Struct std::string::String1.0.0 [−] [src]

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

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 nineteen 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 because 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:

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:

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

Methods

`impl String`
[src]

`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().

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.

`fn from_utf8_lossy(v: &'a [u8]) -> Cow<'a, str>`

Converts a slice of bytes to a string, including invalid characters.

Strings are 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 strings, however: strings are required to be 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.

This function returns a Cow<'a, str>. If our byte slice is invalid UTF-8, then we need to insert the replacement characters, which will change the size of the string, and hence, require a String. But if it's already valid UTF-8, we don't need a new allocation. This return type allows us to handle both cases.

Examples

Basic usage:

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

let sparkle_heart = String::from_utf8_lossy(&sparkle_heart);

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 ptr needs to have been previously allocated by the same allocator the standard library uses.
length needs to be less than or equal to capacity.
capacity needs to be the correct value.

Violating these may cause problems like corrupting the allocator's internal datastructures.

The ownership of ptr 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");
    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`
1.7.0

Extracts a string slice containing the entire string.

`fn as_mut_str(&mut self) -> &mut str`
1.7.0

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.

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

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);

`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);

`fn insert_str(&mut self, idx: usize, string: &str)`

🔬 This is a nightly-only experimental API. (insert_str)

recent addition

Inserts a string slice 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:

#![feature(insert_str)]

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

s.insert_str(0, "foo");

assert_eq!("foobar", 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 split_off(&mut self, mid: usize) -> String`

🔬 This is a nightly-only experimental API. (string_split_off)

Divide one string into two at an index.

The argument, mid, should be a byte offset from the start of the string. It must also be on the boundary of a UTF-8 code point.

The two strings returned go from the start of the string to mid, and from mid to the end of the string.

Panics

Panics if mid is not on a UTF-8 code point boundary, or if it is beyond the last code point of the string.

Examples

let mut hello = String::from("Hello, World!");
let world = hello.split_off(7);
assert_eq!(hello, "Hello, ");
assert_eq!(world, "World!");

`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>`
1.6.0

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>`
1.4.0

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<'a> AddAssign<&'a str> for String`
1.12.0
[src]

`fn add_assign(&mut self, other: &str)`

The method for the += operator

`impl Default for String`
[src]

`fn default() -> String`

Creates an empty String.