Struct alloc::string::String

pub struct String { /* fields omitted */ }

A UTF-8–encoded, growable string.

The String type is the most common string type that has ownership over the contents of the string. It has a close relationship with its borrowed counterpart, the primitive str.

Examples

You can create a String from a literal string with String::from:

let hello = String::from("Hello, world!");

You can append a char to a String with the push method, and append a &str with the push_str method:

let mut hello = String::from("Hello, ");

hello.push('w');
hello.push_str("orld!");

If you have a vector of UTF-8 bytes, you can create a String from it with the from_utf8 method:

// some bytes, in a vector
let sparkle_heart = vec![240, 159, 146, 150];

// We know these bytes are valid, so we'll use `unwrap()`.
let sparkle_heart = String::from_utf8(sparkle_heart).unwrap();

assert_eq!("💖", sparkle_heart);

UTF-8

Strings are always valid UTF-8. This has a few implications, the first of which is that if you need a non-UTF-8 string, consider OsString. It is similar, but without the UTF-8 constraint. The second implication is that you cannot index into a String:

let s = "hello";

println!("The first letter of s is {}", s[0]); // ERROR!!!

Indexing is intended to be a constant-time operation, but UTF-8 encoding does not allow us to do this. Furthermore, it’s not clear what sort of thing the index should return: a byte, a codepoint, or a grapheme cluster. The bytes and chars methods return iterators over the first two, respectively.

Deref

String implements Deref<Target = str>, and so inherits all of str’s methods. In addition, this means that you can pass a String to a function which takes a &str by using an ampersand (&):

fn takes_str(s: &str) { }

let s = String::from("Hello");

takes_str(&s);

This will create a &str from the String and pass it in. This conversion is very inexpensive, and so generally, functions will accept &strs as arguments unless they need a String for some specific reason.

In certain cases Rust doesn’t have enough information to make this conversion, known as Deref coercion. In the following example a string slice &'a str implements the trait TraitExample, and the function example_func takes anything that implements the trait. In this case Rust would need to make two implicit conversions, which Rust doesn’t have the means to do. For that reason, the following example will not compile.

trait TraitExample {}

impl<'a> TraitExample for &'a str {}

fn example_func<A: TraitExample>(example_arg: A) {}

let example_string = String::from("example_string");
example_func(&example_string);

There are two options that would work instead. The first would be to change the line example_func(&example_string); to example_func(example_string.as_str());, using the method as_str() to explicitly extract the string slice containing the string. The second way changes example_func(&example_string); to example_func(&*example_string);. In this case we are dereferencing a String to a str, then referencing the str back to &str. The second way is more idiomatic, however both work to do the conversion explicitly rather than relying on the implicit conversion.

Representation

A String is made up of three components: a pointer to some bytes, a length, and a capacity. The pointer points to an internal buffer String uses to store its data. The length is the number of bytes currently stored in the buffer, and the capacity is the size of the buffer in bytes. As such, the length will always be less than or equal to the capacity.

This buffer is always stored on the heap.

You can look at these with the as_ptr, len, and capacity methods:

use std::mem;

let story = String::from("Once upon a time...");

// Prevent automatically dropping the String's data
let mut story = mem::ManuallyDrop::new(story);

let ptr = story.as_mut_ptr();
let len = story.len();
let capacity = story.capacity();

// story has nineteen bytes
assert_eq!(19, len);

// We can re-build a String out of ptr, len, and capacity. This is all
// unsafe because we are responsible for making sure the components are
// valid:
let s = unsafe { String::from_raw_parts(ptr, len, capacity) } ;

assert_eq!(String::from("Once upon a time..."), s);

If a String has enough capacity, adding elements to it will not re-allocate. For example, consider this program:

let mut s = String::new();

println!("{}", s.capacity());

for _ in 0..5 {
    s.push_str("hello");
    println!("{}", s.capacity());
}

This will output the following:

0
5
10
20
20
40

At first, we have no memory allocated at all, but as we append to the string, it increases its capacity appropriately. If we instead use the with_capacity method to allocate the correct capacity initially:

let mut s = String::with_capacity(25);

println!("{}", s.capacity());

for _ in 0..5 {
    s.push_str("hello");
    println!("{}", s.capacity());
}

We end up with a different output:

25
25
25
25
25
25

Here, there’s no need to allocate more memory inside the loop.

Implementations

impl String

pub const fn new() -> String

Creates a new empty String.

Given that the String is empty, this will not allocate any initial buffer. While that means that this initial operation is very inexpensive, it may cause excessive allocation later when you add data. If you have an idea of how much data the String will hold, consider the with_capacity method to prevent excessive re-allocation.

Examples

Basic usage:

let s = String::new();

pub fn from_utf8(vec: Vec<u8>) -> Result<String, FromUtf8Error>

Converts a vector of bytes to a String.

A string (String) is made of bytes (u8), and a vector of bytes (Vec<u8>) is made of bytes, so this function converts between the two. Not all byte slices are valid Strings, however: String requires that it is valid UTF-8. from_utf8() checks to ensure that the bytes are valid UTF-8, and then does the conversion.

If you are sure that the byte slice is valid UTF-8, and you don’t want to incur the overhead of the validity check, there is an unsafe version of this function, from_utf8_unchecked, which has the same behavior but skips the check.

This method will take care to not copy the vector, for efficiency’s sake.

If you need a &str instead of a String, consider str::from_utf8.

The inverse of this method is into_bytes.

Errors

Returns Err if the slice is not UTF-8 with a description as to why the provided bytes are not UTF-8. The vector you moved in is also included.

Examples

Basic usage:

// some bytes, in a vector
let sparkle_heart = vec![240, 159, 146, 150];

// We know these bytes are valid, so we'll use `unwrap()`.
let sparkle_heart = String::from_utf8(sparkle_heart).unwrap();

assert_eq!("💖", sparkle_heart);

Incorrect bytes:

// some invalid bytes, in a vector
let sparkle_heart = vec![0, 159, 146, 150];

assert!(String::from_utf8(sparkle_heart).is_err());

See the docs for FromUtf8Error for more details on what you can do with this error.

pub fn into_raw_parts(self) -> (*mut u8, usize, usize)

🔬 This is a nightly-only experimental API. (vec_into_raw_parts #65816)

Decomposes a String into its raw components.

Returns the raw pointer to the underlying data, the length of the string (in bytes), and the allocated capacity of the data (in bytes). These are the same arguments in the same order as the arguments to from_raw_parts.

After calling this function, the caller is responsible for the memory previously managed by the String. The only way to do this is to convert the raw pointer, length, and capacity back into a String with the from_raw_parts function, allowing the destructor to perform the cleanup.

Examples

#![feature(vec_into_raw_parts)]
let s = String::from("hello");

let (ptr, len, cap) = s.into_raw_parts();

let rebuilt = unsafe { String::from_raw_parts(ptr, len, cap) };
assert_eq!(rebuilt, "hello");

pub unsafe fn from_raw_parts(
    buf: *mut u8,
    length: usize,
    capacity: usize
) -> String

Creates a new String from a length, capacity, and pointer.

Safety

This is highly unsafe, due to the number of invariants that aren’t checked:

• The memory at buf needs to have been previously allocated by the same allocator the standard library uses, with a required alignment of exactly 1.
• length needs to be less than or equal to capacity.
• capacity needs to be the correct value.
• The first length bytes at buf need to be valid UTF-8.

Violating these may cause problems like corrupting the allocator’s internal data structures.

The ownership of buf is effectively transferred to the String which may then deallocate, reallocate or change the contents of memory pointed to by the pointer at will. Ensure that nothing else uses the pointer after calling this function.

Examples

Basic usage:

use std::mem;

unsafe {
    let s = String::from("hello");

    // Prevent automatically dropping the String's data
    let mut s = mem::ManuallyDrop::new(s);

    let ptr = s.as_mut_ptr();
    let len = s.len();
    let capacity = s.capacity();

    let s = String::from_raw_parts(ptr, len, capacity);

    assert_eq!(String::from("hello"), s);
}

pub unsafe fn from_utf8_unchecked(bytes: Vec<u8>) -> String

Converts a vector of bytes to a String without checking that the string contains valid UTF-8.

See the safe version, from_utf8, for more details.

Safety

This function is unsafe because it does not check that the bytes passed to it are valid UTF-8. If this constraint is violated, it may cause memory unsafety issues with future users of the String, as the rest of the standard library assumes that Strings are valid UTF-8.

Examples

Basic usage:

// some bytes, in a vector
let sparkle_heart = vec![240, 159, 146, 150];

let sparkle_heart = unsafe {
    String::from_utf8_unchecked(sparkle_heart)
};

assert_eq!("💖", sparkle_heart);

pub fn into_bytes(self) -> Vec<u8>

Converts a String into a byte vector.

This consumes the String, so we do not need to copy its contents.

Examples

Basic usage:

let s = String::from("hello");
let bytes = s.into_bytes();

assert_eq!(&[104, 101, 108, 108, 111][..], &bytes[..]);

pub fn as_str(&self) -> &str

Extracts a string slice containing the entire String.

Examples

Basic usage:

let s = String::from("foo");

assert_eq!("foo", s.as_str());

pub fn as_mut_str(&mut self) -> &mut str

Converts a String into a mutable string slice.

Examples

Basic usage:

let mut s = String::from("foobar");
let s_mut_str = s.as_mut_str();

s_mut_str.make_ascii_uppercase();

assert_eq!("FOOBAR", s_mut_str);

pub fn capacity(&self) -> usize

Returns this String’s capacity, in bytes.

Examples

Basic usage:

let s = String::with_capacity(10);

assert!(s.capacity() >= 10);

pub fn try_reserve(&mut self, additional: usize) -> Result<(), TryReserveError>

Tries to reserve capacity for at least additional more elements to be inserted in the given String. The collection may reserve more space to avoid frequent reallocations. After calling reserve, capacity will be greater than or equal to self.len() + additional. Does nothing if capacity is already sufficient.

Errors

If the capacity overflows, or the allocator reports a failure, then an error is returned.

Examples

use std::collections::TryReserveError;

fn process_data(data: &str) -> Result<String, TryReserveError> {
    let mut output = String::new();

    // Pre-reserve the memory, exiting if we can't
    output.try_reserve(data.len())?;

    // Now we know this can't OOM in the middle of our complex work
    output.push_str(data);

    Ok(output)
}

pub fn try_reserve_exact(
    &mut self,
    additional: usize
) -> Result<(), TryReserveError>

Tries to reserve the minimum capacity for exactly additional more elements to be inserted in the given String. After calling reserve_exact, capacity will be greater than or equal to self.len() + additional. Does nothing if the capacity is already sufficient.

Note that the allocator may give the collection more space than it requests. Therefore, capacity can not be relied upon to be precisely minimal. Prefer reserve if future insertions are expected.

Errors

If the capacity overflows, or the allocator reports a failure, then an error is returned.

Examples

use std::collections::TryReserveError;

fn process_data(data: &str) -> Result<String, TryReserveError> {
    let mut output = String::new();

    // Pre-reserve the memory, exiting if we can't
    output.try_reserve(data.len())?;

    // Now we know this can't OOM in the middle of our complex work
    output.push_str(data);

    Ok(output)
}

pub fn as_bytes(&self) -> &[u8]

Returns a byte slice of this String’s contents.

The inverse of this method is from_utf8.

Examples

Basic usage:

let s = String::from("hello");

assert_eq!(&[104, 101, 108, 108, 111], s.as_bytes());

pub fn truncate(&mut self, new_len: usize)

Shortens this String to the specified length.

If new_len is greater than the string’s current length, this has no effect.

Note that this method has no effect on the allocated capacity of the string

Panics

Panics if new_len does not lie on a char boundary.

Examples

Basic usage:

let mut s = String::from("hello");

s.truncate(2);

assert_eq!("he", s);

pub fn pop(&mut self) -> Option<char>

Removes the last character from the string buffer and returns it.

Returns None if this String is empty.

Examples

Basic usage:

let mut s = String::from("foo");

assert_eq!(s.pop(), Some('o'));
assert_eq!(s.pop(), Some('o'));
assert_eq!(s.pop(), Some('f'));

assert_eq!(s.pop(), None);

pub fn remove(&mut self, idx: usize) -> char

Removes a char from this String at a byte position and returns it.

This is an O(n) operation, as it requires copying every element in the buffer.

Panics

Panics if idx is larger than or equal to the String’s length, or if it does not lie on a char boundary.

Examples

Basic usage:

let mut s = String::from("foo");

assert_eq!(s.remove(0), 'f');
assert_eq!(s.remove(1), 'o');
assert_eq!(s.remove(0), 'o');

pub fn retain<F>(&mut self, f: F) where
    F: FnMut(char) -> bool, 

Retains only the characters specified by the predicate.

In other words, remove all characters c such that f(c) returns false. This method operates in place, visiting each character exactly once in the original order, and preserves the order of the retained characters.

Examples

let mut s = String::from("f_o_ob_ar");

s.retain(|c| c != '_');

assert_eq!(s, "foobar");

Because the elements are visited exactly once in the original order, external state may be used to decide which elements to keep.

let mut s = String::from("abcde");
let keep = [false, true, true, false, true];
let mut iter = keep.iter();
s.retain(|_| *iter.next().unwrap());
assert_eq!(s, "bce");

pub unsafe fn as_mut_vec(&mut self) -> &mut Vec<u8>

Returns a mutable reference to the contents of this String.

Safety

This function is unsafe because the returned &mut Vec allows writing bytes which are not valid UTF-8. If this constraint is violated, using the original String after dropping the &mut Vec may violate memory safety, as the rest of the standard library assumes that Strings are valid UTF-8.

Examples

Basic usage:

let mut s = String::from("hello");

unsafe {
    let vec = s.as_mut_vec();
    assert_eq!(&[104, 101, 108, 108, 111][..], &vec[..]);

    vec.reverse();
}
assert_eq!(s, "olleh");

pub fn len(&self) -> usize

Returns the length of this String, in bytes, not chars or graphemes. In other words, it might not be what a human considers the length of the string.

Examples

Basic usage:

let a = String::from("foo");
assert_eq!(a.len(), 3);

let fancy_f = String::from("ƒoo");
assert_eq!(fancy_f.len(), 4);
assert_eq!(fancy_f.chars().count(), 3);

pub fn is_empty(&self) -> bool

Returns true if this String has a length of zero, and false otherwise.

Examples

Basic usage:

let mut v = String::new();
assert!(v.is_empty());

v.push('a');
assert!(!v.is_empty());

pub fn clear(&mut self)

Truncates this String, removing all contents.

While this means the String will have a length of zero, it does not touch its capacity.

Examples

Basic usage:

let mut s = String::from("foo");

s.clear();

assert!(s.is_empty());
assert_eq!(0, s.len());
assert_eq!(3, s.capacity());

pub fn drain<R>(&mut self, range: R) -> Drain<'_> where
    R: RangeBounds<usize>, 

Creates a draining iterator that removes the specified range in the String and yields the removed chars.

Note: The element range is removed even if the iterator is not consumed until the end.

Panics

Panics if the starting point or end point do not lie on a char boundary, or if they’re out of bounds.

Examples

Basic usage:

let mut s = String::from("α is alpha, β is beta");
let beta_offset = s.find('β').unwrap_or(s.len());

// Remove the range up until the β from the string
let t: String = s.drain(..beta_offset).collect();
assert_eq!(t, "α is alpha, ");
assert_eq!(s, "β is beta");

// A full range clears the string
s.drain(..);
assert_eq!(s, "");

Methods from Deref<Target = str>

pub fn try_to_owned(&self) -> Result<String, TryReserveError>

Tries to create a String.

Examples

Basic usage:

let s: &str = "a";
let ss: String = s.try_to_owned().unwrap();

Trait Implementations

impl AsMut<str> for String

fn as_mut(&mut self) -> &mut str

Performs the conversion.

impl AsRef<[u8]> for String

fn as_ref(&self) -> &[u8]

Performs the conversion.

impl AsRef<str> for String

fn as_ref(&self) -> &str

Performs the conversion.

impl Borrow<str> for String

fn borrow(&self) -> &str

Immutably borrows from an owned value.

impl BorrowMut<str> for String

fn borrow_mut(&mut self) -> &mut str

Mutably borrows from an owned value.

impl Debug for String

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Default for String

fn default() -> String

Creates an empty String.

impl Deref for String

type Target = str

The resulting type after dereferencing.

fn deref(&self) -> &str

Dereferences the value.

impl DerefMut for String

fn deref_mut(&mut self) -> &mut str

Mutably dereferences the value.

impl Display for String

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl From<Box<str, Global>> for String

fn from(s: Box<str>) -> String

Converts the given boxed str slice to a String. It is notable that the str slice is owned.

Examples

Basic usage:

let s1: String = String::from("hello world");
let s2: Box<str> = s1.into_boxed_str();
let s3: String = String::from(s2);

assert_eq!("hello world", s3)

impl From<String> for Vec<u8>

fn from(string: String) -> Vec<u8>

Converts the given String to a vector Vec that holds values of type u8.

Examples

Basic usage:

let s1 = String::from("hello world");
let v1 = Vec::from(s1);

for b in v1 {
    println!("{}", b);
}

impl Hash for String

fn hash<H: Hasher>(&self, hasher: &mut H)

Feeds this value into the given Hasher.

fn hash_slice<H>(data: &[Self], state: &mut H) where
    H: Hasher, 

Feeds a slice of this type into the given Hasher.

impl Index<Range<usize>> for String

type Output = str

The returned type after indexing.

fn index(&self, index: Range<usize>) -> &str

Performs the indexing (container[index]) operation.

impl Index<RangeFrom<usize>> for String

type Output = str

The returned type after indexing.

fn index(&self, index: RangeFrom<usize>) -> &str

Performs the indexing (container[index]) operation.

impl Index<RangeFull> for String

type Output = str

The returned type after indexing.

fn index(&self, _index: RangeFull) -> &str

Performs the indexing (container[index]) operation.

impl Index<RangeInclusive<usize>> for String

type Output = str

The returned type after indexing.

fn index(&self, index: RangeInclusive<usize>) -> &str

Performs the indexing (container[index]) operation.

impl Index<RangeTo<usize>> for String

type Output = str

The returned type after indexing.

fn index(&self, index: RangeTo<usize>) -> &str

Performs the indexing (container[index]) operation.

impl Index<RangeToInclusive<usize>> for String

type Output = str

The returned type after indexing.

fn index(&self, index: RangeToInclusive<usize>) -> &str

Performs the indexing (container[index]) operation.

impl IndexMut<Range<usize>> for String

fn index_mut(&mut self, index: Range<usize>) -> &mut str

Performs the mutable indexing (container[index]) operation.

impl IndexMut<RangeFrom<usize>> for String

fn index_mut(&mut self, index: RangeFrom<usize>) -> &mut str

Performs the mutable indexing (container[index]) operation.

impl IndexMut<RangeFull> for String

fn index_mut(&mut self, _index: RangeFull) -> &mut str

Performs the mutable indexing (container[index]) operation.

impl IndexMut<RangeInclusive<usize>> for String

fn index_mut(&mut self, index: RangeInclusive<usize>) -> &mut str

Performs the mutable indexing (container[index]) operation.

impl IndexMut<RangeTo<usize>> for String

fn index_mut(&mut self, index: RangeTo<usize>) -> &mut str

Performs the mutable indexing (container[index]) operation.

impl IndexMut<RangeToInclusive<usize>> for String

fn index_mut(&mut self, index: RangeToInclusive<usize>) -> &mut str

Performs the mutable indexing (container[index]) operation.

impl Ord for String

fn cmp(&self, other: &String) -> Ordering

This method returns an Ordering between self and other.

fn max(self, other: Self) -> Self

Compares and returns the maximum of two values.

fn min(self, other: Self) -> Self

Compares and returns the minimum of two values.

fn clamp(self, min: Self, max: Self) -> Self

Restrict a value to a certain interval.

impl<'a, 'b> PartialEq<&'a str> for String

fn eq(&self, other: &&'a str) -> bool

This method tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &&'a str) -> bool

This method tests for !=.

impl PartialEq<String> for String

fn eq(&self, other: &String) -> bool

This method tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &String) -> bool

This method tests for !=.

impl<'a, 'b> PartialEq<String> for str

fn eq(&self, other: &String) -> bool

This method tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &String) -> bool

This method tests for !=.

impl<'a, 'b> PartialEq<String> for &'a str

fn eq(&self, other: &String) -> bool

This method tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &String) -> bool

This method tests for !=.

impl<'a, 'b> PartialEq<str> for String

fn eq(&self, other: &str) -> bool

This method tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &str) -> bool

This method tests for !=.

impl PartialOrd<String> for String

fn partial_cmp(&self, other: &String) -> Option<Ordering>

This method returns an ordering between self and other values if one exists.

fn lt(&self, other: &Rhs) -> bool

This method tests less than (for self and other) and is used by the < operator.

fn le(&self, other: &Rhs) -> bool

This method tests less than or equal to (for self and other) and is used by the <= operator.

fn gt(&self, other: &Rhs) -> bool

This method tests greater than (for self and other) and is used by the > operator.

fn ge(&self, other: &Rhs) -> bool

This method tests greater than or equal to (for self and other) and is used by the >= operator.

impl<'a, 'b> Pattern<'a> for &'b String

A convenience impl that delegates to the impl for &str.

Examples

assert_eq!(String::from("Hello world").find("world"), Some(6));

type Searcher = <&'b str as Pattern<'a>>::Searcher

🔬 This is a nightly-only experimental API. (pattern #27721)

Associated searcher for this pattern

fn into_searcher(self, haystack: &'a str) -> <&'b str as Pattern<'a>>::Searcher

🔬 This is a nightly-only experimental API. (pattern #27721)

Constructs the associated searcher from self and the haystack to search in.

fn is_contained_in(self, haystack: &'a str) -> bool

🔬 This is a nightly-only experimental API. (pattern #27721)

Checks whether the pattern matches anywhere in the haystack

fn is_prefix_of(self, haystack: &'a str) -> bool

🔬 This is a nightly-only experimental API. (pattern #27721)

Checks whether the pattern matches at the front of the haystack

fn strip_prefix_of(self, haystack: &'a str) -> Option<&'a str>

🔬 This is a nightly-only experimental API. (pattern #27721)

Removes the pattern from the front of haystack, if it matches.

fn is_suffix_of(self, haystack: &'a str) -> bool

🔬 This is a nightly-only experimental API. (pattern #27721)

Checks whether the pattern matches at the back of the haystack

fn strip_suffix_of(self, haystack: &'a str) -> Option<&'a str>

🔬 This is a nightly-only experimental API. (pattern #27721)

Removes the pattern from the back of haystack, if it matches.

impl Eq for String

impl StructuralEq for String