s2`. So what actually happens looks like Figure 4-4.
<img alt="s1 moved to s2" src="img/trpl04-04.svg" class="center" style="width: 50%;" />
Figure 4-4: Representation in memory after `s1` has been invalidated
That solves our problem! With only `s2` valid, when it goes out of scope, it
alone will free the memory, and we’re done.
### Ownership Rules
This leads us to the Ownership Rules:
> 1. Each value in Rust has a variable binding that’s called its *owner*.
> 2. There can only be one owner at a time.
> 3. When the owner goes out of scope, the value will be `drop()`ped.
Furthermore, there’s a design choice that’s implied by this: Rust will never
automatically create "deep" copies of your data. Therefore, any _automatic_
copying can be assumed to be inexpensive.
### Clone
#### Ways Bindings and Data Interact: Clone
But what if we _do_ want to deeply copy the `String`’s data and not just the
`String` itself? There’s a common method for that: `clone()`. We will discuss
methods in the section on [`structs` in Chapter XX][structs]<!-- ignore -->,
but they’re a common enough feature in many programming languages that you have
probably seen them before.
If we _do_ want to deeply copy the `String`’s data and not just the `String`
itself, there’s a common method for that: `clone()`. We will discuss methods in
the section on [`structs` in Chapter XX][structs]<!-- ignore -->, but they’re a
common enough feature in many programming languages that you have probably seen
them before.
[structs]: ch05-01-structs.html
@ -349,7 +357,7 @@ Here’s an example of the `clone()` method in action:
let s1 = String::from("hello");
let s2 = s1.clone();
println!("{}", s1);
println!("s1 = {}, s2 = {}", s1, s2);
```
This will work just fine, and this is how you can explicitly get the behavior
@ -361,13 +369,14 @@ different is going on here.
#### Stack-only Data: Copy
There’s one last wrinkle that we haven’t talked about yet. This code works:
There’s another wrinkle we haven’t talked about yet. This code, that we showed
earlier, works and is valid:
```rust
let x = 5;
let y = x;
println!("{}", x);
println!("x = {}, y = {}", x, y);
```
This seems to contradict what we just learned: we don't have a call to
@ -391,7 +400,8 @@ we will get a compile-time error.
So what types are `Copy`? You can check the documentation for the given type to
be sure, but as a rule of thumb, any group of simple scalar values can be Copy,
but nothing that requires allocation or is some form of resource is `Copy`. Here’s some of the types that are `Copy`:
and nothing that requires allocation or is some form of resource is `Copy`.
Here’s some of the types that are `Copy`:
* All of the integer types, like `u32`.
* The booleans, `true` and `false`.
@ -399,52 +409,29 @@ but nothing that requires allocation or is some form of resource is `Copy`. Here
* Tuples, but only if they contain types which are also `Copy`. `(i32, i32)`
is `Copy`, but `(i32, String)` is not.
### Ownership and functions
### Ownership and Functions
Passing a value to a function has similar semantics as assigning it:
The semantics for passing a value to a function are similar to assigning a
value to a binding. Passing a binding to a function will move or copy, just
like assignment. Here’s an example, with some annotations showing where bindings
go into and out of scope:
Filename: src/main.rs
```rust
fn main() {
let s = String::from("hello");
let s = String::from("hello"); // s comes into scope.
takes_ownership(s);
let x = 5;
makes_copy(x);
}
fn takes_ownership(some_string: String) {
println!("{}", some_string);
}
fn makes_copy(some_integer: i32) {
println!("{}", some_integer);
}
```
Passing a binding to a function will move or copy, just like assignment. Here’s
the same example, but with some annotations showing where things go into and
out of scope:
Filename: src/main.rs
```rust
fn main() {
let s = String::from("hello"); // s goes into scope.
takes_ownership(s); // s moves into the function...
takes_ownership(s); // s's value moves into the function...
// ... and so is no longer valid here.
let x = 5; // x goes into scope.
let x = 5; // x comes into scope.
makes_copy(x); // x would move into the function,
// but i32 is Copy, so it’s okay to still
// use x afterward.
} // Here, x goes out of scope, then s. But since s was moved, nothing special
// happens.
} // Here, x goes out of scope, then s. But since s's value was moved, nothing
// special happens.
fn takes_ownership(some_string: String) { // some_string comes into scope.
println!("{}", some_string);
@ -456,37 +443,14 @@ fn makes_copy(some_integer: i32) { // some_integer comes into scope.
} // Here, some_integer goes out of scope. Nothing special happens.
```
Remember: If we tried to use `s` after the call to `takes_ownership()`, Rust
If we tried to use `s` after the call to `takes_ownership()`, Rust
would throw a compile-time error. These static checks protect us from mistakes.
Try adding code to `main` that uses `s` and `x` to see where you can use them
and where the ownership rules prevent you from doing so.
Returning values can also transfer ownership:
### Return Values and Scope
Filename: src/main.rs
```rust
fn main() {
let s1 = gives_ownership();
let s2 = String::from("hello");
let s3 = takes_and_gives_back(s2);
}
fn gives_ownership() -> String {
let some_string = String::from("hello");
some_string
}
fn takes_and_gives_back(a_string: String) -> String {
a_string
}
```
With similiar annotations:
Returning values can also transfer ownership. Here's an example with similar annotations:
Filename: src/main.rs
@ -521,14 +485,18 @@ fn takes_and_gives_back(a_string: String) -> String { // a_string comes into sco
}
```
It’s the same pattern, every time: assigning something moves it, and when an
owner goes out of scope, if it hasn’t been moved, it will `drop()`.
It’s the same pattern, every time: assigning a value to another binding moves
it, and when heap data values' bindings go out of scope, if the data hasn’t
been moved to be owned by another binding, the value will be cleaned up by
`drop()`.
This might seem a bit tedious, and it is. What if we want to let a function use
a value but not take ownership? It’s quite annoying that anything we pass in
also needs to be passed back if we want to use it again, in addition to any
data resulting from the body of the function that we might want to return as
well. It's _possible_ to return multiple values, using a tuple, like this:
Taking ownership then returning ownership with every function is a bit tedious.
What if we want to let a function use a value but not take ownership? It’s
quite annoying that anything we pass in also needs to be passed back if we want
to use it again, in addition to any data resulting from the body of the
function that we might want to return as well.
It is possible to return multiple values using a tuple, like this:
Filename: src/main.rs
@ -549,5 +517,4 @@ fn calculate_length(s: String) -> (String, usize) {
```
But this is too much ceremony and a lot of work for a concept that should be
common. Luckily for us, Rust has a feature for this concept, and it’s what the
next section is about.
common. Luckily for us, Rust has a feature for this concept: references.

122
src/ch04-02-references-and-borrowing.md
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@ -1,31 +1,12 @@
## References and Borrowing
At the end of the last section, we had some example Rust that wasn’t very
good. Here it is again:
The issue with the tuple code at the end of the last section is that we have to
return the `String` back to the calling function so that we can still use the
`String` after the call to `calculate_length()`, since the `String` was moved
into `calculate_length()`.
Filename: src/main.rs
```rust
fn main() {
let s1 = String::from("hello");
let (s2, len) = calculate_length(s1);
println!("The length of '{}' is {}.", s2, len);
}
fn calculate_length(s: String) -> (String, usize) {
let length = s.len(); // len() returns the length of a String.
(s, length)
}
```
The issue here is that we have to return the `String` back to the calling
function so that we can still use it there, since it was moved when we called
`calculate_length()`.
There is a better way. It looks like this:
Here is how you would use a function without taking ownership of it using
*references:*
Filename: src/main.rs
@ -50,10 +31,11 @@ function return value is gone. Next, note that we pass `&s1` into
`calculate_length()`, and in its definition, we take `&String` rather than
`String`.
These `&`s are called *references*, and they allow you to refer to some value
without taking ownership of it. Here’s a diagram:
These `&`s are *references*, and they allow you to refer to some value
without taking ownership of it. Figure 4-5 shows a diagram of this.
<img alt="&String s pointing at String s1" src="img/trpl04-05.svg" class="center" />
Figure 4-5: `&String s` pointing at `String s1`
Let’s take a closer look at the function call here:
@ -68,9 +50,9 @@ let s1 = String::from("hello");
let len = calculate_length(&s1);
```
The `&s1` syntax lets us create a reference with `s1`. This reference _refers_
to the value of `s1` but does not own it. Because it does not own it, the
value it points to will not be dropped when the reference goes out of scope.
The `&s1` syntax lets us create a reference which _refers_ to the value of `s1`
but does not own it. Because it does not own it, the value it points to will
not be dropped when the reference goes out of scope.
Likewise, the signature of the function uses `&` to indicate that it takes
a reference as an argument. Let’s add some explanatory annotations:
@ -84,17 +66,17 @@ fn calculate_length(s: &String) -> usize { // s is a reference to a String
// it refers to, nothing happens.
```
It’s the same process as before, except that because we don’t have ownership,
we don’t drop what a reference points to when the reference goes out of scope.
This lets us write functions which take references as arguments instead of the
values themselves, so that we won’t need to return them to give back ownership.
It’s the same process as before, but we don’t drop what the reference points to
when it goes out of scope because we don't have ownership. This lets us write
functions which take references as arguments instead of the values themselves,
so that we won’t need to return them to give back ownership.
There’s another word for what references do, and that’s *borrowing*. Just like
with real life, if a person owns something, you can borrow it from them. When
you’re done, you have to give it back.
We call this process *borrowing*. Just like with real life, if a person owns
something, you can borrow it from them, and when you’re done, you have to give
it back.
Speaking of which, what if you try to modify something you borrow from me? Try
this code out. Spoiler alert: it doesn’t work!
So what happens if we try to modify something we're borrowing? Try this code
out. Spoiler alert: it doesn’t work!
Filename: src/main.rs
@ -120,12 +102,12 @@ error: cannot borrow immutable borrowed content `*some_string` as mutable
| ^^^^^^^^^^^
```
Just like bindings are immutable by default, so are references. We’re not
allowed to modify something we have a reference to.
Just as bindings are immutable by default, so are references. We’re not allowed
to modify something we have a reference to.
### Mutable references
### Mutable References
We can fix this bug! Just a small tweak:
We can fix this error with just a small tweak:
Filename: src/main.rs
@ -145,7 +127,9 @@ First, we had to change `s` to be `mut`. Then we had to create a mutable
reference with `&mut s` and accept a mutable reference with `some_string: &mut
String`.
Mutable references have one big restriction, though. This code fails:
Mutable references have one big restriction, though: you can only have one
mutable reference to a particular piece of data in a particular scope. This
code will fail:
Filename: src/main.rs
@ -170,10 +154,15 @@ error[E0499]: cannot borrow `s` as mutable more than once at a time
| - first borrow ends here
```
The error is what it says: you cannot borrow something mutably more than once
at a time. This restriction allows for mutation but in a very controlled
fashion. It is something that new Rustaceans struggle with, because most
languages let you mutate whenever you’d like.
This restriction allows for mutation but in a very controlled fashion. It is
something that new Rustaceans struggle with, because most languages let you
mutate whenever you’d like. The benefit of having this restriction is that Rust
can prevent data races at compile time. A *data race* is a particular type of
race condition where two or more pointers access the same data at the same
time, and at least one of the pointers is being used to write to the data. Data
races cause unpredictable behavior and can be difficult to diagnose and fix
when trying to track them down at runtime; Rust prevents this problem from
happening since it won't even compile code with data races!
As always, we can use `{}`s to create a new scope, allowing for multiple mutable
references, just not _simultaneous_ ones:
@ -217,15 +206,22 @@ error[E0502]: cannot borrow `s` as mutable because it is also borrowed as immuta
Whew! We _also_ cannot have a mutable reference while we have an immutable one.
Users of an immutable reference don’t expect the values to suddenly change out
from under them! Multiple immutable references are okay, however.
from under them! Multiple immutable references are okay, however, since no one
who is just reading the data has the ability to affect anyone else's reading of
the data.
### Dangling references
Even though these errors may be frustrating at times, remember that it's the
Rust compiler pointing out a potential bug earlier (at compile time rather than
at runtime) and showing you exactly where the problem is instead of you having
to track down why sometimes your data isn't what you thought it should be.
In languages with pointers, it’s easy to create a “dangling pointer” by freeing
some memory while keeping around a pointer to that memory. In Rust, by
contrast, the compiler guarantees that references will never be dangling: if we
have a reference to something, the compiler will ensure that it will not go
out of scope before the reference does.
### Dangling References
In languages with pointers, it's easy to make the error of creating a “dangling
pointer” by freeing some memory while keeping around a pointer to that memory.
In Rust, by contrast, the compiler guarantees that references will never be
dangling: if we have a reference to some data, the compiler will ensure that
the data will not go out of scope before the reference to the data does.
Let’s try to create a dangling reference:
@ -259,16 +255,14 @@ error[E0106]: missing lifetime specifier
error: aborting due to previous error
```
This error message refers to a feature we haven’t learned about yet,
*lifetimes*. The message does contain the key to why this code is a problem,
though:
This error message refers to a feature we haven’t learned about yet:
*lifetimes*. We'll discuss lifetimes in detail in Chapter XX, but, disregarding
the parts about lifetimes, the message does contain the key to why this code is
a problem: `this function’s return type contains a borrowed value, but there is
no value for it to be borrowed from`.
```bash
this function’s return type contains a borrowed value, but there is no value
for it to be borrowed from
```
Let’s examine exactly what happens with `dangle()`:
Let’s have a closer look at exactly what's happenening at each stage of our
`dangle()` code:
```rust,ignore
fn dangle() -> &String { // dangle returns a reference to a String

141
src/ch04-03-slices.md
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@ -1,13 +1,13 @@
## Slices
So far, we’ve talked about types that have ownership, like `String`, and ones
that don’t, like `&String`. There is another kind of type which does not have
ownership: slices. Slices let you reference a contiguous sequence of elements
in a collection rather than the whole collection itself.
There is another data type which does not have ownership: slices. Slices let
you reference a contiguous sequence of elements in a collection rather than the
whole collection itself.
Here’s a small programming problem: write a function which takes a string
and returns the first word you find. If we don’t find a space in the string,
then the whole string is a word, so the whole thing should be returned.
Here’s a small programming problem: write a function which takes a string and
returns the first word it finds in that string. If it doesn’t find a space in
the string, it means the whole string is one word, so the whole thing should be
returned.
Let’s think about the signature of this function:
@ -50,16 +50,16 @@ array of bytes using the `.as_bytes()` method.
for (i, &item) in bytes.iter().enumerate() {
```
We will be discussing iterators in more detail in Chapter XX, but for
now, know that `iter()` is a method that returns each element in a
collection, and `enumerate()` modifies the result of `iter()` and returns
a tuple instead. The first element of the tuple is the index, and the
second element is a reference to the element itself. This is a bit
nicer than calculating the index ourselves.
We will be discussing iterators in more detail in Chapter XX, but for now, know
that `iter()` is a method that returns each element in a collection, and
`enumerate()` modifies the result of `iter()` and returns each element as part
of a tuple instead, where the first element of the tuple is the index, and the
second element is a reference to the element itself. This is a bit nicer than
calculating the index ourselves.
Since it’s a tuple, we can use patterns, just like elsewhere in Rust. So we
match against the tuple with i for the index and &item for a single byte. Since
we get a reference from `.iter().enumerate()`, we use `&` in the pattern.
match against the tuple with `i` for the index and `&item` for a single byte.
Since we get a reference from `.iter().enumerate()`, we use `&` in the pattern.
```rust,ignore
if item == b' ' {
@ -70,13 +70,15 @@ s.len()
```
We search for the byte that represents the space, using the byte literal
syntax. If we find one, we return the position. Otherwise, we return the length
of the string, using `s.len()`.
syntax. If we find a space, we return the position. Otherwise, we return the
length of the string, using `s.len()`.
This works, but there’s a problem. We’re returning a `usize` on its own, but
it’s only a meaningful number in the context of the `&String`. In other
words, because it’s a separate value from the `String`, there’s no guarantee
that it will still be valid in the future. Consider this:
We now have a way to find out the index of the end of the first word in the
string, but there’s a problem. We’re returning a `usize` on its own, but it’s
only a meaningful number in the context of the `&String`. In other words,
because it’s a separate value from the `String`, there’s no guarantee that it
will still be valid in the future. Consider this program that uses this
`first_word()` function:
Filename: src/main.rs
@ -105,6 +107,12 @@ fn main() {
}
```
This program compiles without any errors, and also would if we used `word`
after calling `s.clear()`. `word` isn't connected to the state of `s` at all,
so `word` still contains the value `5`. We could use that `5` with `s` to try
to extract the first word out, but this would be a bug since the contents of
`s` have changed since we saved `5` in `word`.
This is bad! It’s even worse if we wanted to write a `second_word()`
function. Its signature would have to look like this:
@ -112,15 +120,16 @@ function. Its signature would have to look like this:
fn second_word(s: &String) -> (usize, usize) {
```
Now we’re tracking both a start _and_ an ending index. Even more chances for
things to go wrong. We now have three unrelated variable bindings floating
Now we’re tracking both a start _and_ an ending index, and we have even more
values that were calculated from data in a particular state but aren't tied to
that state at all. We now have three unrelated variable bindings floating
around which need to be kept in sync.
Luckily, Rust has a solution to this problem: string slices.
## String slices
### String Slices
A string slice looks like this:
A string slice is a reference to part of a `String`, and looks like this:
```rust
let s = String::from("hello world");
@ -129,19 +138,20 @@ let hello = &s[0..5];
let world = &s[6..11];
```
This looks just like taking a reference to the whole `String`, but with the
extra `[0..5]` bit. Instead of being a reference to the entire `String`, it’s a
This is similar to taking a reference to the whole `String`, but with the
extra `[0..5]` bit. Rather than a reference to the entire `String`, it’s a
reference to an internal position in the `String` and the number of elements
that it refers to.
We can create slices with a range of `[starting_index..ending_index]`, but the
We create slices with a range of `[starting_index..ending_index]`, but the
slice data structure actually stores the starting position and the length of the
slice. So in the case of `let world = &s[6..11];`, `world` would be a slice that
contains a pointer to the 6th byte of `s` and a length value of 5.
In other words, it looks like this:
Figure 4-7 shows this in a diagram:
<img alt="world containing a pointer to the 6th byte of String s and a length 5" src="img/trpl04-06.svg" class="center" style="width: 50%;" />
Figure 4-7: Two string slices referring to parts of a `String`
With Rust’s `..` range syntax, if you want to start at the first index (zero),
you can drop the value before the `..`. In other words, these are equal:
@ -178,7 +188,8 @@ let slice = &s[0..len];
let slice = &s[..];
```
With this in mind, let’s re-write `first_word()` to return a slice:
With this in mind, let’s re-write `first_word()` to return a slice. The type
that signifies "string slice" is written as `&str`:
Filename: src/main.rs
@ -196,20 +207,28 @@ fn first_word(s: &String) -> &str {
}
```
Now we have a single value, the `&str`, pronounced "string slice". It stores
both elements that we care about: a reference to the starting point of the
slice and the number of elements in the slice.
We get the index for the end of the word in the same way as before, by looking
for the first occurrence of a space. When we find a space, we return a string
slice using the start of the string and the index of the space as the starting
and ending indices.
This would also work for a `second_word()`:
Now when we call `first_word()`, we get back a single value that is tied to the
underlying data. The value is made up of a reference to the starting point of
the slice and the number of elements in the slice.
Returning a slice would also work for a `second_word()` function:
```rust,ignore
fn second_word(s: &String) -> &str {
```
We now have a straightforward API that’s much harder to mess up.
But what about our error condition from before? Slices also fix that. Using
the slice version of `first_word()` will throw an error:
We now have a straightforward API that’s much harder to mess up. Remember our
bug from before, when we got the first word but then cleared the string so that
our first word was invalid? That code was logically incorrect but didn't show
any immediate errors-- the problems would show up later, if we kept trying to
use the first word index with an emptied string. Slices make this bug
impossible, and let us know we have a problem with our code much sooner. Using
the slice version of `first_word()` will throw a compile time error:
Filename: src/main.rs
@ -223,7 +242,7 @@ fn main() {
}
```
Here’s the error:
Here’s the compiler error:
```bash
17:6 error: cannot borrow `s` as mutable because it is also borrowed as
@ -241,13 +260,13 @@ fn main() {
^
```
Remember the borrowing rules? If we have an immutable reference to something,
we cannot also take a mutable reference. Since `clear()` needs to truncate the
`String`, it tries to take a mutable reference, which fails. Not only has Rust
made our API easier to use, but it’s also eliminated an entire class of errors
at compile time!
Remember from the borrowing rules that if we have an immutable reference to
something, we cannot also take a mutable reference. Since `clear()` needs to
truncate the `String`, it tries to take a mutable reference, which fails. Not
only has Rust made our API easier to use, but it’s also eliminated an entire
class of errors at compile time!
### String literals are slices
#### String Literals are Slices
Remember how we talked about string literals being stored inside of the binary
itself? Now that we know about slices, we can now properly understand string
@ -261,7 +280,7 @@ The type of `s` here is `&str`: It’s a slice, pointing to that specific point
of the binary. This is also why string literals are immutable; `&str` is an
immutable reference.
### String slices as arguments
#### String Slices as Arguments
Knowing that you can take slices of both literals and `String`s leads us to
one more improvement on `first_word()`, and that’s its signature:
@ -270,17 +289,16 @@ one more improvement on `first_word()`, and that’s its signature:
fn first_word(s: &String) -> &str {
```
A more experienced Rustacean would write this one instead:
A more experienced Rustacean would write this one instead because it allows us
to use the same function on both `String`s and `&str`s:
```rust,ignore
fn first_word(s: &str) -> &str {
```
Why is this? Well, we aren’t trying to modify `s` at all. And we can take
a string slice that’s the full length of a `String`, so we haven’t lost
the ability to talk about full `String`s. And additionally, we can take
string slices of string literals too, so this function is more useful, but
with no loss of functionality:
If we have a string slice, we can pass that as the argument directly. If we
have a `String`, we can pass a slice of the entire `String`. This makes our API
more general and useful without losing any functionality:
Filename: src/main.rs
@ -313,17 +331,17 @@ fn main() {
}
```
## Other slices
### Other Slices
String slices, as you might imagine, are specific to strings. But there’s a more
general slice type, too. Consider arrays:
general slice type, too. Consider this array:
```rust
let a = [1, 2, 3, 4, 5];
```
Just like we may want to refer to a part of a string, we may want to refer to
part of an array:
part of an array, and would do so like this:
```rust
let a = [1, 2, 3, 4, 5];
@ -335,3 +353,16 @@ This slice has the type `&[i32]`. It works the exact same way as string slices
do, by storing a reference to the first element and a length. You’ll use this
kind of slice for all sorts of other collections. We’ll discuss these in detail
when we talk about vectors in Chapter XX.
## Summary
The concepts of ownership, borrowing, and slices are what ensure memory safety
in Rust programs at compile time. Rust is a language that gives you control
over your memory usage like other systems programming languages, but having the
owner of data automatically clean up that data when the owner goes out of scope
means you don't have to write and debug extra code to get this control.
Ownership affects how lots of other parts of Rust work, so we will be talking
about these concepts further throughout the rest of the book. Let's move on to
the next chapter where we'll look at grouping pieces of data together in a
`struct`.