We talked about strings in Chapter 4, but we’ll look at them in more depth now. New Rustaceans commonly get stuck on strings for a combination of three reasons: Rust’s propensity for exposing possible errors, strings being a more complicated data structure than many programmers give them credit for, and UTF-8. These factors combine in a way that can seem difficult when you’re coming from other programming languages.
We discuss strings in the context of collections because strings are
implemented as a collection of bytes, plus some methods to provide useful
functionality when those bytes are interpreted as text. In this section, we’ll
talk about the operations on String that every collection type has, such as
creating, updating, and reading. We’ll also discuss the ways in which String
is different from the other collections, namely how indexing into a String is
complicated by the differences between how people and computers interpret
String data.
We’ll first define what we mean by the term string. Rust has only one string
type in the core language, which is the string slice str that is usually seen
in its borrowed form &str. In Chapter 4, we talked about string slices,
which are references to some UTF-8 encoded string data stored elsewhere. String
literals, for example, are stored in the program’s binary and are therefore
string slices.
The String type, which is provided by Rust’s standard library rather than
coded into the core language, is a growable, mutable, owned, UTF-8 encoded
string type. When Rustaceans refer to “strings” in Rust, they might be
referring to either the String or the string slice &str types, not just one
of those types. Although this section is largely about String, both types are
used heavily in Rust’s standard library, and both String and string slices
are UTF-8 encoded.
Many of the same operations available with Vec<T> are available with String
as well because String is actually implemented as a wrapper around a vector
of bytes with some extra guarantees, restrictions, and capabilities. An example
of a function that works the same way with Vec<T> and String is the new
function to create an instance, shown in Listing 8-11.
{{#rustdoc_include ../listings/ch08-common-collections/listing-08-11/src/main.rs:here}}This line creates a new, empty string called s, into which we can then load
data. Often, we’ll have some initial data with which we want to start the
string. For that, we use the to_string method, which is available on any type
that implements the Display trait, as string literals do. Listing 8-12 shows
two examples.
{{#rustdoc_include ../listings/ch08-common-collections/listing-08-12/src/main.rs:here}}This code creates a string containing initial contents.
We can also use the function String::from to create a String from a string
literal. The code in Listing 8-13 is equivalent to the code in Listing 8-12
that uses to_string.
{{#rustdoc_include ../listings/ch08-common-collections/listing-08-13/src/main.rs:here}}Because strings are used for so many things, we can use many different generic
APIs for strings, providing us with a lot of options. Some of them can seem
redundant, but they all have their place! In this case, String::from and
to_string do the same thing, so which one you choose is a matter of style and
readability.
Remember that strings are UTF-8 encoded, so we can include any properly encoded data in them, as shown in Listing 8-14.
{{#rustdoc_include ../listings/ch08-common-collections/listing-08-14/src/main.rs:here}}All of these are valid String values.
A String can grow in size and its contents can change, just like the contents
of a Vec<T>, if you push more data into it. In addition, you can conveniently
use the + operator or the format! macro to concatenate String values.
We can grow a String by using the push_str method to append a string slice,
as shown in Listing 8-15.
{{#rustdoc_include ../listings/ch08-common-collections/listing-08-15/src/main.rs:here}}After these two lines, s will contain foobar. The push_str method takes a
string slice because we don’t necessarily want to take ownership of the
parameter. For example, in the code in Listing 8-16, we want to be able to use
s2 after appending its contents to s1.
{{#rustdoc_include ../listings/ch08-common-collections/listing-08-16/src/main.rs:here}}If the push_str method took ownership of s2, we wouldn’t be able to print
its value on the last line. However, this code works as we’d expect!
The push method takes a single character as a parameter and adds it to the
String. Listing 8-17 adds the letter l to a String using the push
method.
{{#rustdoc_include ../listings/ch08-common-collections/listing-08-17/src/main.rs:here}}As a result, s will contain lol.
Often, you’ll want to combine two existing strings. One way to do so is to use
the + operator, as shown in Listing 8-18.
{{#rustdoc_include ../listings/ch08-common-collections/listing-08-18/src/main.rs:here}}The string s3 will contain Hello, world!. The reason s1 is no longer
valid after the addition, and the reason we used a reference to s2, has to do
with the signature of the method that’s called when we use the + operator.
The + operator uses the add method, whose signature looks something like
this:
fn add(self, s: &str) -> String {In the standard library, you’ll see add defined using generics and associated
types. Here, we’ve substituted in concrete types, which is what happens when we
call this method with String values. We’ll discuss generics in Chapter 10.
This signature gives us the clues we need in order to understand the tricky
bits of the + operator.
First, s2 has an &, meaning that we’re adding a reference of the second
string to the first string. This is because of the s parameter in the add
function: we can only add a &str to a String; we can’t add two String
values together. But wait—the type of &s2 is &String, not &str, as
specified in the second parameter to add. So why does Listing 8-18 compile?
The reason we’re able to use &s2 in the call to add is that the compiler
can coerce the &String argument into a &str. When we call the add
method, Rust uses a deref coercion, which here turns &s2 into &s2[..].
We’ll discuss deref coercion in more depth in Chapter 15. Because add does
not take ownership of the s parameter, s2 will still be a valid String
after this operation.
Second, we can see in the signature that add takes ownership of self
because self does not have an &. This means s1 in Listing 8-18 will be
moved into the add call and will no longer be valid after that. So, although
let s3 = s1 + &s2; looks like it will copy both strings and create a new one,
this statement actually takes ownership of s1, appends a copy of the contents
of s2, and then returns ownership of the result. In other words, it looks
like it’s making a lot of copies, but it isn’t; the implementation is more
efficient than copying.
If we need to concatenate multiple strings, the behavior of the + operator
gets unwieldy:
{{#rustdoc_include ../listings/ch08-common-collections/no-listing-01-concat-multiple-strings/src/main.rs:here}}At this point, s will be tic-tac-toe. With all of the + and "
characters, it’s difficult to see what’s going on. For combining strings in
more complicated ways, we can instead use the format! macro:
{{#rustdoc_include ../listings/ch08-common-collections/no-listing-02-format/src/main.rs:here}}This code also sets s to tic-tac-toe. The format! macro works like
println!, but instead of printing the output to the screen, it returns a
String with the contents. The version of the code using format! is much
easier to read, and the code generated by the format! macro uses references
so that this call doesn’t take ownership of any of its parameters.
In many other programming languages, accessing individual characters in a
string by referencing them by index is a valid and common operation. However,
if you try to access parts of a String using indexing syntax in Rust, you’ll
get an error. Consider the invalid code in Listing 8-19.
{{#rustdoc_include ../listings/ch08-common-collections/listing-08-19/src/main.rs:here}}This code will result in the following error:
{{#include ../listings/ch08-common-collections/listing-08-19/output.txt}}The error and the note tell the story: Rust strings don’t support indexing. But why not? To answer that question, we need to discuss how Rust stores strings in memory.
A String is a wrapper over a Vec<u8>. Let’s look at some of our properly
encoded UTF-8 example strings from Listing 8-14. First, this one:
{{#rustdoc_include ../listings/ch08-common-collections/listing-08-14/src/main.rs:spanish}}In this case, len will be 4, which means the vector storing the string
"Hola" is 4 bytes long. Each of these letters takes one byte when encoded in
UTF-8. The following line, however, may surprise you (note that this string
begins with the capital Cyrillic letter Ze, not the number 3):
{{#rustdoc_include ../listings/ch08-common-collections/listing-08-14/src/main.rs:russian}}If you were asked how long the string is, you might say 12. In fact, Rust’s answer is 24: that’s the number of bytes it takes to encode “Здравствуйте” in UTF-8, because each Unicode scalar value in that string takes 2 bytes of storage. Therefore, an index into the string’s bytes will not always correlate to a valid Unicode scalar value. To demonstrate, consider this invalid Rust code:
let hello = "Здравствуйте";
let answer = &hello[0];You already know that answer will not be З, the first letter. When encoded
in UTF-8, the first byte of З is 208 and the second is 151, so it would
seem that answer should in fact be 208, but 208 is not a valid character
on its own. Returning 208 is likely not what a user would want if they asked
for the first letter of this string; however, that’s the only data that Rust
has at byte index 0. Users generally don’t want the byte value returned, even
if the string contains only Latin letters: if &"hi"[0] were valid code that
returned the byte value, it would return 104, not h.
The answer, then, is that to avoid returning an unexpected value and causing bugs that might not be discovered immediately, Rust doesn’t compile this code at all and prevents misunderstandings early in the development process.
Another point about UTF-8 is that there are actually three relevant ways to look at strings from Rust’s perspective: as bytes, scalar values, and grapheme clusters (the closest thing to what we would call letters).
If we look at the Hindi word “नमस्ते” written in the Devanagari script, it is
stored as a vector of u8 values that looks like this:
[224, 164, 168, 224, 164, 174, 224, 164, 184, 224, 165, 141, 224, 164, 164,
224, 165, 135]
That’s 18 bytes and is how computers ultimately store this data. If we look at
them as Unicode scalar values, which are what Rust’s char type is, those
bytes look like this:
['न', 'म', 'स', '्', 'त', 'े']
There are six char values here, but the fourth and sixth are not letters:
they’re diacritics that don’t make sense on their own. Finally, if we look at
them as grapheme clusters, we’d get what a person would call the four letters
that make up the Hindi word:
["न", "म", "स्", "ते"]
Rust provides different ways of interpreting the raw string data that computers store so that each program can choose the interpretation it needs, no matter what human language the data is in.
A final reason Rust doesn’t allow us to index into a String to get a
character is that indexing operations are expected to always take constant time
(O(1)). But it isn’t possible to guarantee that performance with a String,
because Rust would have to walk through the contents from the beginning to the
index to determine how many valid characters there were.
Indexing into a string is often a bad idea because it’s not clear what the return type of the string-indexing operation should be: a byte value, a character, a grapheme cluster, or a string slice. If you really need to use indices to create string slices, therefore, Rust asks you to be more specific.
Rather than indexing using [] with a single number, you can use [] with a
range to create a string slice containing particular bytes:
let hello = "Здравствуйте";
let s = &hello[0..4];Here, s will be a &str that contains the first four bytes of the string.
Earlier, we mentioned that each of these characters was two bytes, which means
s will be Зд.
If we were to try to slice only part of a character’s bytes with something like
&hello[0..1], Rust would panic at runtime in the same way as if an invalid
index were accessed in a vector:
{{#include ../listings/ch08-common-collections/output-only-01-not-char-boundary/output.txt}}You should use caution when creating string slices with ranges, because doing so can crash your program.
The best way to operate on pieces of strings is to be explicit about whether
you want characters or bytes. For individual Unicode scalar values, use the
chars method. Calling chars on “Зд” separates out and returns two values of
type char, and you can iterate over the result to access each element:
for c in "Зд".chars() {
println!("{c}");
}This code will print the following:
З
д
Alternatively, the bytes method returns each raw byte, which might be
appropriate for your domain:
for b in "Зд".bytes() {
println!("{b}");
}This code will print the four bytes that make up this string:
208
151
208
180
But be sure to remember that valid Unicode scalar values may be made up of more than one byte.
Getting grapheme clusters from strings, as with the Devanagari script, is complex, so this functionality is not provided by the standard library. Crates are available on crates.io if this is the functionality you need.
To summarize, strings are complicated. Different programming languages make
different choices about how to present this complexity to the programmer. Rust
has chosen to make the correct handling of String data the default behavior
for all Rust programs, which means programmers have to put more thought into
handling UTF-8 data up front. This trade-off exposes more of the complexity of
strings than is apparent in other programming languages, but it prevents you
from having to handle errors involving non-ASCII characters later in your
development life cycle.
The good news is that the standard library offers a lot of functionality built
off the String and &str types to help handle these complex situations
correctly. Be sure to check out the documentation for useful methods like
contains for searching in a string and replace for substituting parts of a
string with another string.
Let’s switch to something a bit less complex: hash maps!