Struct std::string::String
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pub struct String {
vec: Vec<u8>,
}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. Furtheremore, it's not clear what sort of
thing the index should return: a byte, a codepoint, or a grapheme cluster.
The as_bytes() and chars() methods return iterators over the first
two, respectively.
Deref
Strings implement Deref<Target=str>, and so inherit all of str's
methods. In addition, this means that you can pass a String to any
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.
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..."); let ptr = story.as_ptr(); let len = story.len(); let capacity = story.capacity(); // story has thirteen bytes assert_eq!(19, len); // Now that we have our parts, we throw the story away. mem::forget(story); // We can re-build a String out of ptr, len, and capacity. This is all // unsafe becuase we are responsible for making sure the components are // valid: let s = unsafe { String::from_raw_parts(ptr as *mut _, 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.
Fields
vec |
Methods
impl String
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, but 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();
fn with_capacity(capacity: usize) -> String
Creates a new empty String with a particular capacity.
Strings have an internal buffer to hold their data. The capacity is
the length of that buffer, and can be queried with the capacity()
method. This method creates an empty String, but one with an initial
buffer that can hold capacity bytes. This is useful when you may be
appending a bunch of data to the String, reducing the number of
reallocations it needs to do.
If the given capacity is 0, no allocation will occur, and this method
is identical to the new() method.
Examples
Basic usage:
let mut s = String::with_capacity(10); // The String contains no chars, even though it has capacity for more assert_eq!(s.len(), 0); // These are all done without reallocating... let cap = s.capacity(); for i in 0..10 { s.push('a'); } assert_eq!(s.capacity(), cap); // ...but this may make the vector reallocate s.push('a');
fn from_utf8(vec: Vec<u8>) -> Result<String, FromUtf8Error>
Converts a vector of bytes to a String.
A string slice (&str) 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().
Failure
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.
fn from_utf8_lossy(v: &'a [u8]) -> Cow<'a, str>
Converts a slice of bytes to a String, including invalid characters.
A string slice (&str) is made of bytes (u8), and a slice of
bytes (&[u8]) is made of bytes, so this function converts between
the two. Not all byte slices are valid string slices, however: &str
requires that it is valid UTF-8. During this conversion,
from_utf8_lossy() will replace any invalid UTF-8 sequences with
U+FFFD REPLACEMENT CHARACTER, which looks like this: �
If you are sure that the byte slice is valid UTF-8, and you don't want
to incur the overhead of the conversion, there is an unsafe version
of this function, from_utf8_unchecked(), which has the same behavior
but skips the checks.
If you need a &str instead of a String, consider
str::from_utf8().
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 let input = b"Hello \xF0\x90\x80World"; let output = String::from_utf8_lossy(input); assert_eq!("Hello �World", output);
fn from_utf16(v: &[u16]) -> Result<String, FromUtf16Error>
Decode a UTF-16 encoded vector v into a String, returning Err
if v contains any invalid data.
Examples
Basic usage:
// 𝄞music let v = &[0xD834, 0xDD1E, 0x006d, 0x0075, 0x0073, 0x0069, 0x0063]; assert_eq!(String::from("𝄞music"), String::from_utf16(v).unwrap()); // 𝄞mu<invalid>ic let v = &[0xD834, 0xDD1E, 0x006d, 0x0075, 0xD800, 0x0069, 0x0063]; assert!(String::from_utf16(v).is_err());
fn from_utf16_lossy(v: &[u16]) -> String
Decode a UTF-16 encoded vector v into a string, replacing
invalid data with the replacement character (U+FFFD).
Examples
Basic usage:
// 𝄞mus<invalid>ic<invalid> let v = &[0xD834, 0xDD1E, 0x006d, 0x0075, 0x0073, 0xDD1E, 0x0069, 0x0063, 0xD834]; assert_eq!(String::from("𝄞mus\u{FFFD}ic\u{FFFD}"), String::from_utf16_lossy(v));
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
ptrneeds to have been previously allocated by the same allocator the standard library uses. lengthneeds to be less than or equal tocapacity.capacityneeds to be the correct value.
Violating these may cause problems like corrupting the allocator's internal datastructures.
Examples
Basic usage:
use std::mem; unsafe { let s = String::from("hello"); let ptr = s.as_ptr(); let len = s.len(); let capacity = s.capacity(); mem::forget(s); let s = String::from_raw_parts(ptr as *mut _, len, capacity); assert_eq!(String::from("hello"), s); }
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);
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[..]);
fn as_str(&self) -> &str
Extracts a string slice containing the entire string.
fn as_mut_str(&mut self) -> &mut str
Extracts a string slice containing the entire string.
fn push_str(&mut self, string: &str)
Appends a given string slice onto the end of this String.
Examples
Basic usage:
let mut s = String::from("foo"); s.push_str("bar"); assert_eq!("foobar", s);
fn capacity(&self) -> usize
Returns this String's capacity, in bytes.
Examples
Basic usage:
let s = String::with_capacity(10); assert!(s.capacity() >= 10);
fn reserve(&mut self, additional: usize)
Ensures that this String's capacity is at least additional bytes
larger than its length.
The capacity may be increased by more than additional bytes if it
chooses, to prevent frequent reallocations.
If you do not want this "at least" behavior, see the reserve_exact()
method.
Panics
Panics if the new capacity overflows usize.
Examples
Basic usage:
let mut s = String::new(); s.reserve(10); assert!(s.capacity() >= 10);
This may not actually increase the capacity:
let mut s = String::with_capacity(10); s.push('a'); s.push('b'); // s now has a length of 2 and a capacity of 10 assert_eq!(2, s.len()); assert_eq!(10, s.capacity()); // Since we already have an extra 8 capacity, calling this... s.reserve(8); // ... doesn't actually increase. assert_eq!(10, s.capacity());
fn reserve_exact(&mut self, additional: usize)
Ensures that this String's capacity is additional bytes
larger than its length.
Consider using the reserve() method unless you absolutely know
better than the allocator.
Panics
Panics if the new capacity overflows usize.
Examples
Basic usage:
let mut s = String::new(); s.reserve_exact(10); assert!(s.capacity() >= 10);
This may not actually increase the capacity:
let mut s = String::with_capacity(10); s.push('a'); s.push('b'); // s now has a length of 2 and a capacity of 10 assert_eq!(2, s.len()); assert_eq!(10, s.capacity()); // Since we already have an extra 8 capacity, calling this... s.reserve_exact(8); // ... doesn't actually increase. assert_eq!(10, s.capacity());
fn shrink_to_fit(&mut self)
Shrinks the capacity of this String to match its length.
Examples
Basic usage:
let mut s = String::from("foo"); s.reserve(100); assert!(s.capacity() >= 100); s.shrink_to_fit(); assert_eq!(3, s.capacity());
fn push(&mut self, ch: char)
Appends the given char to the end of this String.
Examples
Basic usage:
let mut s = String::from("abc"); s.push('1'); s.push('2'); s.push('3'); assert_eq!("abc123", s);
fn as_bytes(&self) -> &[u8]
Returns a byte slice of this String's contents.
Examples
Basic usage:
let s = String::from("hello"); assert_eq!(&[104, 101, 108, 108, 111], s.as_bytes());
fn truncate(&mut self, new_len: usize)
Shortens this String to the specified length.
Panics
Panics if new_len > current length, or 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);
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);
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');
fn insert(&mut self, idx: usize, ch: char)
Inserts a character into this String at a byte position.
This is an O(n) operation as it requires copying every element in the
buffer.
Panics
Panics if idx is larger than the String's length, or if it does not
lie on a char boundary.
Examples
Basic usage:
let mut s = String::with_capacity(3); s.insert(0, 'f'); s.insert(1, 'o'); s.insert(2, 'o'); assert_eq!("foo", s);
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 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:
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");
fn len(&self) -> usize
Returns the length of this String, in bytes.
Examples
Basic usage:
let a = String::from("foo"); assert_eq!(a.len(), 3);
fn is_empty(&self) -> bool
Returns true if this String has a length of zero.
Returns false otherwise.
Examples
Basic usage:
let mut v = String::new(); assert!(v.is_empty()); v.push('a'); assert!(!v.is_empty());
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());
fn drain<R>(&mut self, range: R) -> Drain where R: RangeArgument<usize>
Create 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, "");
fn into_boxed_str(self) -> Box<str>
Converts this String into a Box<str>.
This will drop any excess capacity.
Examples
Basic usage:
let s = String::from("hello"); let b = s.into_boxed_str();
Trait Implementations
impl Borrow<str> for String
fn borrow(&self) -> &str
impl Clone for String
fn clone(&self) -> String
fn clone_from(&mut self, source: &String)
impl FromIterator<char> for String
fn from_iter<I>(iterable: I) -> String where I: IntoIterator<Item=char>
impl<'a> FromIterator<&'a str> for String
fn from_iter<I>(iterable: I) -> String where I: IntoIterator<Item=&'a str>
impl FromIterator<String> for String
fn from_iter<I>(iterable: I) -> String where I: IntoIterator<Item=String>
impl Extend<char> for String
fn extend<I>(&mut self, iterable: I) where I: IntoIterator<Item=char>
impl<'a> Extend<&'a char> for String
fn extend<I>(&mut self, iterable: I) where I: IntoIterator<Item=&'a char>
impl<'a> Extend<&'a str> for String
fn extend<I>(&mut self, iterable: I) where I: IntoIterator<Item=&'a str>
impl Extend<String> for String
fn extend<I>(&mut self, iterable: I) where I: IntoIterator<Item=String>
impl<'a, 'b> Pattern<'a> for &'b String
A convenience impl that delegates to the impl for &str