Struct std::vec::Vec
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pub struct Vec<T> {
buf: RawVec<T>,
len: usize,
}A growable list type, written Vec<T> but pronounced 'vector.'
Examples
let mut vec = Vec::new(); vec.push(1); vec.push(2); assert_eq!(vec.len(), 2); assert_eq!(vec[0], 1); assert_eq!(vec.pop(), Some(2)); assert_eq!(vec.len(), 1); vec[0] = 7; assert_eq!(vec[0], 7); vec.extend([1, 2, 3].iter().cloned()); for x in &vec { println!("{}", x); } assert_eq!(vec, [7, 1, 2, 3]);
The vec! macro is provided to make initialization more convenient:
let mut vec = vec![1, 2, 3]; vec.push(4); assert_eq!(vec, [1, 2, 3, 4]);
It can also initialize each element of a Vec<T> with a given value:
let vec = vec![0; 5]; assert_eq!(vec, [0, 0, 0, 0, 0]);
Use a Vec<T> as an efficient stack:
let mut stack = Vec::new(); stack.push(1); stack.push(2); stack.push(3); while let Some(top) = stack.pop() { // Prints 3, 2, 1 println!("{}", top); }
Capacity and reallocation
The capacity of a vector is the amount of space allocated for any future elements that will be added onto the vector. This is not to be confused with the length of a vector, which specifies the number of actual elements within the vector. If a vector's length exceeds its capacity, its capacity will automatically be increased, but its elements will have to be reallocated.
For example, a vector with capacity 10 and length 0 would be an empty vector
with space for 10 more elements. Pushing 10 or fewer elements onto the
vector will not change its capacity or cause reallocation to occur. However,
if the vector's length is increased to 11, it will have to reallocate, which
can be slow. For this reason, it is recommended to use Vec::with_capacity
whenever possible to specify how big the vector is expected to get.
Guarantees
Due to its incredibly fundamental nature, Vec makes a lot of guarantees
about its design. This ensures that it's as low-overhead as possible in
the general case, and can be correctly manipulated in primitive ways
by unsafe code. Note that these guarantees refer to an unqualified Vec<T>.
If additional type parameters are added (e.g. to support custom allocators),
overriding their defaults may change the behavior.
Most fundamentally, Vec is and always will be a (pointer, capacity, length) triplet. No more, no less. The order of these fields is completely unspecified, and you should use the appropriate methods to modify these. The pointer will never be null, so this type is null-pointer-optimized.
However, the pointer may not actually point to allocated memory. In particular,
if you construct a Vec with capacity 0 via Vec::new(), vec![],
Vec::with_capacity(0), or by calling shrink_to_fit() on an empty Vec, it
will not allocate memory. Similarly, if you store zero-sized types inside
a Vec, it will not allocate space for them. Note that in this case the
Vec may not report a capacity() of 0. Vec will allocate if and only
if mem::size_of::<T>() * capacity() > 0. In general, Vec's allocation
details are subtle enough that it is strongly recommended that you only
free memory allocated by a Vec by creating a new Vec and dropping it.
If a Vec has allocated memory, then the memory it points to is on the heap
(as defined by the allocator Rust is configured to use by default), and its
pointer points to len() initialized elements in order (what you would see
if you coerced it to a slice), followed by capacity() - len() logically
uninitialized elements.
Vec will never perform a "small optimization" where elements are actually stored on the stack for two reasons:
It would make it more difficult for unsafe code to correctly manipulate a Vec. The contents of a Vec wouldn't have a stable address if it were only moved, and it would be more difficult to determine if a Vec had actually allocated memory.
It would penalize the general case, incurring an additional branch on every access.
Vec will never automatically shrink itself, even if completely empty. This
ensures no unnecessary allocations or deallocations occur. Emptying a Vec
and then filling it back up to the same len() should incur no calls to
the allocator. If you wish to free up unused memory, use shrink_to_fit.
push and insert will never (re)allocate if the reported capacity is
sufficient. push and insert will (re)allocate if len() == capacity().
That is, the reported capacity is completely accurate, and can be relied on.
It can even be used to manually free the memory allocated by a Vec if
desired. Bulk insertion methods may reallocate, even when not necessary.
Vec does not guarantee any particular growth strategy when reallocating
when full, nor when reserve is called. The current strategy is basic
and it may prove desirable to use a non-constant growth factor. Whatever
strategy is used will of course guarantee O(1) amortized push.
vec![x; n], vec![a, b, c, d], and Vec::with_capacity(n), will all
produce a Vec with exactly the requested capacity. If len() == capacity(),
(as is the case for the vec! macro), then a Vec<T> can be converted
to and from a Box<[T]> without reallocating or moving the elements.
Vec will not specifically overwrite any data that is removed from it, but also won't specifically preserve it. Its uninitialized memory is scratch space that it may use however it wants. It will generally just do whatever is most efficient or otherwise easy to implement. Do not rely on removed data to be erased for security purposes. Even if you drop a Vec, its buffer may simply be reused by another Vec. Even if you zero a Vec's memory first, that may not actually happen because the optimizer does not consider this a side-effect that must be preserved.
Vec does not currently guarantee the order in which elements are dropped (the order has changed in the past, and may change again).
Fields
buf | |
len |
Methods
impl<T> Vec<T>
fn new() -> Vec<T>
Constructs a new, empty Vec<T>.
The vector will not allocate until elements are pushed onto it.
Examples
let mut vec: Vec<i32> = Vec::new();
fn with_capacity(capacity: usize) -> Vec<T>
Constructs a new, empty Vec<T> with the specified capacity.
The vector will be able to hold exactly capacity elements without
reallocating. If capacity is 0, the vector will not allocate.
It is important to note that this function does not specify the length
of the returned vector, but only the capacity. (For an explanation of
the difference between length and capacity, see the main Vec<T> docs
above, 'Capacity and reallocation'.)
Examples
let mut vec = Vec::with_capacity(10); // The vector contains no items, even though it has capacity for more assert_eq!(vec.len(), 0); // These are all done without reallocating... for i in 0..10 { vec.push(i); } // ...but this may make the vector reallocate vec.push(11);
unsafe fn from_raw_parts(ptr: *mut T, length: usize, capacity: usize) -> Vec<T>
Creates a Vec<T> directly from the raw components of another vector.
Safety
This is highly unsafe, due to the number of invariants that aren't checked:
ptrneeds to have been previously allocated viaString/Vec<T>(at least, it's highly likely to be incorrect if it wasn't).lengthneeds to be the length that less than or equal tocapacity.capacityneeds to be the capacity that the pointer was allocated with.
Violating these may cause problems like corrupting the allocator's internal datastructures.
Examples
use std::ptr; use std::mem; fn main() { let mut v = vec![1, 2, 3]; // Pull out the various important pieces of information about `v` let p = v.as_mut_ptr(); let len = v.len(); let cap = v.capacity(); unsafe { // Cast `v` into the void: no destructor run, so we are in // complete control of the allocation to which `p` points. mem::forget(v); // Overwrite memory with 4, 5, 6 for i in 0..len as isize { ptr::write(p.offset(i), 4 + i); } // Put everything back together into a Vec let rebuilt = Vec::from_raw_parts(p, len, cap); assert_eq!(rebuilt, [4, 5, 6]); } }
fn capacity(&self) -> usize
Returns the number of elements the vector can hold without reallocating.
Examples
let vec: Vec<i32> = Vec::with_capacity(10); assert_eq!(vec.capacity(), 10);
fn reserve(&mut self, additional: usize)
Reserves capacity for at least additional more elements to be inserted
in the given Vec<T>. The collection may reserve more space to avoid
frequent reallocations.
Panics
Panics if the new capacity overflows usize.
Examples
let mut vec = vec![1]; vec.reserve(10); assert!(vec.capacity() >= 11);
fn reserve_exact(&mut self, additional: usize)
Reserves the minimum capacity for exactly additional more elements to
be inserted in the given Vec<T>. Does nothing if the capacity is already
sufficient.
Note that the allocator may give the collection more space than it
requests. Therefore capacity can not be relied upon to be precisely
minimal. Prefer reserve if future insertions are expected.
Panics
Panics if the new capacity overflows usize.
Examples
let mut vec = vec![1]; vec.reserve_exact(10); assert!(vec.capacity() >= 11);
fn shrink_to_fit(&mut self)
Shrinks the capacity of the vector as much as possible.
It will drop down as close as possible to the length but the allocator may still inform the vector that there is space for a few more elements.
Examples
let mut vec = Vec::with_capacity(10); vec.extend([1, 2, 3].iter().cloned()); assert_eq!(vec.capacity(), 10); vec.shrink_to_fit(); assert!(vec.capacity() >= 3);
fn into_boxed_slice(self) -> Box<[T]>
Converts the vector into Box<[T]>.
Note that this will drop any excess capacity. Calling this and
converting back to a vector with into_vec() is equivalent to calling
shrink_to_fit().
fn truncate(&mut self, len: usize)
Shorten a vector to be len elements long, dropping excess elements.
If len is greater than the vector's current length, this has no
effect.
Examples
let mut vec = vec![1, 2, 3, 4, 5]; vec.truncate(2); assert_eq!(vec, [1, 2]);
fn as_slice(&self) -> &[T]
Extracts a slice containing the entire vector.
Equivalent to &s[..].
fn as_mut_slice(&mut self) -> &mut [T]
Extracts a mutable slice of the entire vector.
Equivalent to &mut s[..].
unsafe fn set_len(&mut self, len: usize)
Sets the length of a vector.
This will explicitly set the size of the vector, without actually modifying its buffers, so it is up to the caller to ensure that the vector is actually the specified size.
Examples
let mut v = vec![1, 2, 3, 4]; unsafe { v.set_len(1); }
fn swap_remove(&mut self, index: usize) -> T
Removes an element from anywhere in the vector and return it, replacing it with the last element.
This does not preserve ordering, but is O(1).
Panics
Panics if index is out of bounds.
Examples
let mut v = vec!["foo", "bar", "baz", "qux"]; assert_eq!(v.swap_remove(1), "bar"); assert_eq!(v, ["foo", "qux", "baz"]); assert_eq!(v.swap_remove(0), "foo"); assert_eq!(v, ["baz", "qux"]);
fn insert(&mut self, index: usize, element: T)
Inserts an element at position index within the vector, shifting all
elements after position i one position to the right.
Panics
Panics if index is greater than the vector's length.
Examples
let mut vec = vec![1, 2, 3]; vec.insert(1, 4); assert_eq!(vec, [1, 4, 2, 3]); vec.insert(4, 5); assert_eq!(vec, [1, 4, 2, 3, 5]);
fn remove(&mut self, index: usize) -> T
Removes and returns the element at position index within the vector,
shifting all elements after position index one position to the left.
Panics
Panics if index is out of bounds.
Examples
let mut v = vec![1, 2, 3]; assert_eq!(v.remove(1), 2); assert_eq!(v, [1, 3]);
fn retain<F>(&mut self, f: F) where F: FnMut(&T) -> bool
Retains only the elements specified by the predicate.
In other words, remove all elements e such that f(&e) returns false.
This method operates in place and preserves the order of the retained
elements.
Examples
let mut vec = vec![1, 2, 3, 4]; vec.retain(|&x| x%2 == 0); assert_eq!(vec, [2, 4]);
fn push(&mut self, value: T)
Appends an element to the back of a collection.
Panics
Panics if the number of elements in the vector overflows a usize.
Examples
let mut vec = vec![1, 2]; vec.push(3); assert_eq!(vec, [1, 2, 3]);
fn pop(&mut self) -> Option<T>
Removes the last element from a vector and returns it, or None if it
is empty.
Examples
let mut vec = vec![1, 2, 3]; assert_eq!(vec.pop(), Some(3)); assert_eq!(vec, [1, 2]);
fn append(&mut self, other: &mut Vec<T>)
Moves all the elements of other into Self, leaving other empty.
Panics
Panics if the number of elements in the vector overflows a usize.
Examples
let mut vec = vec![1, 2, 3]; let mut vec2 = vec![4, 5, 6]; vec.append(&mut vec2); assert_eq!(vec, [1, 2, 3, 4, 5, 6]); assert_eq!(vec2, []);
fn drain<R>(&mut self, range: R) -> Drain<T> where R: RangeArgument<usize>
Create a draining iterator that removes the specified range in the vector and yields the removed items.
Note 1: The element range is removed even if the iterator is not consumed until the end.
Note 2: It is unspecified how many elements are removed from the vector,
if the Drain value is leaked.
Panics
Panics if the starting point is greater than the end point or if the end point is greater than the length of the vector.
Examples
let mut v = vec![1, 2, 3]; let u: Vec<_> = v.drain(1..).collect(); assert_eq!(v, &[1]); assert_eq!(u, &[2, 3]); // A full range clears the vector v.drain(..); assert_eq!(v, &[]);
fn clear(&mut self)
Clears the vector, removing all values.
Examples
let mut v = vec![1, 2, 3]; v.clear(); assert!(v.is_empty());
fn len(&self) -> usize
Returns the number of elements in the vector.
Examples
let a = vec![1, 2, 3]; assert_eq!(a.len(), 3);
fn is_empty(&self) -> bool
Returns true if the vector contains no elements.
Examples
let mut v = Vec::new(); assert!(v.is_empty()); v.push(1); assert!(!v.is_empty());
fn split_off(&mut self, at: usize) -> Vec<T>
Splits the collection into two at the given index.
Returns a newly allocated Self. self contains elements [0, at),
and the returned Self contains elements [at, len).
Note that the capacity of self does not change.
Panics
Panics if at > len.
Examples
let mut vec = vec![1,2,3]; let vec2 = vec.split_off(1); assert_eq!(vec, [1]); assert_eq!(vec2, [2, 3]);
impl<T> Vec<T> where T: Clone
fn resize(&mut self, new_len: usize, value: T)
Resizes the Vec in-place so that len() is equal to new_len.
If new_len is greater than len(), the Vec is extended by the
difference, with each additional slot filled with value.
If new_len is less than len(), the Vec is simply truncated.
Examples
let mut vec = vec!["hello"]; vec.resize(3, "world"); assert_eq!(vec, ["hello", "world", "world"]); let mut vec = vec![1, 2, 3, 4]; vec.resize(2, 0); assert_eq!(vec, [1, 2]);
fn push_all(&mut self, other: &[T])
: renamed to extend_from_slice
fn extend_from_slice(&mut self, other: &[T])
Appends all elements in a slice to the Vec.
Iterates over the slice other, clones each element, and then appends
it to this Vec. The other vector is traversed in-order.
Note that this function is same as extend except that it is
specialized to work with slices instead. If and when Rust gets
specialization this function will likely be deprecated (but still
available).
Examples
let mut vec = vec![1]; vec.extend_from_slice(&[2, 3, 4]); assert_eq!(vec, [1, 2, 3, 4]);
impl<T> Vec<T> where T: PartialEq<T>
fn dedup(&mut self)
Removes consecutive repeated elements in the vector.
If the vector is sorted, this removes all duplicates.
Examples
let mut vec = vec![1, 2, 2, 3, 2]; vec.dedup(); assert_eq!(vec, [1, 2, 3, 2]);