1 //! A UTF-8–encoded, growable string.
3 //! This module contains the [`String`] type, the [`ToString`] trait for
4 //! converting to strings, and several error types that may result from
5 //! working with [`String`]s.
9 //! There are multiple ways to create a new [`String`] from a string literal:
12 //! let s = "Hello".to_string();
14 //! let s = String::from("world");
15 //! let s: String = "also this".into();
18 //! You can create a new [`String`] from an existing one by concatenating with
22 //! let s = "Hello".to_string();
24 //! let message = s + " world!";
27 //! If you have a vector of valid UTF-8 bytes, you can make a [`String`] out of
28 //! it. You can do the reverse too.
31 //! let sparkle_heart = vec![240, 159, 146, 150];
33 //! // We know these bytes are valid, so we'll use `unwrap()`.
34 //! let sparkle_heart = String::from_utf8(sparkle_heart).unwrap();
36 //! assert_eq!("💖", sparkle_heart);
38 //! let bytes = sparkle_heart.into_bytes();
40 //! assert_eq!(bytes, [240, 159, 146, 150]);
43 #![stable(feature = "rust1", since = "1.0.0")]
45 use core::char::{decode_utf16, REPLACEMENT_CHARACTER};
48 use core::iter::{FromIterator, FusedIterator};
49 use core::ops::Bound::{Excluded, Included, Unbounded};
50 use core::ops::{self, Add, AddAssign, Index, IndexMut, Range, RangeBounds};
52 use core::str::{lossy, pattern::Pattern};
54 use crate::borrow::{Cow, ToOwned};
55 use crate::boxed::Box;
56 use crate::collections::TryReserveError;
57 use crate::str::{self, from_boxed_utf8_unchecked, Chars, FromStr, Utf8Error};
60 /// A UTF-8–encoded, growable string.
62 /// The `String` type is the most common string type that has ownership over the
63 /// contents of the string. It has a close relationship with its borrowed
64 /// counterpart, the primitive [`str`].
68 /// You can create a `String` from [a literal string][`str`] with [`String::from`]:
70 /// [`String::from`]: From::from
73 /// let hello = String::from("Hello, world!");
76 /// You can append a [`char`] to a `String` with the [`push`] method, and
77 /// append a [`&str`] with the [`push_str`] method:
80 /// let mut hello = String::from("Hello, ");
83 /// hello.push_str("orld!");
86 /// [`push`]: String::push
87 /// [`push_str`]: String::push_str
89 /// If you have a vector of UTF-8 bytes, you can create a `String` from it with
90 /// the [`from_utf8`] method:
93 /// // some bytes, in a vector
94 /// let sparkle_heart = vec![240, 159, 146, 150];
96 /// // We know these bytes are valid, so we'll use `unwrap()`.
97 /// let sparkle_heart = String::from_utf8(sparkle_heart).unwrap();
99 /// assert_eq!("💖", sparkle_heart);
102 /// [`from_utf8`]: String::from_utf8
106 /// `String`s are always valid UTF-8. This has a few implications, the first of
107 /// which is that if you need a non-UTF-8 string, consider [`OsString`]. It is
108 /// similar, but without the UTF-8 constraint. The second implication is that
109 /// you cannot index into a `String`:
111 /// ```compile_fail,E0277
114 /// println!("The first letter of s is {}", s[0]); // ERROR!!!
117 /// [`OsString`]: ../../std/ffi/struct.OsString.html
119 /// Indexing is intended to be a constant-time operation, but UTF-8 encoding
120 /// does not allow us to do this. Furthermore, it's not clear what sort of
121 /// thing the index should return: a byte, a codepoint, or a grapheme cluster.
122 /// The [`bytes`] and [`chars`] methods return iterators over the first
123 /// two, respectively.
125 /// [`bytes`]: str::bytes
126 /// [`chars`]: str::chars
130 /// `String`s implement [`Deref`]`<Target=str>`, and so inherit all of [`str`]'s
131 /// methods. In addition, this means that you can pass a `String` to a
132 /// function which takes a [`&str`] by using an ampersand (`&`):
135 /// fn takes_str(s: &str) { }
137 /// let s = String::from("Hello");
142 /// This will create a [`&str`] from the `String` and pass it in. This
143 /// conversion is very inexpensive, and so generally, functions will accept
144 /// [`&str`]s as arguments unless they need a `String` for some specific
147 /// In certain cases Rust doesn't have enough information to make this
148 /// conversion, known as [`Deref`] coercion. In the following example a string
149 /// slice [`&'a str`][`&str`] implements the trait `TraitExample`, and the function
150 /// `example_func` takes anything that implements the trait. In this case Rust
151 /// would need to make two implicit conversions, which Rust doesn't have the
152 /// means to do. For that reason, the following example will not compile.
154 /// ```compile_fail,E0277
155 /// trait TraitExample {}
157 /// impl<'a> TraitExample for &'a str {}
159 /// fn example_func<A: TraitExample>(example_arg: A) {}
161 /// let example_string = String::from("example_string");
162 /// example_func(&example_string);
165 /// There are two options that would work instead. The first would be to
166 /// change the line `example_func(&example_string);` to
167 /// `example_func(example_string.as_str());`, using the method [`as_str()`]
168 /// to explicitly extract the string slice containing the string. The second
169 /// way changes `example_func(&example_string);` to
170 /// `example_func(&*example_string);`. In this case we are dereferencing a
171 /// `String` to a [`str`][`&str`], then referencing the [`str`][`&str`] back to
172 /// [`&str`]. The second way is more idiomatic, however both work to do the
173 /// conversion explicitly rather than relying on the implicit conversion.
177 /// A `String` is made up of three components: a pointer to some bytes, a
178 /// length, and a capacity. The pointer points to an internal buffer `String`
179 /// uses to store its data. The length is the number of bytes currently stored
180 /// in the buffer, and the capacity is the size of the buffer in bytes. As such,
181 /// the length will always be less than or equal to the capacity.
183 /// This buffer is always stored on the heap.
185 /// You can look at these with the [`as_ptr`], [`len`], and [`capacity`]
191 /// let story = String::from("Once upon a time...");
193 // FIXME Update this when vec_into_raw_parts is stabilized
194 /// // Prevent automatically dropping the String's data
195 /// let mut story = mem::ManuallyDrop::new(story);
197 /// let ptr = story.as_mut_ptr();
198 /// let len = story.len();
199 /// let capacity = story.capacity();
201 /// // story has nineteen bytes
202 /// assert_eq!(19, len);
204 /// // We can re-build a String out of ptr, len, and capacity. This is all
205 /// // unsafe because we are responsible for making sure the components are
207 /// let s = unsafe { String::from_raw_parts(ptr, len, capacity) } ;
209 /// assert_eq!(String::from("Once upon a time..."), s);
212 /// [`as_ptr`]: str::as_ptr
213 /// [`len`]: String::len
214 /// [`capacity`]: String::capacity
216 /// If a `String` has enough capacity, adding elements to it will not
217 /// re-allocate. For example, consider this program:
220 /// let mut s = String::new();
222 /// println!("{}", s.capacity());
225 /// s.push_str("hello");
226 /// println!("{}", s.capacity());
230 /// This will output the following:
241 /// At first, we have no memory allocated at all, but as we append to the
242 /// string, it increases its capacity appropriately. If we instead use the
243 /// [`with_capacity`] method to allocate the correct capacity initially:
246 /// let mut s = String::with_capacity(25);
248 /// println!("{}", s.capacity());
251 /// s.push_str("hello");
252 /// println!("{}", s.capacity());
256 /// [`with_capacity`]: String::with_capacity
258 /// We end up with a different output:
269 /// Here, there's no need to allocate more memory inside the loop.
271 /// [`str`]: prim@str
272 /// [`&str`]: prim@str
273 /// [`Deref`]: core::ops::Deref
274 /// [`as_str()`]: String::as_str
275 #[derive(PartialOrd, Eq, Ord)]
276 #[cfg_attr(not(test), rustc_diagnostic_item = "string_type")]
277 #[stable(feature = "rust1", since = "1.0.0")]
282 /// A possible error value when converting a `String` from a UTF-8 byte vector.
284 /// This type is the error type for the [`from_utf8`] method on [`String`]. It
285 /// is designed in such a way to carefully avoid reallocations: the
286 /// [`into_bytes`] method will give back the byte vector that was used in the
287 /// conversion attempt.
289 /// [`from_utf8`]: String::from_utf8
290 /// [`into_bytes`]: FromUtf8Error::into_bytes
292 /// The [`Utf8Error`] type provided by [`std::str`] represents an error that may
293 /// occur when converting a slice of [`u8`]s to a [`&str`]. In this sense, it's
294 /// an analogue to `FromUtf8Error`, and you can get one from a `FromUtf8Error`
295 /// through the [`utf8_error`] method.
297 /// [`Utf8Error`]: core::str::Utf8Error
298 /// [`std::str`]: core::str
299 /// [`&str`]: prim@str
300 /// [`utf8_error`]: Self::utf8_error
307 /// // some invalid bytes, in a vector
308 /// let bytes = vec![0, 159];
310 /// let value = String::from_utf8(bytes);
312 /// assert!(value.is_err());
313 /// assert_eq!(vec![0, 159], value.unwrap_err().into_bytes());
315 #[stable(feature = "rust1", since = "1.0.0")]
316 #[derive(Debug, Clone, PartialEq, Eq)]
317 pub struct FromUtf8Error {
322 /// A possible error value when converting a `String` from a UTF-16 byte slice.
324 /// This type is the error type for the [`from_utf16`] method on [`String`].
326 /// [`from_utf16`]: String::from_utf16
332 /// // 𝄞mu<invalid>ic
333 /// let v = &[0xD834, 0xDD1E, 0x006d, 0x0075,
334 /// 0xD800, 0x0069, 0x0063];
336 /// assert!(String::from_utf16(v).is_err());
338 #[stable(feature = "rust1", since = "1.0.0")]
340 pub struct FromUtf16Error(());
343 /// Creates a new empty `String`.
345 /// Given that the `String` is empty, this will not allocate any initial
346 /// buffer. While that means that this initial operation is very
347 /// inexpensive, it may cause excessive allocation later when you add
348 /// data. If you have an idea of how much data the `String` will hold,
349 /// consider the [`with_capacity`] method to prevent excessive
352 /// [`with_capacity`]: String::with_capacity
359 /// let s = String::new();
362 #[rustc_const_stable(feature = "const_string_new", since = "1.32.0")]
363 #[stable(feature = "rust1", since = "1.0.0")]
364 pub const fn new() -> String {
365 String { vec: Vec::new() }
368 /// Creates a new empty `String` with a particular capacity.
370 /// `String`s have an internal buffer to hold their data. The capacity is
371 /// the length of that buffer, and can be queried with the [`capacity`]
372 /// method. This method creates an empty `String`, but one with an initial
373 /// buffer that can hold `capacity` bytes. This is useful when you may be
374 /// appending a bunch of data to the `String`, reducing the number of
375 /// reallocations it needs to do.
377 /// [`capacity`]: String::capacity
379 /// If the given capacity is `0`, no allocation will occur, and this method
380 /// is identical to the [`new`] method.
382 /// [`new`]: String::new
389 /// let mut s = String::with_capacity(10);
391 /// // The String contains no chars, even though it has capacity for more
392 /// assert_eq!(s.len(), 0);
394 /// // These are all done without reallocating...
395 /// let cap = s.capacity();
400 /// assert_eq!(s.capacity(), cap);
402 /// // ...but this may make the string reallocate
406 #[doc(alias = "alloc")]
407 #[doc(alias = "malloc")]
408 #[stable(feature = "rust1", since = "1.0.0")]
409 pub fn with_capacity(capacity: usize) -> String {
410 String { vec: Vec::with_capacity(capacity) }
413 // HACK(japaric): with cfg(test) the inherent `[T]::to_vec` method, which is
414 // required for this method definition, is not available. Since we don't
415 // require this method for testing purposes, I'll just stub it
416 // NB see the slice::hack module in slice.rs for more information
419 pub fn from_str(_: &str) -> String {
420 panic!("not available with cfg(test)");
423 /// Converts a vector of bytes to a `String`.
425 /// A string ([`String`]) is made of bytes ([`u8`]), and a vector of bytes
426 /// ([`Vec<u8>`]) is made of bytes, so this function converts between the
427 /// two. Not all byte slices are valid `String`s, however: `String`
428 /// requires that it is valid UTF-8. `from_utf8()` checks to ensure that
429 /// the bytes are valid UTF-8, and then does the conversion.
431 /// If you are sure that the byte slice is valid UTF-8, and you don't want
432 /// to incur the overhead of the validity check, there is an unsafe version
433 /// of this function, [`from_utf8_unchecked`], which has the same behavior
434 /// but skips the check.
436 /// This method will take care to not copy the vector, for efficiency's
439 /// If you need a [`&str`] instead of a `String`, consider
440 /// [`str::from_utf8`].
442 /// The inverse of this method is [`into_bytes`].
446 /// Returns [`Err`] if the slice is not UTF-8 with a description as to why the
447 /// provided bytes are not UTF-8. The vector you moved in is also included.
454 /// // some bytes, in a vector
455 /// let sparkle_heart = vec![240, 159, 146, 150];
457 /// // We know these bytes are valid, so we'll use `unwrap()`.
458 /// let sparkle_heart = String::from_utf8(sparkle_heart).unwrap();
460 /// assert_eq!("💖", sparkle_heart);
466 /// // some invalid bytes, in a vector
467 /// let sparkle_heart = vec![0, 159, 146, 150];
469 /// assert!(String::from_utf8(sparkle_heart).is_err());
472 /// See the docs for [`FromUtf8Error`] for more details on what you can do
475 /// [`from_utf8_unchecked`]: String::from_utf8_unchecked
476 /// [`Vec<u8>`]: crate::vec::Vec
477 /// [`&str`]: prim@str
478 /// [`into_bytes`]: String::into_bytes
480 #[stable(feature = "rust1", since = "1.0.0")]
481 pub fn from_utf8(vec: Vec<u8>) -> Result<String, FromUtf8Error> {
482 match str::from_utf8(&vec) {
483 Ok(..) => Ok(String { vec }),
484 Err(e) => Err(FromUtf8Error { bytes: vec, error: e }),
488 /// Converts a slice of bytes to a string, including invalid characters.
490 /// Strings are made of bytes ([`u8`]), and a slice of bytes
491 /// ([`&[u8]`][byteslice]) is made of bytes, so this function converts
492 /// between the two. Not all byte slices are valid strings, however: strings
493 /// are required to be valid UTF-8. During this conversion,
494 /// `from_utf8_lossy()` will replace any invalid UTF-8 sequences with
495 /// [`U+FFFD REPLACEMENT CHARACTER`][U+FFFD], which looks like this: �
497 /// [byteslice]: ../../std/primitive.slice.html
498 /// [U+FFFD]: core::char::REPLACEMENT_CHARACTER
500 /// If you are sure that the byte slice is valid UTF-8, and you don't want
501 /// to incur the overhead of the conversion, there is an unsafe version
502 /// of this function, [`from_utf8_unchecked`], which has the same behavior
503 /// but skips the checks.
505 /// [`from_utf8_unchecked`]: String::from_utf8_unchecked
507 /// This function returns a [`Cow<'a, str>`]. If our byte slice is invalid
508 /// UTF-8, then we need to insert the replacement characters, which will
509 /// change the size of the string, and hence, require a `String`. But if
510 /// it's already valid UTF-8, we don't need a new allocation. This return
511 /// type allows us to handle both cases.
513 /// [`Cow<'a, str>`]: crate::borrow::Cow
520 /// // some bytes, in a vector
521 /// let sparkle_heart = vec![240, 159, 146, 150];
523 /// let sparkle_heart = String::from_utf8_lossy(&sparkle_heart);
525 /// assert_eq!("💖", sparkle_heart);
531 /// // some invalid bytes
532 /// let input = b"Hello \xF0\x90\x80World";
533 /// let output = String::from_utf8_lossy(input);
535 /// assert_eq!("Hello �World", output);
537 #[stable(feature = "rust1", since = "1.0.0")]
538 pub fn from_utf8_lossy(v: &[u8]) -> Cow<'_, str> {
539 let mut iter = lossy::Utf8Lossy::from_bytes(v).chunks();
541 let (first_valid, first_broken) = if let Some(chunk) = iter.next() {
542 let lossy::Utf8LossyChunk { valid, broken } = chunk;
543 if valid.len() == v.len() {
544 debug_assert!(broken.is_empty());
545 return Cow::Borrowed(valid);
549 return Cow::Borrowed("");
552 const REPLACEMENT: &str = "\u{FFFD}";
554 let mut res = String::with_capacity(v.len());
555 res.push_str(first_valid);
556 if !first_broken.is_empty() {
557 res.push_str(REPLACEMENT);
560 for lossy::Utf8LossyChunk { valid, broken } in iter {
562 if !broken.is_empty() {
563 res.push_str(REPLACEMENT);
570 /// Decode a UTF-16–encoded vector `v` into a `String`, returning [`Err`]
571 /// if `v` contains any invalid data.
579 /// let v = &[0xD834, 0xDD1E, 0x006d, 0x0075,
580 /// 0x0073, 0x0069, 0x0063];
581 /// assert_eq!(String::from("𝄞music"),
582 /// String::from_utf16(v).unwrap());
584 /// // 𝄞mu<invalid>ic
585 /// let v = &[0xD834, 0xDD1E, 0x006d, 0x0075,
586 /// 0xD800, 0x0069, 0x0063];
587 /// assert!(String::from_utf16(v).is_err());
589 #[stable(feature = "rust1", since = "1.0.0")]
590 pub fn from_utf16(v: &[u16]) -> Result<String, FromUtf16Error> {
591 // This isn't done via collect::<Result<_, _>>() for performance reasons.
592 // FIXME: the function can be simplified again when #48994 is closed.
593 let mut ret = String::with_capacity(v.len());
594 for c in decode_utf16(v.iter().cloned()) {
598 return Err(FromUtf16Error(()));
604 /// Decode a UTF-16–encoded slice `v` into a `String`, replacing
605 /// invalid data with [the replacement character (`U+FFFD`)][U+FFFD].
607 /// Unlike [`from_utf8_lossy`] which returns a [`Cow<'a, str>`],
608 /// `from_utf16_lossy` returns a `String` since the UTF-16 to UTF-8
609 /// conversion requires a memory allocation.
611 /// [`from_utf8_lossy`]: String::from_utf8_lossy
612 /// [`Cow<'a, str>`]: crate::borrow::Cow
613 /// [U+FFFD]: core::char::REPLACEMENT_CHARACTER
620 /// // 𝄞mus<invalid>ic<invalid>
621 /// let v = &[0xD834, 0xDD1E, 0x006d, 0x0075,
622 /// 0x0073, 0xDD1E, 0x0069, 0x0063,
625 /// assert_eq!(String::from("𝄞mus\u{FFFD}ic\u{FFFD}"),
626 /// String::from_utf16_lossy(v));
629 #[stable(feature = "rust1", since = "1.0.0")]
630 pub fn from_utf16_lossy(v: &[u16]) -> String {
631 decode_utf16(v.iter().cloned()).map(|r| r.unwrap_or(REPLACEMENT_CHARACTER)).collect()
634 /// Decomposes a `String` into its raw components.
636 /// Returns the raw pointer to the underlying data, the length of
637 /// the string (in bytes), and the allocated capacity of the data
638 /// (in bytes). These are the same arguments in the same order as
639 /// the arguments to [`from_raw_parts`].
641 /// After calling this function, the caller is responsible for the
642 /// memory previously managed by the `String`. The only way to do
643 /// this is to convert the raw pointer, length, and capacity back
644 /// into a `String` with the [`from_raw_parts`] function, allowing
645 /// the destructor to perform the cleanup.
647 /// [`from_raw_parts`]: String::from_raw_parts
652 /// #![feature(vec_into_raw_parts)]
653 /// let s = String::from("hello");
655 /// let (ptr, len, cap) = s.into_raw_parts();
657 /// let rebuilt = unsafe { String::from_raw_parts(ptr, len, cap) };
658 /// assert_eq!(rebuilt, "hello");
660 #[unstable(feature = "vec_into_raw_parts", reason = "new API", issue = "65816")]
661 pub fn into_raw_parts(self) -> (*mut u8, usize, usize) {
662 self.vec.into_raw_parts()
665 /// Creates a new `String` from a length, capacity, and pointer.
669 /// This is highly unsafe, due to the number of invariants that aren't
672 /// * The memory at `buf` needs to have been previously allocated by the
673 /// same allocator the standard library uses, with a required alignment of exactly 1.
674 /// * `length` needs to be less than or equal to `capacity`.
675 /// * `capacity` needs to be the correct value.
676 /// * The first `length` bytes at `buf` need to be valid UTF-8.
678 /// Violating these may cause problems like corrupting the allocator's
679 /// internal data structures.
681 /// The ownership of `buf` is effectively transferred to the
682 /// `String` which may then deallocate, reallocate or change the
683 /// contents of memory pointed to by the pointer at will. Ensure
684 /// that nothing else uses the pointer after calling this
695 /// let s = String::from("hello");
697 // FIXME Update this when vec_into_raw_parts is stabilized
698 /// // Prevent automatically dropping the String's data
699 /// let mut s = mem::ManuallyDrop::new(s);
701 /// let ptr = s.as_mut_ptr();
702 /// let len = s.len();
703 /// let capacity = s.capacity();
705 /// let s = String::from_raw_parts(ptr, len, capacity);
707 /// assert_eq!(String::from("hello"), s);
711 #[stable(feature = "rust1", since = "1.0.0")]
712 pub unsafe fn from_raw_parts(buf: *mut u8, length: usize, capacity: usize) -> String {
713 unsafe { String { vec: Vec::from_raw_parts(buf, length, capacity) } }
716 /// Converts a vector of bytes to a `String` without checking that the
717 /// string contains valid UTF-8.
719 /// See the safe version, [`from_utf8`], for more details.
721 /// [`from_utf8`]: String::from_utf8
725 /// This function is unsafe because it does not check that the bytes passed
726 /// to it are valid UTF-8. If this constraint is violated, it may cause
727 /// memory unsafety issues with future users of the `String`, as the rest of
728 /// the standard library assumes that `String`s are valid UTF-8.
735 /// // some bytes, in a vector
736 /// let sparkle_heart = vec![240, 159, 146, 150];
738 /// let sparkle_heart = unsafe {
739 /// String::from_utf8_unchecked(sparkle_heart)
742 /// assert_eq!("💖", sparkle_heart);
745 #[stable(feature = "rust1", since = "1.0.0")]
746 pub unsafe fn from_utf8_unchecked(bytes: Vec<u8>) -> String {
747 String { vec: bytes }
750 /// Converts a `String` into a byte vector.
752 /// This consumes the `String`, so we do not need to copy its contents.
759 /// let s = String::from("hello");
760 /// let bytes = s.into_bytes();
762 /// assert_eq!(&[104, 101, 108, 108, 111][..], &bytes[..]);
765 #[stable(feature = "rust1", since = "1.0.0")]
766 pub fn into_bytes(self) -> Vec<u8> {
770 /// Extracts a string slice containing the entire `String`.
777 /// let s = String::from("foo");
779 /// assert_eq!("foo", s.as_str());
782 #[stable(feature = "string_as_str", since = "1.7.0")]
783 pub fn as_str(&self) -> &str {
787 /// Converts a `String` into a mutable string slice.
794 /// let mut s = String::from("foobar");
795 /// let s_mut_str = s.as_mut_str();
797 /// s_mut_str.make_ascii_uppercase();
799 /// assert_eq!("FOOBAR", s_mut_str);
802 #[stable(feature = "string_as_str", since = "1.7.0")]
803 pub fn as_mut_str(&mut self) -> &mut str {
807 /// Appends a given string slice onto the end of this `String`.
814 /// let mut s = String::from("foo");
816 /// s.push_str("bar");
818 /// assert_eq!("foobar", s);
821 #[stable(feature = "rust1", since = "1.0.0")]
822 pub fn push_str(&mut self, string: &str) {
823 self.vec.extend_from_slice(string.as_bytes())
826 /// Returns this `String`'s capacity, in bytes.
833 /// let s = String::with_capacity(10);
835 /// assert!(s.capacity() >= 10);
838 #[stable(feature = "rust1", since = "1.0.0")]
839 pub fn capacity(&self) -> usize {
843 /// Ensures that this `String`'s capacity is at least `additional` bytes
844 /// larger than its length.
846 /// The capacity may be increased by more than `additional` bytes if it
847 /// chooses, to prevent frequent reallocations.
849 /// If you do not want this "at least" behavior, see the [`reserve_exact`]
854 /// Panics if the new capacity overflows [`usize`].
856 /// [`reserve_exact`]: String::reserve_exact
863 /// let mut s = String::new();
867 /// assert!(s.capacity() >= 10);
870 /// This may not actually increase the capacity:
873 /// let mut s = String::with_capacity(10);
877 /// // s now has a length of 2 and a capacity of 10
878 /// assert_eq!(2, s.len());
879 /// assert_eq!(10, s.capacity());
881 /// // Since we already have an extra 8 capacity, calling this...
884 /// // ... doesn't actually increase.
885 /// assert_eq!(10, s.capacity());
888 #[stable(feature = "rust1", since = "1.0.0")]
889 pub fn reserve(&mut self, additional: usize) {
890 self.vec.reserve(additional)
893 /// Ensures that this `String`'s capacity is `additional` bytes
894 /// larger than its length.
896 /// Consider using the [`reserve`] method unless you absolutely know
897 /// better than the allocator.
899 /// [`reserve`]: String::reserve
903 /// Panics if the new capacity overflows `usize`.
910 /// let mut s = String::new();
912 /// s.reserve_exact(10);
914 /// assert!(s.capacity() >= 10);
917 /// This may not actually increase the capacity:
920 /// let mut s = String::with_capacity(10);
924 /// // s now has a length of 2 and a capacity of 10
925 /// assert_eq!(2, s.len());
926 /// assert_eq!(10, s.capacity());
928 /// // Since we already have an extra 8 capacity, calling this...
929 /// s.reserve_exact(8);
931 /// // ... doesn't actually increase.
932 /// assert_eq!(10, s.capacity());
935 #[stable(feature = "rust1", since = "1.0.0")]
936 pub fn reserve_exact(&mut self, additional: usize) {
937 self.vec.reserve_exact(additional)
940 /// Tries to reserve capacity for at least `additional` more elements to be inserted
941 /// in the given `String`. The collection may reserve more space to avoid
942 /// frequent reallocations. After calling `reserve`, capacity will be
943 /// greater than or equal to `self.len() + additional`. Does nothing if
944 /// capacity is already sufficient.
948 /// If the capacity overflows, or the allocator reports a failure, then an error
954 /// #![feature(try_reserve)]
955 /// use std::collections::TryReserveError;
957 /// fn process_data(data: &str) -> Result<String, TryReserveError> {
958 /// let mut output = String::new();
960 /// // Pre-reserve the memory, exiting if we can't
961 /// output.try_reserve(data.len())?;
963 /// // Now we know this can't OOM in the middle of our complex work
964 /// output.push_str(data);
968 /// # process_data("rust").expect("why is the test harness OOMing on 4 bytes?");
970 #[unstable(feature = "try_reserve", reason = "new API", issue = "48043")]
971 pub fn try_reserve(&mut self, additional: usize) -> Result<(), TryReserveError> {
972 self.vec.try_reserve(additional)
975 /// Tries to reserves the minimum capacity for exactly `additional` more elements to
976 /// be inserted in the given `String`. After calling `reserve_exact`,
977 /// capacity will be greater than or equal to `self.len() + additional`.
978 /// Does nothing if the capacity is already sufficient.
980 /// Note that the allocator may give the collection more space than it
981 /// requests. Therefore, capacity can not be relied upon to be precisely
982 /// minimal. Prefer `reserve` if future insertions are expected.
986 /// If the capacity overflows, or the allocator reports a failure, then an error
992 /// #![feature(try_reserve)]
993 /// use std::collections::TryReserveError;
995 /// fn process_data(data: &str) -> Result<String, TryReserveError> {
996 /// let mut output = String::new();
998 /// // Pre-reserve the memory, exiting if we can't
999 /// output.try_reserve(data.len())?;
1001 /// // Now we know this can't OOM in the middle of our complex work
1002 /// output.push_str(data);
1006 /// # process_data("rust").expect("why is the test harness OOMing on 4 bytes?");
1008 #[unstable(feature = "try_reserve", reason = "new API", issue = "48043")]
1009 pub fn try_reserve_exact(&mut self, additional: usize) -> Result<(), TryReserveError> {
1010 self.vec.try_reserve_exact(additional)
1013 /// Shrinks the capacity of this `String` to match its length.
1020 /// let mut s = String::from("foo");
1023 /// assert!(s.capacity() >= 100);
1025 /// s.shrink_to_fit();
1026 /// assert_eq!(3, s.capacity());
1029 #[stable(feature = "rust1", since = "1.0.0")]
1030 pub fn shrink_to_fit(&mut self) {
1031 self.vec.shrink_to_fit()
1034 /// Shrinks the capacity of this `String` with a lower bound.
1036 /// The capacity will remain at least as large as both the length
1037 /// and the supplied value.
1039 /// Panics if the current capacity is smaller than the supplied
1040 /// minimum capacity.
1045 /// #![feature(shrink_to)]
1046 /// let mut s = String::from("foo");
1049 /// assert!(s.capacity() >= 100);
1051 /// s.shrink_to(10);
1052 /// assert!(s.capacity() >= 10);
1054 /// assert!(s.capacity() >= 3);
1057 #[unstable(feature = "shrink_to", reason = "new API", issue = "56431")]
1058 pub fn shrink_to(&mut self, min_capacity: usize) {
1059 self.vec.shrink_to(min_capacity)
1062 /// Appends the given [`char`] to the end of this `String`.
1069 /// let mut s = String::from("abc");
1075 /// assert_eq!("abc123", s);
1078 #[stable(feature = "rust1", since = "1.0.0")]
1079 pub fn push(&mut self, ch: char) {
1080 match ch.len_utf8() {
1081 1 => self.vec.push(ch as u8),
1082 _ => self.vec.extend_from_slice(ch.encode_utf8(&mut [0; 4]).as_bytes()),
1086 /// Returns a byte slice of this `String`'s contents.
1088 /// The inverse of this method is [`from_utf8`].
1090 /// [`from_utf8`]: String::from_utf8
1097 /// let s = String::from("hello");
1099 /// assert_eq!(&[104, 101, 108, 108, 111], s.as_bytes());
1102 #[stable(feature = "rust1", since = "1.0.0")]
1103 pub fn as_bytes(&self) -> &[u8] {
1107 /// Shortens this `String` to the specified length.
1109 /// If `new_len` is greater than the string's current length, this has no
1112 /// Note that this method has no effect on the allocated capacity
1117 /// Panics if `new_len` does not lie on a [`char`] boundary.
1124 /// let mut s = String::from("hello");
1128 /// assert_eq!("he", s);
1131 #[stable(feature = "rust1", since = "1.0.0")]
1132 pub fn truncate(&mut self, new_len: usize) {
1133 if new_len <= self.len() {
1134 assert!(self.is_char_boundary(new_len));
1135 self.vec.truncate(new_len)
1139 /// Removes the last character from the string buffer and returns it.
1141 /// Returns [`None`] if this `String` is empty.
1148 /// let mut s = String::from("foo");
1150 /// assert_eq!(s.pop(), Some('o'));
1151 /// assert_eq!(s.pop(), Some('o'));
1152 /// assert_eq!(s.pop(), Some('f'));
1154 /// assert_eq!(s.pop(), None);
1157 #[stable(feature = "rust1", since = "1.0.0")]
1158 pub fn pop(&mut self) -> Option<char> {
1159 let ch = self.chars().rev().next()?;
1160 let newlen = self.len() - ch.len_utf8();
1162 self.vec.set_len(newlen);
1167 /// Removes a [`char`] from this `String` at a byte position and returns it.
1169 /// This is an *O*(*n*) operation, as it requires copying every element in the
1174 /// Panics if `idx` is larger than or equal to the `String`'s length,
1175 /// or if it does not lie on a [`char`] boundary.
1182 /// let mut s = String::from("foo");
1184 /// assert_eq!(s.remove(0), 'f');
1185 /// assert_eq!(s.remove(1), 'o');
1186 /// assert_eq!(s.remove(0), 'o');
1189 #[stable(feature = "rust1", since = "1.0.0")]
1190 pub fn remove(&mut self, idx: usize) -> char {
1191 let ch = match self[idx..].chars().next() {
1193 None => panic!("cannot remove a char from the end of a string"),
1196 let next = idx + ch.len_utf8();
1197 let len = self.len();
1199 ptr::copy(self.vec.as_ptr().add(next), self.vec.as_mut_ptr().add(idx), len - next);
1200 self.vec.set_len(len - (next - idx));
1205 /// Retains only the characters specified by the predicate.
1207 /// In other words, remove all characters `c` such that `f(c)` returns `false`.
1208 /// This method operates in place, visiting each character exactly once in the
1209 /// original order, and preserves the order of the retained characters.
1214 /// let mut s = String::from("f_o_ob_ar");
1216 /// s.retain(|c| c != '_');
1218 /// assert_eq!(s, "foobar");
1221 /// The exact order may be useful for tracking external state, like an index.
1224 /// let mut s = String::from("abcde");
1225 /// let keep = [false, true, true, false, true];
1227 /// s.retain(|_| (keep[i], i += 1).0);
1228 /// assert_eq!(s, "bce");
1231 #[stable(feature = "string_retain", since = "1.26.0")]
1232 pub fn retain<F>(&mut self, mut f: F)
1234 F: FnMut(char) -> bool,
1236 let len = self.len();
1237 let mut del_bytes = 0;
1241 self.vec.set_len(0);
1245 let ch = unsafe { self.get_unchecked(idx..len).chars().next().unwrap() };
1246 let ch_len = ch.len_utf8();
1249 del_bytes += ch_len;
1250 } else if del_bytes > 0 {
1253 self.vec.as_ptr().add(idx),
1254 self.vec.as_mut_ptr().add(idx - del_bytes),
1260 // Point idx to the next char
1265 self.vec.set_len(len - del_bytes);
1269 /// Inserts a character into this `String` at a byte position.
1271 /// This is an *O*(*n*) operation as it requires copying every element in the
1276 /// Panics if `idx` is larger than the `String`'s length, or if it does not
1277 /// lie on a [`char`] boundary.
1284 /// let mut s = String::with_capacity(3);
1286 /// s.insert(0, 'f');
1287 /// s.insert(1, 'o');
1288 /// s.insert(2, 'o');
1290 /// assert_eq!("foo", s);
1293 #[stable(feature = "rust1", since = "1.0.0")]
1294 pub fn insert(&mut self, idx: usize, ch: char) {
1295 assert!(self.is_char_boundary(idx));
1296 let mut bits = [0; 4];
1297 let bits = ch.encode_utf8(&mut bits).as_bytes();
1300 self.insert_bytes(idx, bits);
1304 unsafe fn insert_bytes(&mut self, idx: usize, bytes: &[u8]) {
1305 let len = self.len();
1306 let amt = bytes.len();
1307 self.vec.reserve(amt);
1310 ptr::copy(self.vec.as_ptr().add(idx), self.vec.as_mut_ptr().add(idx + amt), len - idx);
1311 ptr::copy(bytes.as_ptr(), self.vec.as_mut_ptr().add(idx), amt);
1312 self.vec.set_len(len + amt);
1316 /// Inserts a string slice into this `String` at a byte position.
1318 /// This is an *O*(*n*) operation as it requires copying every element in the
1323 /// Panics if `idx` is larger than the `String`'s length, or if it does not
1324 /// lie on a [`char`] boundary.
1331 /// let mut s = String::from("bar");
1333 /// s.insert_str(0, "foo");
1335 /// assert_eq!("foobar", s);
1338 #[stable(feature = "insert_str", since = "1.16.0")]
1339 pub fn insert_str(&mut self, idx: usize, string: &str) {
1340 assert!(self.is_char_boundary(idx));
1343 self.insert_bytes(idx, string.as_bytes());
1347 /// Returns a mutable reference to the contents of this `String`.
1351 /// This function is unsafe because it does not check that the bytes passed
1352 /// to it are valid UTF-8. If this constraint is violated, it may cause
1353 /// memory unsafety issues with future users of the `String`, as the rest of
1354 /// the standard library assumes that `String`s are valid UTF-8.
1361 /// let mut s = String::from("hello");
1364 /// let vec = s.as_mut_vec();
1365 /// assert_eq!(&[104, 101, 108, 108, 111][..], &vec[..]);
1369 /// assert_eq!(s, "olleh");
1372 #[stable(feature = "rust1", since = "1.0.0")]
1373 pub unsafe fn as_mut_vec(&mut self) -> &mut Vec<u8> {
1377 /// Returns the length of this `String`, in bytes, not [`char`]s or
1378 /// graphemes. In other words, it may not be what a human considers the
1379 /// length of the string.
1386 /// let a = String::from("foo");
1387 /// assert_eq!(a.len(), 3);
1389 /// let fancy_f = String::from("ƒoo");
1390 /// assert_eq!(fancy_f.len(), 4);
1391 /// assert_eq!(fancy_f.chars().count(), 3);
1393 #[doc(alias = "length")]
1395 #[stable(feature = "rust1", since = "1.0.0")]
1396 pub fn len(&self) -> usize {
1400 /// Returns `true` if this `String` has a length of zero, and `false` otherwise.
1407 /// let mut v = String::new();
1408 /// assert!(v.is_empty());
1411 /// assert!(!v.is_empty());
1414 #[stable(feature = "rust1", since = "1.0.0")]
1415 pub fn is_empty(&self) -> bool {
1419 /// Splits the string into two at the given byte index.
1421 /// Returns a newly allocated `String`. `self` contains bytes `[0, at)`, and
1422 /// the returned `String` contains bytes `[at, len)`. `at` must be on the
1423 /// boundary of a UTF-8 code point.
1425 /// Note that the capacity of `self` does not change.
1429 /// Panics if `at` is not on a `UTF-8` code point boundary, or if it is beyond the last
1430 /// code point of the string.
1436 /// let mut hello = String::from("Hello, World!");
1437 /// let world = hello.split_off(7);
1438 /// assert_eq!(hello, "Hello, ");
1439 /// assert_eq!(world, "World!");
1443 #[stable(feature = "string_split_off", since = "1.16.0")]
1444 #[must_use = "use `.truncate()` if you don't need the other half"]
1445 pub fn split_off(&mut self, at: usize) -> String {
1446 assert!(self.is_char_boundary(at));
1447 let other = self.vec.split_off(at);
1448 unsafe { String::from_utf8_unchecked(other) }
1451 /// Truncates this `String`, removing all contents.
1453 /// While this means the `String` will have a length of zero, it does not
1454 /// touch its capacity.
1461 /// let mut s = String::from("foo");
1465 /// assert!(s.is_empty());
1466 /// assert_eq!(0, s.len());
1467 /// assert_eq!(3, s.capacity());
1470 #[stable(feature = "rust1", since = "1.0.0")]
1471 pub fn clear(&mut self) {
1475 /// Creates a draining iterator that removes the specified range in the `String`
1476 /// and yields the removed `chars`.
1478 /// Note: The element range is removed even if the iterator is not
1479 /// consumed until the end.
1483 /// Panics if the starting point or end point do not lie on a [`char`]
1484 /// boundary, or if they're out of bounds.
1491 /// let mut s = String::from("α is alpha, β is beta");
1492 /// let beta_offset = s.find('β').unwrap_or(s.len());
1494 /// // Remove the range up until the β from the string
1495 /// let t: String = s.drain(..beta_offset).collect();
1496 /// assert_eq!(t, "α is alpha, ");
1497 /// assert_eq!(s, "β is beta");
1499 /// // A full range clears the string
1501 /// assert_eq!(s, "");
1503 #[stable(feature = "drain", since = "1.6.0")]
1504 pub fn drain<R>(&mut self, range: R) -> Drain<'_>
1506 R: RangeBounds<usize>,
1510 // The String version of Drain does not have the memory safety issues
1511 // of the vector version. The data is just plain bytes.
1512 // Because the range removal happens in Drop, if the Drain iterator is leaked,
1513 // the removal will not happen.
1514 let Range { start, end } = range.assert_len(self.len());
1515 assert!(self.is_char_boundary(start));
1516 assert!(self.is_char_boundary(end));
1518 // Take out two simultaneous borrows. The &mut String won't be accessed
1519 // until iteration is over, in Drop.
1520 let self_ptr = self as *mut _;
1521 // SAFETY: `assert_len` and `is_char_boundary` do the appropriate bounds checks.
1522 let chars_iter = unsafe { self.get_unchecked(start..end) }.chars();
1524 Drain { start, end, iter: chars_iter, string: self_ptr }
1527 /// Removes the specified range in the string,
1528 /// and replaces it with the given string.
1529 /// The given string doesn't need to be the same length as the range.
1533 /// Panics if the starting point or end point do not lie on a [`char`]
1534 /// boundary, or if they're out of bounds.
1541 /// let mut s = String::from("α is alpha, β is beta");
1542 /// let beta_offset = s.find('β').unwrap_or(s.len());
1544 /// // Replace the range up until the β from the string
1545 /// s.replace_range(..beta_offset, "Α is capital alpha; ");
1546 /// assert_eq!(s, "Α is capital alpha; β is beta");
1548 #[stable(feature = "splice", since = "1.27.0")]
1549 pub fn replace_range<R>(&mut self, range: R, replace_with: &str)
1551 R: RangeBounds<usize>,
1555 // Replace_range does not have the memory safety issues of a vector Splice.
1556 // of the vector version. The data is just plain bytes.
1558 // WARNING: Inlining this variable would be unsound (#81138)
1559 let start = range.start_bound();
1561 Included(&n) => assert!(self.is_char_boundary(n)),
1562 Excluded(&n) => assert!(self.is_char_boundary(n + 1)),
1565 // WARNING: Inlining this variable would be unsound (#81138)
1566 let end = range.end_bound();
1568 Included(&n) => assert!(self.is_char_boundary(n + 1)),
1569 Excluded(&n) => assert!(self.is_char_boundary(n)),
1573 // Using `range` again would be unsound (#81138)
1574 // We assume the bounds reported by `range` remain the same, but
1575 // an adversarial implementation could change between calls
1576 unsafe { self.as_mut_vec() }.splice((start, end), replace_with.bytes());
1579 /// Converts this `String` into a [`Box`]`<`[`str`]`>`.
1581 /// This will drop any excess capacity.
1583 /// [`str`]: prim@str
1590 /// let s = String::from("hello");
1592 /// let b = s.into_boxed_str();
1594 #[stable(feature = "box_str", since = "1.4.0")]
1596 pub fn into_boxed_str(self) -> Box<str> {
1597 let slice = self.vec.into_boxed_slice();
1598 unsafe { from_boxed_utf8_unchecked(slice) }
1602 impl FromUtf8Error {
1603 /// Returns a slice of [`u8`]s bytes that were attempted to convert to a `String`.
1610 /// // some invalid bytes, in a vector
1611 /// let bytes = vec![0, 159];
1613 /// let value = String::from_utf8(bytes);
1615 /// assert_eq!(&[0, 159], value.unwrap_err().as_bytes());
1617 #[stable(feature = "from_utf8_error_as_bytes", since = "1.26.0")]
1618 pub fn as_bytes(&self) -> &[u8] {
1622 /// Returns the bytes that were attempted to convert to a `String`.
1624 /// This method is carefully constructed to avoid allocation. It will
1625 /// consume the error, moving out the bytes, so that a copy of the bytes
1626 /// does not need to be made.
1633 /// // some invalid bytes, in a vector
1634 /// let bytes = vec![0, 159];
1636 /// let value = String::from_utf8(bytes);
1638 /// assert_eq!(vec![0, 159], value.unwrap_err().into_bytes());
1640 #[stable(feature = "rust1", since = "1.0.0")]
1641 pub fn into_bytes(self) -> Vec<u8> {
1645 /// Fetch a `Utf8Error` to get more details about the conversion failure.
1647 /// The [`Utf8Error`] type provided by [`std::str`] represents an error that may
1648 /// occur when converting a slice of [`u8`]s to a [`&str`]. In this sense, it's
1649 /// an analogue to `FromUtf8Error`. See its documentation for more details
1652 /// [`std::str`]: core::str
1653 /// [`&str`]: prim@str
1660 /// // some invalid bytes, in a vector
1661 /// let bytes = vec![0, 159];
1663 /// let error = String::from_utf8(bytes).unwrap_err().utf8_error();
1665 /// // the first byte is invalid here
1666 /// assert_eq!(1, error.valid_up_to());
1668 #[stable(feature = "rust1", since = "1.0.0")]
1669 pub fn utf8_error(&self) -> Utf8Error {
1674 #[stable(feature = "rust1", since = "1.0.0")]
1675 impl fmt::Display for FromUtf8Error {
1676 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1677 fmt::Display::fmt(&self.error, f)
1681 #[stable(feature = "rust1", since = "1.0.0")]
1682 impl fmt::Display for FromUtf16Error {
1683 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1684 fmt::Display::fmt("invalid utf-16: lone surrogate found", f)
1688 #[stable(feature = "rust1", since = "1.0.0")]
1689 impl Clone for String {
1690 fn clone(&self) -> Self {
1691 String { vec: self.vec.clone() }
1694 fn clone_from(&mut self, source: &Self) {
1695 self.vec.clone_from(&source.vec);
1699 #[stable(feature = "rust1", since = "1.0.0")]
1700 impl FromIterator<char> for String {
1701 fn from_iter<I: IntoIterator<Item = char>>(iter: I) -> String {
1702 let mut buf = String::new();
1708 #[stable(feature = "string_from_iter_by_ref", since = "1.17.0")]
1709 impl<'a> FromIterator<&'a char> for String {
1710 fn from_iter<I: IntoIterator<Item = &'a char>>(iter: I) -> String {
1711 let mut buf = String::new();
1717 #[stable(feature = "rust1", since = "1.0.0")]
1718 impl<'a> FromIterator<&'a str> for String {
1719 fn from_iter<I: IntoIterator<Item = &'a str>>(iter: I) -> String {
1720 let mut buf = String::new();
1726 #[stable(feature = "extend_string", since = "1.4.0")]
1727 impl FromIterator<String> for String {
1728 fn from_iter<I: IntoIterator<Item = String>>(iter: I) -> String {
1729 let mut iterator = iter.into_iter();
1731 // Because we're iterating over `String`s, we can avoid at least
1732 // one allocation by getting the first string from the iterator
1733 // and appending to it all the subsequent strings.
1734 match iterator.next() {
1735 None => String::new(),
1737 buf.extend(iterator);
1744 #[stable(feature = "box_str2", since = "1.45.0")]
1745 impl FromIterator<Box<str>> for String {
1746 fn from_iter<I: IntoIterator<Item = Box<str>>>(iter: I) -> String {
1747 let mut buf = String::new();
1753 #[stable(feature = "herd_cows", since = "1.19.0")]
1754 impl<'a> FromIterator<Cow<'a, str>> for String {
1755 fn from_iter<I: IntoIterator<Item = Cow<'a, str>>>(iter: I) -> String {
1756 let mut iterator = iter.into_iter();
1758 // Because we're iterating over CoWs, we can (potentially) avoid at least
1759 // one allocation by getting the first item and appending to it all the
1760 // subsequent items.
1761 match iterator.next() {
1762 None => String::new(),
1764 let mut buf = cow.into_owned();
1765 buf.extend(iterator);
1772 #[stable(feature = "rust1", since = "1.0.0")]
1773 impl Extend<char> for String {
1774 fn extend<I: IntoIterator<Item = char>>(&mut self, iter: I) {
1775 let iterator = iter.into_iter();
1776 let (lower_bound, _) = iterator.size_hint();
1777 self.reserve(lower_bound);
1778 iterator.for_each(move |c| self.push(c));
1782 fn extend_one(&mut self, c: char) {
1787 fn extend_reserve(&mut self, additional: usize) {
1788 self.reserve(additional);
1792 #[stable(feature = "extend_ref", since = "1.2.0")]
1793 impl<'a> Extend<&'a char> for String {
1794 fn extend<I: IntoIterator<Item = &'a char>>(&mut self, iter: I) {
1795 self.extend(iter.into_iter().cloned());
1799 fn extend_one(&mut self, &c: &'a char) {
1804 fn extend_reserve(&mut self, additional: usize) {
1805 self.reserve(additional);
1809 #[stable(feature = "rust1", since = "1.0.0")]
1810 impl<'a> Extend<&'a str> for String {
1811 fn extend<I: IntoIterator<Item = &'a str>>(&mut self, iter: I) {
1812 iter.into_iter().for_each(move |s| self.push_str(s));
1816 fn extend_one(&mut self, s: &'a str) {
1821 #[stable(feature = "box_str2", since = "1.45.0")]
1822 impl Extend<Box<str>> for String {
1823 fn extend<I: IntoIterator<Item = Box<str>>>(&mut self, iter: I) {
1824 iter.into_iter().for_each(move |s| self.push_str(&s));
1828 #[stable(feature = "extend_string", since = "1.4.0")]
1829 impl Extend<String> for String {
1830 fn extend<I: IntoIterator<Item = String>>(&mut self, iter: I) {
1831 iter.into_iter().for_each(move |s| self.push_str(&s));
1835 fn extend_one(&mut self, s: String) {
1840 #[stable(feature = "herd_cows", since = "1.19.0")]
1841 impl<'a> Extend<Cow<'a, str>> for String {
1842 fn extend<I: IntoIterator<Item = Cow<'a, str>>>(&mut self, iter: I) {
1843 iter.into_iter().for_each(move |s| self.push_str(&s));
1847 fn extend_one(&mut self, s: Cow<'a, str>) {
1852 /// A convenience impl that delegates to the impl for `&str`.
1857 /// assert_eq!(String::from("Hello world").find("world"), Some(6));
1860 feature = "pattern",
1861 reason = "API not fully fleshed out and ready to be stabilized",
1864 impl<'a, 'b> Pattern<'a> for &'b String {
1865 type Searcher = <&'b str as Pattern<'a>>::Searcher;
1867 fn into_searcher(self, haystack: &'a str) -> <&'b str as Pattern<'a>>::Searcher {
1868 self[..].into_searcher(haystack)
1872 fn is_contained_in(self, haystack: &'a str) -> bool {
1873 self[..].is_contained_in(haystack)
1877 fn is_prefix_of(self, haystack: &'a str) -> bool {
1878 self[..].is_prefix_of(haystack)
1882 fn strip_prefix_of(self, haystack: &'a str) -> Option<&'a str> {
1883 self[..].strip_prefix_of(haystack)
1887 fn is_suffix_of(self, haystack: &'a str) -> bool {
1888 self[..].is_suffix_of(haystack)
1892 fn strip_suffix_of(self, haystack: &'a str) -> Option<&'a str> {
1893 self[..].strip_suffix_of(haystack)
1897 #[stable(feature = "rust1", since = "1.0.0")]
1898 impl PartialEq for String {
1900 fn eq(&self, other: &String) -> bool {
1901 PartialEq::eq(&self[..], &other[..])
1904 fn ne(&self, other: &String) -> bool {
1905 PartialEq::ne(&self[..], &other[..])
1909 macro_rules! impl_eq {
1910 ($lhs:ty, $rhs: ty) => {
1911 #[stable(feature = "rust1", since = "1.0.0")]
1912 #[allow(unused_lifetimes)]
1913 impl<'a, 'b> PartialEq<$rhs> for $lhs {
1915 fn eq(&self, other: &$rhs) -> bool {
1916 PartialEq::eq(&self[..], &other[..])
1919 fn ne(&self, other: &$rhs) -> bool {
1920 PartialEq::ne(&self[..], &other[..])
1924 #[stable(feature = "rust1", since = "1.0.0")]
1925 #[allow(unused_lifetimes)]
1926 impl<'a, 'b> PartialEq<$lhs> for $rhs {
1928 fn eq(&self, other: &$lhs) -> bool {
1929 PartialEq::eq(&self[..], &other[..])
1932 fn ne(&self, other: &$lhs) -> bool {
1933 PartialEq::ne(&self[..], &other[..])
1939 impl_eq! { String, str }
1940 impl_eq! { String, &'a str }
1941 impl_eq! { Cow<'a, str>, str }
1942 impl_eq! { Cow<'a, str>, &'b str }
1943 impl_eq! { Cow<'a, str>, String }
1945 #[stable(feature = "rust1", since = "1.0.0")]
1946 impl Default for String {
1947 /// Creates an empty `String`.
1949 fn default() -> String {
1954 #[stable(feature = "rust1", since = "1.0.0")]
1955 impl fmt::Display for String {
1957 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1958 fmt::Display::fmt(&**self, f)
1962 #[stable(feature = "rust1", since = "1.0.0")]
1963 impl fmt::Debug for String {
1965 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1966 fmt::Debug::fmt(&**self, f)
1970 #[stable(feature = "rust1", since = "1.0.0")]
1971 impl hash::Hash for String {
1973 fn hash<H: hash::Hasher>(&self, hasher: &mut H) {
1974 (**self).hash(hasher)
1978 /// Implements the `+` operator for concatenating two strings.
1980 /// This consumes the `String` on the left-hand side and re-uses its buffer (growing it if
1981 /// necessary). This is done to avoid allocating a new `String` and copying the entire contents on
1982 /// every operation, which would lead to *O*(*n*^2) running time when building an *n*-byte string by
1983 /// repeated concatenation.
1985 /// The string on the right-hand side is only borrowed; its contents are copied into the returned
1990 /// Concatenating two `String`s takes the first by value and borrows the second:
1993 /// let a = String::from("hello");
1994 /// let b = String::from(" world");
1996 /// // `a` is moved and can no longer be used here.
1999 /// If you want to keep using the first `String`, you can clone it and append to the clone instead:
2002 /// let a = String::from("hello");
2003 /// let b = String::from(" world");
2004 /// let c = a.clone() + &b;
2005 /// // `a` is still valid here.
2008 /// Concatenating `&str` slices can be done by converting the first to a `String`:
2011 /// let a = "hello";
2012 /// let b = " world";
2013 /// let c = a.to_string() + b;
2015 #[stable(feature = "rust1", since = "1.0.0")]
2016 impl Add<&str> for String {
2017 type Output = String;
2020 fn add(mut self, other: &str) -> String {
2021 self.push_str(other);
2026 /// Implements the `+=` operator for appending to a `String`.
2028 /// This has the same behavior as the [`push_str`][String::push_str] method.
2029 #[stable(feature = "stringaddassign", since = "1.12.0")]
2030 impl AddAssign<&str> for String {
2032 fn add_assign(&mut self, other: &str) {
2033 self.push_str(other);
2037 #[stable(feature = "rust1", since = "1.0.0")]
2038 impl ops::Index<ops::Range<usize>> for String {
2042 fn index(&self, index: ops::Range<usize>) -> &str {
2046 #[stable(feature = "rust1", since = "1.0.0")]
2047 impl ops::Index<ops::RangeTo<usize>> for String {
2051 fn index(&self, index: ops::RangeTo<usize>) -> &str {
2055 #[stable(feature = "rust1", since = "1.0.0")]
2056 impl ops::Index<ops::RangeFrom<usize>> for String {
2060 fn index(&self, index: ops::RangeFrom<usize>) -> &str {
2064 #[stable(feature = "rust1", since = "1.0.0")]
2065 impl ops::Index<ops::RangeFull> for String {
2069 fn index(&self, _index: ops::RangeFull) -> &str {
2070 unsafe { str::from_utf8_unchecked(&self.vec) }
2073 #[stable(feature = "inclusive_range", since = "1.26.0")]
2074 impl ops::Index<ops::RangeInclusive<usize>> for String {
2078 fn index(&self, index: ops::RangeInclusive<usize>) -> &str {
2079 Index::index(&**self, index)
2082 #[stable(feature = "inclusive_range", since = "1.26.0")]
2083 impl ops::Index<ops::RangeToInclusive<usize>> for String {
2087 fn index(&self, index: ops::RangeToInclusive<usize>) -> &str {
2088 Index::index(&**self, index)
2092 #[stable(feature = "derefmut_for_string", since = "1.3.0")]
2093 impl ops::IndexMut<ops::Range<usize>> for String {
2095 fn index_mut(&mut self, index: ops::Range<usize>) -> &mut str {
2096 &mut self[..][index]
2099 #[stable(feature = "derefmut_for_string", since = "1.3.0")]
2100 impl ops::IndexMut<ops::RangeTo<usize>> for String {
2102 fn index_mut(&mut self, index: ops::RangeTo<usize>) -> &mut str {
2103 &mut self[..][index]
2106 #[stable(feature = "derefmut_for_string", since = "1.3.0")]
2107 impl ops::IndexMut<ops::RangeFrom<usize>> for String {
2109 fn index_mut(&mut self, index: ops::RangeFrom<usize>) -> &mut str {
2110 &mut self[..][index]
2113 #[stable(feature = "derefmut_for_string", since = "1.3.0")]
2114 impl ops::IndexMut<ops::RangeFull> for String {
2116 fn index_mut(&mut self, _index: ops::RangeFull) -> &mut str {
2117 unsafe { str::from_utf8_unchecked_mut(&mut *self.vec) }
2120 #[stable(feature = "inclusive_range", since = "1.26.0")]
2121 impl ops::IndexMut<ops::RangeInclusive<usize>> for String {
2123 fn index_mut(&mut self, index: ops::RangeInclusive<usize>) -> &mut str {
2124 IndexMut::index_mut(&mut **self, index)
2127 #[stable(feature = "inclusive_range", since = "1.26.0")]
2128 impl ops::IndexMut<ops::RangeToInclusive<usize>> for String {
2130 fn index_mut(&mut self, index: ops::RangeToInclusive<usize>) -> &mut str {
2131 IndexMut::index_mut(&mut **self, index)
2135 #[stable(feature = "rust1", since = "1.0.0")]
2136 impl ops::Deref for String {
2140 fn deref(&self) -> &str {
2141 unsafe { str::from_utf8_unchecked(&self.vec) }
2145 #[stable(feature = "derefmut_for_string", since = "1.3.0")]
2146 impl ops::DerefMut for String {
2148 fn deref_mut(&mut self) -> &mut str {
2149 unsafe { str::from_utf8_unchecked_mut(&mut *self.vec) }
2153 /// A type alias for [`Infallible`].
2155 /// This alias exists for backwards compatibility, and may be eventually deprecated.
2157 /// [`Infallible`]: core::convert::Infallible
2158 #[stable(feature = "str_parse_error", since = "1.5.0")]
2159 pub type ParseError = core::convert::Infallible;
2161 #[stable(feature = "rust1", since = "1.0.0")]
2162 impl FromStr for String {
2163 type Err = core::convert::Infallible;
2165 fn from_str(s: &str) -> Result<String, Self::Err> {
2170 /// A trait for converting a value to a `String`.
2172 /// This trait is automatically implemented for any type which implements the
2173 /// [`Display`] trait. As such, `ToString` shouldn't be implemented directly:
2174 /// [`Display`] should be implemented instead, and you get the `ToString`
2175 /// implementation for free.
2177 /// [`Display`]: fmt::Display
2178 #[stable(feature = "rust1", since = "1.0.0")]
2179 pub trait ToString {
2180 /// Converts the given value to a `String`.
2188 /// let five = String::from("5");
2190 /// assert_eq!(five, i.to_string());
2192 #[rustc_conversion_suggestion]
2193 #[stable(feature = "rust1", since = "1.0.0")]
2194 fn to_string(&self) -> String;
2199 /// In this implementation, the `to_string` method panics
2200 /// if the `Display` implementation returns an error.
2201 /// This indicates an incorrect `Display` implementation
2202 /// since `fmt::Write for String` never returns an error itself.
2203 #[stable(feature = "rust1", since = "1.0.0")]
2204 impl<T: fmt::Display + ?Sized> ToString for T {
2205 // A common guideline is to not inline generic functions. However,
2206 // removing `#[inline]` from this method causes non-negligible regressions.
2207 // See <https://github.com/rust-lang/rust/pull/74852>, the last attempt
2208 // to try to remove it.
2210 default fn to_string(&self) -> String {
2212 let mut buf = String::new();
2213 buf.write_fmt(format_args!("{}", self))
2214 .expect("a Display implementation returned an error unexpectedly");
2219 #[stable(feature = "char_to_string_specialization", since = "1.46.0")]
2220 impl ToString for char {
2222 fn to_string(&self) -> String {
2223 String::from(self.encode_utf8(&mut [0; 4]))
2227 #[stable(feature = "u8_to_string_specialization", since = "1.999.0")]
2228 impl ToString for u8 {
2230 fn to_string(&self) -> String {
2231 let mut result = String::with_capacity(3);
2234 result.push((b'0' + n / 100) as char);
2237 if !result.is_empty() || n >= 10 {
2238 result.push((b'0' + n / 10) as char);
2241 result.push((b'0' + n) as char);
2246 #[stable(feature = "i8_to_string_specialization", since = "1.999.0")]
2247 impl ToString for i8 {
2249 fn to_string(&self) -> String {
2250 let mut vec = vec![0; 4];
2252 if self.is_negative() {
2256 let mut n = self.unsigned_abs();
2263 debug_assert!(n < 100);
2264 vec[free] = b'0' + n / 10;
2268 debug_assert!(n < 10);
2269 vec[free] = b'0' + n;
2273 // SAFETY: Vec only contains ascii so valid utf8
2274 unsafe { String::from_utf8_unchecked(vec) }
2278 #[stable(feature = "str_to_string_specialization", since = "1.9.0")]
2279 impl ToString for str {
2281 fn to_string(&self) -> String {
2286 #[stable(feature = "cow_str_to_string_specialization", since = "1.17.0")]
2287 impl ToString for Cow<'_, str> {
2289 fn to_string(&self) -> String {
2294 #[stable(feature = "string_to_string_specialization", since = "1.17.0")]
2295 impl ToString for String {
2297 fn to_string(&self) -> String {
2302 #[stable(feature = "rust1", since = "1.0.0")]
2303 impl AsRef<str> for String {
2305 fn as_ref(&self) -> &str {
2310 #[stable(feature = "string_as_mut", since = "1.43.0")]
2311 impl AsMut<str> for String {
2313 fn as_mut(&mut self) -> &mut str {
2318 #[stable(feature = "rust1", since = "1.0.0")]
2319 impl AsRef<[u8]> for String {
2321 fn as_ref(&self) -> &[u8] {
2326 #[stable(feature = "rust1", since = "1.0.0")]
2327 impl From<&str> for String {
2329 fn from(s: &str) -> String {
2334 #[stable(feature = "from_mut_str_for_string", since = "1.44.0")]
2335 impl From<&mut str> for String {
2336 /// Converts a `&mut str` into a `String`.
2338 /// The result is allocated on the heap.
2340 fn from(s: &mut str) -> String {
2345 #[stable(feature = "from_ref_string", since = "1.35.0")]
2346 impl From<&String> for String {
2348 fn from(s: &String) -> String {
2353 // note: test pulls in libstd, which causes errors here
2355 #[stable(feature = "string_from_box", since = "1.18.0")]
2356 impl From<Box<str>> for String {
2357 /// Converts the given boxed `str` slice to a `String`.
2358 /// It is notable that the `str` slice is owned.
2365 /// let s1: String = String::from("hello world");
2366 /// let s2: Box<str> = s1.into_boxed_str();
2367 /// let s3: String = String::from(s2);
2369 /// assert_eq!("hello world", s3)
2371 fn from(s: Box<str>) -> String {
2376 #[stable(feature = "box_from_str", since = "1.20.0")]
2377 impl From<String> for Box<str> {
2378 /// Converts the given `String` to a boxed `str` slice that is owned.
2385 /// let s1: String = String::from("hello world");
2386 /// let s2: Box<str> = Box::from(s1);
2387 /// let s3: String = String::from(s2);
2389 /// assert_eq!("hello world", s3)
2391 fn from(s: String) -> Box<str> {
2396 #[stable(feature = "string_from_cow_str", since = "1.14.0")]
2397 impl<'a> From<Cow<'a, str>> for String {
2398 fn from(s: Cow<'a, str>) -> String {
2403 #[stable(feature = "rust1", since = "1.0.0")]
2404 impl<'a> From<&'a str> for Cow<'a, str> {
2406 fn from(s: &'a str) -> Cow<'a, str> {
2411 #[stable(feature = "rust1", since = "1.0.0")]
2412 impl<'a> From<String> for Cow<'a, str> {
2414 fn from(s: String) -> Cow<'a, str> {
2419 #[stable(feature = "cow_from_string_ref", since = "1.28.0")]
2420 impl<'a> From<&'a String> for Cow<'a, str> {
2422 fn from(s: &'a String) -> Cow<'a, str> {
2423 Cow::Borrowed(s.as_str())
2427 #[stable(feature = "cow_str_from_iter", since = "1.12.0")]
2428 impl<'a> FromIterator<char> for Cow<'a, str> {
2429 fn from_iter<I: IntoIterator<Item = char>>(it: I) -> Cow<'a, str> {
2430 Cow::Owned(FromIterator::from_iter(it))
2434 #[stable(feature = "cow_str_from_iter", since = "1.12.0")]
2435 impl<'a, 'b> FromIterator<&'b str> for Cow<'a, str> {
2436 fn from_iter<I: IntoIterator<Item = &'b str>>(it: I) -> Cow<'a, str> {
2437 Cow::Owned(FromIterator::from_iter(it))
2441 #[stable(feature = "cow_str_from_iter", since = "1.12.0")]
2442 impl<'a> FromIterator<String> for Cow<'a, str> {
2443 fn from_iter<I: IntoIterator<Item = String>>(it: I) -> Cow<'a, str> {
2444 Cow::Owned(FromIterator::from_iter(it))
2448 #[stable(feature = "from_string_for_vec_u8", since = "1.14.0")]
2449 impl From<String> for Vec<u8> {
2450 /// Converts the given `String` to a vector `Vec` that holds values of type `u8`.
2457 /// let s1 = String::from("hello world");
2458 /// let v1 = Vec::from(s1);
2461 /// println!("{}", b);
2464 fn from(string: String) -> Vec<u8> {
2469 #[stable(feature = "rust1", since = "1.0.0")]
2470 impl fmt::Write for String {
2472 fn write_str(&mut self, s: &str) -> fmt::Result {
2478 fn write_char(&mut self, c: char) -> fmt::Result {
2484 /// A draining iterator for `String`.
2486 /// This struct is created by the [`drain`] method on [`String`]. See its
2487 /// documentation for more.
2489 /// [`drain`]: String::drain
2490 #[stable(feature = "drain", since = "1.6.0")]
2491 pub struct Drain<'a> {
2492 /// Will be used as &'a mut String in the destructor
2493 string: *mut String,
2494 /// Start of part to remove
2496 /// End of part to remove
2498 /// Current remaining range to remove
2502 #[stable(feature = "collection_debug", since = "1.17.0")]
2503 impl fmt::Debug for Drain<'_> {
2504 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2505 f.debug_tuple("Drain").field(&self.as_str()).finish()
2509 #[stable(feature = "drain", since = "1.6.0")]
2510 unsafe impl Sync for Drain<'_> {}
2511 #[stable(feature = "drain", since = "1.6.0")]
2512 unsafe impl Send for Drain<'_> {}
2514 #[stable(feature = "drain", since = "1.6.0")]
2515 impl Drop for Drain<'_> {
2516 fn drop(&mut self) {
2518 // Use Vec::drain. "Reaffirm" the bounds checks to avoid
2519 // panic code being inserted again.
2520 let self_vec = (*self.string).as_mut_vec();
2521 if self.start <= self.end && self.end <= self_vec.len() {
2522 self_vec.drain(self.start..self.end);
2528 impl<'a> Drain<'a> {
2529 /// Returns the remaining (sub)string of this iterator as a slice.
2534 /// #![feature(string_drain_as_str)]
2535 /// let mut s = String::from("abc");
2536 /// let mut drain = s.drain(..);
2537 /// assert_eq!(drain.as_str(), "abc");
2538 /// let _ = drain.next().unwrap();
2539 /// assert_eq!(drain.as_str(), "bc");
2541 #[unstable(feature = "string_drain_as_str", issue = "76905")] // Note: uncomment AsRef impls below when stabilizing.
2542 pub fn as_str(&self) -> &str {
2547 // Uncomment when stabilizing `string_drain_as_str`.
2548 // #[unstable(feature = "string_drain_as_str", issue = "76905")]
2549 // impl<'a> AsRef<str> for Drain<'a> {
2550 // fn as_ref(&self) -> &str {
2555 // #[unstable(feature = "string_drain_as_str", issue = "76905")]
2556 // impl<'a> AsRef<[u8]> for Drain<'a> {
2557 // fn as_ref(&self) -> &[u8] {
2558 // self.as_str().as_bytes()
2562 #[stable(feature = "drain", since = "1.6.0")]
2563 impl Iterator for Drain<'_> {
2567 fn next(&mut self) -> Option<char> {
2571 fn size_hint(&self) -> (usize, Option<usize>) {
2572 self.iter.size_hint()
2576 fn last(mut self) -> Option<char> {
2581 #[stable(feature = "drain", since = "1.6.0")]
2582 impl DoubleEndedIterator for Drain<'_> {
2584 fn next_back(&mut self) -> Option<char> {
2585 self.iter.next_back()
2589 #[stable(feature = "fused", since = "1.26.0")]
2590 impl FusedIterator for Drain<'_> {}
2592 #[stable(feature = "from_char_for_string", since = "1.46.0")]
2593 impl From<char> for String {
2595 fn from(c: char) -> Self {