pub struct String { /* private fields */ }
Expand description
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!");
RunYou 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!");
RunIf 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);
RunUTF-8
String
s 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!!!
RunIndexing 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);
RunThis will create a &str
from the String
and pass it in. This
conversion is very inexpensive, and so generally, functions will accept
&str
s 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);
RunThere 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);
RunIf 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());
}
RunThis 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());
}
RunWe 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
sourceimpl String
impl String
const: 1.39.0 · sourcepub const fn new() -> 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();
Runsourcepub fn from_utf8(vec: Vec<u8>) -> Result<String, FromUtf8Error>
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 String
s, 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);
RunIncorrect bytes:
// some invalid bytes, in a vector
let sparkle_heart = vec![0, 159, 146, 150];
assert!(String::from_utf8(sparkle_heart).is_err());
RunSee the docs for FromUtf8Error
for more details on what you can do
with this error.
sourcepub fn into_raw_parts(self) -> (*mut u8, usize, usize)
pub fn into_raw_parts(self) -> (*mut u8, usize, usize)
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");
Runsourcepub unsafe fn from_raw_parts(
buf: *mut u8,
length: usize,
capacity: usize
) -> String
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 tocapacity
.capacity
needs to be the correct value.- The first
length
bytes atbuf
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);
}
Runsourcepub unsafe fn from_utf8_unchecked(bytes: Vec<u8>) -> String
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 String
s 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);
Runsourcepub fn into_bytes(self) -> Vec<u8>
pub fn into_bytes(self) -> Vec<u8>
1.7.0 · sourcepub fn as_mut_str(&mut self) -> &mut str
pub fn as_mut_str(&mut self) -> &mut str
1.57.0 · sourcepub fn try_reserve(&mut self, additional: usize) -> Result<(), TryReserveError>
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)
}
Run1.57.0 · sourcepub fn try_reserve_exact(
&mut self,
additional: usize
) -> Result<(), TryReserveError>
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 try_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_exact(data.len())?;
// Now we know this can't OOM in the middle of our complex work
output.push_str(data);
Ok(output)
}
Runsourcepub fn truncate(&mut self, new_len: usize)
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);
Runsourcepub fn remove(&mut self, idx: usize) -> char
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');
Run1.26.0 · sourcepub fn retain<F>(&mut self, f: F) where
F: FnMut(char) -> bool,
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");
RunBecause 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");
Runsourcepub unsafe fn as_mut_vec(&mut self) -> &mut Vec<u8>
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 String
s 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");
Runsourcepub fn len(&self) -> usize
pub fn len(&self) -> usize
Returns the length of this String
, in bytes, not char
s 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);
Run1.6.0 · sourcepub fn drain<R>(&mut self, range: R) -> Drain<'_>ⓘNotable traits for Drain<'_>impl Iterator for Drain<'_> type Item = char;
where
R: RangeBounds<usize>,
pub fn drain<R>(&mut self, range: R) -> Drain<'_>ⓘNotable traits for Drain<'_>impl Iterator for Drain<'_> type Item = char;
where
R: RangeBounds<usize>,
Removes the specified range from the string in bulk, returning all removed characters as an iterator.
The returned iterator keeps a mutable borrow on the string to optimize its implementation.
Panics
Panics if the starting point or end point do not lie on a char
boundary, or if they’re out of bounds.
Leaking
If the returned iterator goes out of scope without being dropped (due to
core::mem::forget
, for example), the string may still contain a copy
of any drained characters, or may have lost characters arbitrarily,
including characters outside the range.
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, like `clear()` does
s.drain(..);
assert_eq!(s, "");
RunMethods from Deref<Target = str>
sourcepub fn try_to_owned(&self) -> Result<String, TryReserveError>
pub fn try_to_owned(&self) -> Result<String, TryReserveError>
Trait Implementations
1.36.0 · sourceimpl BorrowMut<str> for String
impl BorrowMut<str> for String
sourcefn borrow_mut(&mut self) -> &mut str
fn borrow_mut(&mut self) -> &mut str
Mutably borrows from an owned value. Read more
1.26.0 · sourceimpl Index<RangeInclusive<usize>> for String
impl Index<RangeInclusive<usize>> for String
1.26.0 · sourceimpl Index<RangeToInclusive<usize>> for String
impl Index<RangeToInclusive<usize>> for String
1.26.0 · sourceimpl IndexMut<RangeInclusive<usize>> for String
impl IndexMut<RangeInclusive<usize>> for String
1.26.0 · sourceimpl IndexMut<RangeToInclusive<usize>> for String
impl IndexMut<RangeToInclusive<usize>> for String
sourceimpl Ord for String
impl Ord for String
sourceimpl PartialOrd<String> for String
impl PartialOrd<String> for String
sourcefn partial_cmp(&self, other: &String) -> Option<Ordering>
fn partial_cmp(&self, other: &String) -> Option<Ordering>
This method returns an ordering between self
and other
values if one exists. Read more
sourcefn lt(&self, other: &Rhs) -> bool
fn lt(&self, other: &Rhs) -> bool
This method tests less than (for self
and other
) and is used by the <
operator. Read more
sourcefn le(&self, other: &Rhs) -> bool
fn le(&self, other: &Rhs) -> bool
This method tests less than or equal to (for self
and other
) and is used by the <=
operator. Read more
sourceimpl<'a, 'b> Pattern<'a> for &'b String
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));
Runsourcefn into_searcher(self, haystack: &'a str) -> <&'b str as Pattern<'a>>::Searcher
fn into_searcher(self, haystack: &'a str) -> <&'b str as Pattern<'a>>::Searcher
Constructs the associated searcher from
self
and the haystack
to search in. Read more
sourcefn is_contained_in(self, haystack: &'a str) -> bool
fn is_contained_in(self, haystack: &'a str) -> bool
Checks whether the pattern matches anywhere in the haystack
sourcefn is_prefix_of(self, haystack: &'a str) -> bool
fn is_prefix_of(self, haystack: &'a str) -> bool
Checks whether the pattern matches at the front of the haystack
sourcefn strip_prefix_of(self, haystack: &'a str) -> Option<&'a str>
fn strip_prefix_of(self, haystack: &'a str) -> Option<&'a str>
Removes the pattern from the front of haystack, if it matches.
sourcefn is_suffix_of(self, haystack: &'a str) -> bool
fn is_suffix_of(self, haystack: &'a str) -> bool
Checks whether the pattern matches at the back of the haystack
impl Eq for String
impl StructuralEq for String
Auto Trait Implementations
impl RefUnwindSafe for String
impl Send for String
impl Sync for String
impl Unpin for String
impl UnwindSafe for String
Blanket Implementations
sourceimpl<T> BorrowMut<T> for T where
T: ?Sized,
impl<T> BorrowMut<T> for T where
T: ?Sized,
const: unstable · sourcepub fn borrow_mut(&mut self) -> &mut T
pub fn borrow_mut(&mut self) -> &mut T
Mutably borrows from an owned value. Read more