# Struct alloc::collections::vec_deque::VecDeque

1.0.0 · source · [−]

    pub struct VecDeque<T, A: Allocator = Global> { /* private fields */ }

Expand description

A double-ended queue implemented with a growable ring buffer.

The “default” usage of this type as a queue is to use `push_back` to add to the queue, and `pop_front` to remove from the queue. `extend` and `append` push onto the back in this manner, and iterating over `VecDeque` goes front to back.

A `VecDeque` with a known list of items can be initialized from an array:

    use std::collections::VecDeque;

    let deq = VecDeque::from([-1, 0, 1]);

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Since `VecDeque` is a ring buffer, its elements are not necessarily contiguous in memory. If you want to access the elements as a single slice, such as for efficient sorting, you can use `make_contiguous`. It rotates the `VecDeque` so that its elements do not wrap, and returns a mutable slice to the now-contiguous element sequence.

## Implementations

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### impl<T> VecDeque<T>

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#### pub fn new() -> VecDeque<T>

Creates an empty deque.

##### Examples

    use std::collections::VecDeque;

    let deque: VecDeque<u32> = VecDeque::new();

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#### pub fn with_capacity(capacity: usize) -> VecDeque<T>

Creates an empty deque with space for at least `capacity` elements.

##### Examples

    use std::collections::VecDeque;

    let deque: VecDeque<u32> = VecDeque::with_capacity(10);

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### impl<T, A: Allocator> VecDeque<T, A>

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#### pub fn new_in(alloc: A) -> VecDeque<T, A>

🔬This is a nightly-only experimental API. (`allocator_api` #32838)

Creates an empty deque.

##### Examples

    use std::collections::VecDeque;

    let deque: VecDeque<u32> = VecDeque::new();

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#### pub fn with_capacity_in(capacity: usize, alloc: A) -> VecDeque<T, A>

🔬This is a nightly-only experimental API. (`allocator_api` #32838)

Creates an empty deque with space for at least `capacity` elements.

##### Examples

    use std::collections::VecDeque;

    let deque: VecDeque<u32> = VecDeque::with_capacity(10);

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#### pub fn get(&self, index: usize) -> Option<&T>

Provides a reference to the element at the given index.

Element at index 0 is the front of the queue.

##### Examples

    use std::collections::VecDeque;

    let mut buf = VecDeque::new();
    buf.push_back(3);
    buf.push_back(4);
    buf.push_back(5);
    assert_eq!(buf.get(1), Some(&4));

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#### pub fn get_mut(&mut self, index: usize) -> Option<&mut T>

Provides a mutable reference to the element at the given index.

Element at index 0 is the front of the queue.

##### Examples

    use std::collections::VecDeque;

    let mut buf = VecDeque::new();
    buf.push_back(3);
    buf.push_back(4);
    buf.push_back(5);
    if let Some(elem) = buf.get_mut(1) {
        *elem = 7;
    }

    assert_eq!(buf[1], 7);

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#### pub fn swap(&mut self, i: usize, j: usize)

Swaps elements at indices `i` and `j`.

`i` and `j` may be equal.

Element at index 0 is the front of the queue.

##### Panics

Panics if either index is out of bounds.

##### Examples

    use std::collections::VecDeque;

    let mut buf = VecDeque::new();
    buf.push_back(3);
    buf.push_back(4);
    buf.push_back(5);
    assert_eq!(buf, [3, 4, 5]);
    buf.swap(0, 2);
    assert_eq!(buf, [5, 4, 3]);

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#### pub fn capacity(&self) -> usize

Returns the number of elements the deque can hold without reallocating.

##### Examples

    use std::collections::VecDeque;

    let buf: VecDeque<i32> = VecDeque::with_capacity(10);
    assert!(buf.capacity() >= 10);

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#### pub fn reserve_exact(&mut self, additional: usize)

Reserves the minimum capacity for at least `additional` more elements to be inserted in the given deque. 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

    use std::collections::VecDeque;

    let mut buf: VecDeque<i32> = [1].into();
    buf.reserve_exact(10);
    assert!(buf.capacity() >= 11);

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#### pub fn reserve(&mut self, additional: usize)

Reserves capacity for at least `additional` more elements to be inserted in the given deque. The collection may reserve more space to speculatively avoid frequent reallocations.

##### Panics

Panics if the new capacity overflows `usize`.

##### Examples

    use std::collections::VecDeque;

    let mut buf: VecDeque<i32> = [1].into();
    buf.reserve(10);
    assert!(buf.capacity() >= 11);

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1.57.0 · source

#### pub fn try_reserve_exact(
&mut self,
additional: usize
) -> Result<(), TryReserveError>

Tries to reserve the minimum capacity for at least `additional` more elements to be inserted in the given deque. After calling `try_reserve_exact`, capacity will be greater than or equal to `self.len() + additional` if it returns `Ok(())`. Does nothing if the capacity is already sufficient.

Note that the allocator may give the collection more space than it requests. Therefore, capacity can not be relied upon to be precisely minimal. Prefer `try_reserve` if future insertions are expected.

##### Errors

If the capacity overflows `usize`, or the allocator reports a failure, then an error is returned.

##### Examples

    use std::collections::TryReserveError;
    use std::collections::VecDeque;

    fn process_data(data: &[u32]) -> Result<VecDeque<u32>, TryReserveError> {
        let mut output = VecDeque::new();

        // Pre-reserve the memory, exiting if we can't
        output.try_reserve_exact(data.len())?;

        // Now we know this can't OOM(Out-Of-Memory) in the middle of our complex work
        output.extend(data.iter().map(|&val| {
            val * 2 + 5 // very complicated
        }));

        Ok(output)
    }

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1.57.0 · source

#### pub fn try_reserve(&mut self, additional: usize) -> Result<(), TryReserveError>

Tries to reserve capacity for at least `additional` more elements to be inserted in the given deque. The collection may reserve more space to speculatively avoid frequent reallocations. After calling `try_reserve`, capacity will be greater than or equal to `self.len() + additional` if it returns `Ok(())`. Does nothing if capacity is already sufficient. This method preserves the contents even if an error occurs.

##### Errors

If the capacity overflows `usize`, or the allocator reports a failure, then an error is returned.

##### Examples

    use std::collections::TryReserveError;
    use std::collections::VecDeque;

    fn process_data(data: &[u32]) -> Result<VecDeque<u32>, TryReserveError> {
        let mut output = VecDeque::new();

        // Pre-reserve the memory, exiting if we can't
        output.try_reserve(data.len())?;

        // Now we know this can't OOM in the middle of our complex work
        output.extend(data.iter().map(|&val| {
            val * 2 + 5 // very complicated
        }));

        Ok(output)
    }

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1.5.0 · source

#### pub fn shrink_to_fit(&mut self)

Shrinks the capacity of the deque as much as possible.

It will drop down as close as possible to the length but the allocator may still inform the deque that there is space for a few more elements.

##### Examples

    use std::collections::VecDeque;

    let mut buf = VecDeque::with_capacity(15);
    buf.extend(0..4);
    assert_eq!(buf.capacity(), 15);
    buf.shrink_to_fit();
    assert!(buf.capacity() >= 4);

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1.56.0 · source

#### pub fn shrink_to(&mut self, min_capacity: usize)

Shrinks the capacity of the deque with a lower bound.

The capacity will remain at least as large as both the length and the supplied value.

If the current capacity is less than the lower limit, this is a no-op.

##### Examples

    use std::collections::VecDeque;

    let mut buf = VecDeque::with_capacity(15);
    buf.extend(0..4);
    assert_eq!(buf.capacity(), 15);
    buf.shrink_to(6);
    assert!(buf.capacity() >= 6);
    buf.shrink_to(0);
    assert!(buf.capacity() >= 4);

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1.16.0 · source

#### pub fn truncate(&mut self, len: usize)

Shortens the deque, keeping the first `len` elements and dropping the rest.

If `len` is greater than the deque’s current length, this has no effect.

##### Examples

    use std::collections::VecDeque;

    let mut buf = VecDeque::new();
    buf.push_back(5);
    buf.push_back(10);
    buf.push_back(15);
    assert_eq!(buf, [5, 10, 15]);
    buf.truncate(1);
    assert_eq!(buf, [5]);

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#### pub fn allocator(&self) -> &A

🔬This is a nightly-only experimental API. (`allocator_api` #32838)

Returns a reference to the underlying allocator.

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#### pub fn iter(&self) -> Iter<'_, T>ⓘNotable traits for Iter<'a, T>`impl<'a, T> Iterator for Iter<'a, T> type Item = &'a T;`

Returns a front-to-back iterator.

##### Examples

    use std::collections::VecDeque;

    let mut buf = VecDeque::new();
    buf.push_back(5);
    buf.push_back(3);
    buf.push_back(4);
    let b: &[_] = &[&5, &3, &4];
    let c: Vec<&i32> = buf.iter().collect();
    assert_eq!(&c[..], b);

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#### pub fn iter_mut(&mut self) -> IterMut<'_, T>ⓘNotable traits for IterMut<'a, T>`impl<'a, T> Iterator for IterMut<'a, T> type Item = &'a mut T;`

Returns a front-to-back iterator that returns mutable references.

##### Examples

    use std::collections::VecDeque;

    let mut buf = VecDeque::new();
    buf.push_back(5);
    buf.push_back(3);
    buf.push_back(4);
    for num in buf.iter_mut() {
        *num = *num - 2;
    }
    let b: &[_] = &[&mut 3, &mut 1, &mut 2];
    assert_eq!(&buf.iter_mut().collect::<Vec<&mut i32>>()[..], b);

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1.5.0 · source

#### pub fn as_slices(&self) -> (&[T], &[T])

Returns a pair of slices which contain, in order, the contents of the deque.

If `make_contiguous` was previously called, all elements of the deque will be in the first slice and the second slice will be empty.

##### Examples

    use std::collections::VecDeque;

    let mut deque = VecDeque::new();

    deque.push_back(0);
    deque.push_back(1);
    deque.push_back(2);

    assert_eq!(deque.as_slices(), (&[0, 1, 2][..], &[][..]));

    deque.push_front(10);
    deque.push_front(9);

    assert_eq!(deque.as_slices(), (&[9, 10][..], &[0, 1, 2][..]));

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1.5.0 · source

#### pub fn as_mut_slices(&mut self) -> (&mut [T], &mut [T])

Returns a pair of slices which contain, in order, the contents of the deque.

If `make_contiguous` was previously called, all elements of the deque will be in the first slice and the second slice will be empty.

##### Examples

    use std::collections::VecDeque;

    let mut deque = VecDeque::new();

    deque.push_back(0);
    deque.push_back(1);

    deque.push_front(10);
    deque.push_front(9);

    deque.as_mut_slices().0[0] = 42;
    deque.as_mut_slices().1[0] = 24;
    assert_eq!(deque.as_slices(), (&[42, 10][..], &[24, 1][..]));

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#### pub fn len(&self) -> usize

Returns the number of elements in the deque.

##### Examples

    use std::collections::VecDeque;

    let mut deque = VecDeque::new();
    assert_eq!(deque.len(), 0);
    deque.push_back(1);
    assert_eq!(deque.len(), 1);

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#### pub fn is_empty(&self) -> bool

Returns `true` if the deque is empty.

##### Examples

    use std::collections::VecDeque;

    let mut deque = VecDeque::new();
    assert!(deque.is_empty());
    deque.push_front(1);
    assert!(!deque.is_empty());

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1.51.0 · source

#### pub fn range<R>(&self, range: R) -> Iter<'_, T>ⓘNotable traits for Iter<'a, T>`impl<'a, T> Iterator for Iter<'a, T> type Item = &'a T;`where
R: RangeBounds<usize>,

Creates an iterator that covers the specified range in the deque.

##### Panics

Panics if the starting point is greater than the end point or if the end point is greater than the length of the deque.

##### Examples

    use std::collections::VecDeque;

    let deque: VecDeque<_> = [1, 2, 3].into();
    let range = deque.range(2..).copied().collect::<VecDeque<_>>();
    assert_eq!(range, [3]);

    // A full range covers all contents
    let all = deque.range(..);
    assert_eq!(all.len(), 3);

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1.51.0 · source

#### pub fn range_mut<R>(&mut self, range: R) -> IterMut<'_, T>ⓘNotable traits for IterMut<'a, T>`impl<'a, T> Iterator for IterMut<'a, T> type Item = &'a mut T;`where
R: RangeBounds<usize>,

Creates an iterator that covers the specified mutable range in the deque.

##### Panics

Panics if the starting point is greater than the end point or if the end point is greater than the length of the deque.

##### Examples

    use std::collections::VecDeque;

    let mut deque: VecDeque<_> = [1, 2, 3].into();
    for v in deque.range_mut(2..) {
      *v *= 2;
    }
    assert_eq!(deque, [1, 2, 6]);

    // A full range covers all contents
    for v in deque.range_mut(..) {
      *v *= 2;
    }
    assert_eq!(deque, [2, 4, 12]);

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1.6.0 · source

#### pub fn drain<R>(&mut self, range: R) -> Drain<'_, T, A>ⓘNotable traits for Drain<'_, T, A>`impl<T, A: Allocator> Iterator for Drain<'_, T, A> type Item = T;`where
R: RangeBounds<usize>,

Removes the specified range from the deque in bulk, returning all removed elements as an iterator. If the iterator is dropped before being fully consumed, it drops the remaining removed elements.

The returned iterator keeps a mutable borrow on the queue to optimize its implementation.

##### Panics

Panics if the starting point is greater than the end point or if the end point is greater than the length of the deque.

##### Leaking

If the returned iterator goes out of scope without being dropped (due to `mem::forget`, for example), the deque may have lost and leaked elements arbitrarily, including elements outside the range.

##### Examples

    use std::collections::VecDeque;

    let mut deque: VecDeque<_> = [1, 2, 3].into();
    let drained = deque.drain(2..).collect::<VecDeque<_>>();
    assert_eq!(drained, [3]);
    assert_eq!(deque, [1, 2]);

    // A full range clears all contents, like `clear()` does
    deque.drain(..);
    assert!(deque.is_empty());

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#### pub fn clear(&mut self)

Clears the deque, removing all values.

##### Examples

    use std::collections::VecDeque;

    let mut deque = VecDeque::new();
    deque.push_back(1);
    deque.clear();
    assert!(deque.is_empty());

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1.12.0 · source

#### pub fn contains(&self, x: &T) -> bool where
T: PartialEq<T>,

Returns `true` if the deque contains an element equal to the given value.

This operation is O(n).

Note that if you have a sorted `VecDeque`, `binary_search` may be faster.

##### Examples

    use std::collections::VecDeque;

    let mut deque: VecDeque<u32> = VecDeque::new();

    deque.push_back(0);
    deque.push_back(1);

    assert_eq!(deque.contains(&1), true);
    assert_eq!(deque.contains(&10), false);

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#### pub fn front(&self) -> Option<&T>

Provides a reference to the front element, or `None` if the deque is empty.

##### Examples

    use std::collections::VecDeque;

    let mut d = VecDeque::new();
    assert_eq!(d.front(), None);

    d.push_back(1);
    d.push_back(2);
    assert_eq!(d.front(), Some(&1));

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#### pub fn front_mut(&mut self) -> Option<&mut T>

Provides a mutable reference to the front element, or `None` if the deque is empty.

##### Examples

    use std::collections::VecDeque;

    let mut d = VecDeque::new();
    assert_eq!(d.front_mut(), None);

    d.push_back(1);
    d.push_back(2);
    match d.front_mut() {
        Some(x) => *x = 9,
        None => (),
    }
    assert_eq!(d.front(), Some(&9));

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#### pub fn back(&self) -> Option<&T>

Provides a reference to the back element, or `None` if the deque is empty.

##### Examples

    use std::collections::VecDeque;

    let mut d = VecDeque::new();
    assert_eq!(d.back(), None);

    d.push_back(1);
    d.push_back(2);
    assert_eq!(d.back(), Some(&2));

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#### pub fn back_mut(&mut self) -> Option<&mut T>

Provides a mutable reference to the back element, or `None` if the deque is empty.

##### Examples

    use std::collections::VecDeque;

    let mut d = VecDeque::new();
    assert_eq!(d.back(), None);

    d.push_back(1);
    d.push_back(2);
    match d.back_mut() {
        Some(x) => *x = 9,
        None => (),
    }
    assert_eq!(d.back(), Some(&9));

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#### pub fn pop_front(&mut self) -> Option<T>

Removes the first element and returns it, or `None` if the deque is empty.

##### Examples

    use std::collections::VecDeque;

    let mut d = VecDeque::new();
    d.push_back(1);
    d.push_back(2);

    assert_eq!(d.pop_front(), Some(1));
    assert_eq!(d.pop_front(), Some(2));
    assert_eq!(d.pop_front(), None);

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#### pub fn pop_back(&mut self) -> Option<T>

Removes the last element from the deque and returns it, or `None` if it is empty.

##### Examples

    use std::collections::VecDeque;

    let mut buf = VecDeque::new();
    assert_eq!(buf.pop_back(), None);
    buf.push_back(1);
    buf.push_back(3);
    assert_eq!(buf.pop_back(), Some(3));

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#### pub fn push_front(&mut self, value: T)

Prepends an element to the deque.

##### Examples

    use std::collections::VecDeque;

    let mut d = VecDeque::new();
    d.push_front(1);
    d.push_front(2);
    assert_eq!(d.front(), Some(&2));

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#### pub fn push_back(&mut self, value: T)

Appends an element to the back of the deque.

##### Examples

    use std::collections::VecDeque;

    let mut buf = VecDeque::new();
    buf.push_back(1);
    buf.push_back(3);
    assert_eq!(3, *buf.back().unwrap());

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1.5.0 · source

#### pub fn swap_remove_front(&mut self, index: usize) -> Option<T>

Removes an element from anywhere in the deque and returns it, replacing it with the first element.

This does not preserve ordering, but is O(1).

Returns `None` if `index` is out of bounds.

Element at index 0 is the front of the queue.

##### Examples

    use std::collections::VecDeque;

    let mut buf = VecDeque::new();
    assert_eq!(buf.swap_remove_front(0), None);
    buf.push_back(1);
    buf.push_back(2);
    buf.push_back(3);
    assert_eq!(buf, [1, 2, 3]);

    assert_eq!(buf.swap_remove_front(2), Some(3));
    assert_eq!(buf, [2, 1]);

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1.5.0 · source

#### pub fn swap_remove_back(&mut self, index: usize) -> Option<T>

Removes an element from anywhere in the deque and returns it, replacing it with the last element.

This does not preserve ordering, but is O(1).

Returns `None` if `index` is out of bounds.

Element at index 0 is the front of the queue.

##### Examples

    use std::collections::VecDeque;

    let mut buf = VecDeque::new();
    assert_eq!(buf.swap_remove_back(0), None);
    buf.push_back(1);
    buf.push_back(2);
    buf.push_back(3);
    assert_eq!(buf, [1, 2, 3]);

    assert_eq!(buf.swap_remove_back(0), Some(1));
    assert_eq!(buf, [3, 2]);

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1.5.0 · source

#### pub fn insert(&mut self, index: usize, value: T)

Inserts an element at `index` within the deque, shifting all elements with indices greater than or equal to `index` towards the back.

Element at index 0 is the front of the queue.

##### Panics

Panics if `index` is greater than deque’s length

##### Examples

    use std::collections::VecDeque;

    let mut vec_deque = VecDeque::new();
    vec_deque.push_back('a');
    vec_deque.push_back('b');
    vec_deque.push_back('c');
    assert_eq!(vec_deque, &['a', 'b', 'c']);

    vec_deque.insert(1, 'd');
    assert_eq!(vec_deque, &['a', 'd', 'b', 'c']);

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#### pub fn remove(&mut self, index: usize) -> Option<T>

Removes and returns the element at `index` from the deque. Whichever end is closer to the removal point will be moved to make room, and all the affected elements will be moved to new positions. Returns `None` if `index` is out of bounds.

Element at index 0 is the front of the queue.

##### Examples

    use std::collections::VecDeque;

    let mut buf = VecDeque::new();
    buf.push_back(1);
    buf.push_back(2);
    buf.push_back(3);
    assert_eq!(buf, [1, 2, 3]);

    assert_eq!(buf.remove(1), Some(2));
    assert_eq!(buf, [1, 3]);

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1.4.0 · source

#### pub fn split_off(&mut self, at: usize) -> Selfwhere
A: Clone,

Splits the deque into two at the given index.

Returns a newly allocated `VecDeque`. `self` contains elements `[0, at)`, and the returned deque contains elements `[at, len)`.

Note that the capacity of `self` does not change.

Element at index 0 is the front of the queue.

##### Panics

Panics if `at > len`.

##### Examples

    use std::collections::VecDeque;

    let mut buf: VecDeque<_> = [1, 2, 3].into();
    let buf2 = buf.split_off(1);
    assert_eq!(buf, [1]);
    assert_eq!(buf2, [2, 3]);

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1.4.0 · source

#### pub fn append(&mut self, other: &mut Self)

Moves all the elements of `other` into `self`, leaving `other` empty.

##### Panics

Panics if the new number of elements in self overflows a `usize`.

##### Examples

    use std::collections::VecDeque;

    let mut buf: VecDeque<_> = [1, 2].into();
    let mut buf2: VecDeque<_> = [3, 4].into();
    buf.append(&mut buf2);
    assert_eq!(buf, [1, 2, 3, 4]);
    assert_eq!(buf2, []);

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1.4.0 · source

#### pub 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` for which `f(&e)` returns false. This method operates in place, visiting each element exactly once in the original order, and preserves the order of the retained elements.

##### Examples

    use std::collections::VecDeque;

    let mut buf = VecDeque::new();
    buf.extend(1..5);
    buf.retain(|&x| x % 2 == 0);
    assert_eq!(buf, [2, 4]);

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Because the elements are visited exactly once in the original order, external state may be used to decide which elements to keep.

    use std::collections::VecDeque;

    let mut buf = VecDeque::new();
    buf.extend(1..6);

    let keep = [false, true, true, false, true];
    let mut iter = keep.iter();
    buf.retain(|_| *iter.next().unwrap());
    assert_eq!(buf, [2, 3, 5]);

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1.61.0 · source

#### pub fn retain_mut<F>(&mut self, f: F)where
F: FnMut(&mut T) -> bool,

Retains only the elements specified by the predicate.

In other words, remove all elements `e` for which `f(&e)` returns false. This method operates in place, visiting each element exactly once in the original order, and preserves the order of the retained elements.

##### Examples

    use std::collections::VecDeque;

    let mut buf = VecDeque::new();
    buf.extend(1..5);
    buf.retain_mut(|x| if *x % 2 == 0 {
        *x += 1;
        true
    } else {
        false
    });
    assert_eq!(buf, [3, 5]);

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1.33.0 · source

#### pub fn resize_with(&mut self, new_len: usize, generator: impl FnMut() -> T)

Modifies the deque in-place so that `len()` is equal to `new_len`, either by removing excess elements from the back or by appending elements generated by calling `generator` to the back.

##### Examples

    use std::collections::VecDeque;

    let mut buf = VecDeque::new();
    buf.push_back(5);
    buf.push_back(10);
    buf.push_back(15);
    assert_eq!(buf, [5, 10, 15]);

    buf.resize_with(5, Default::default);
    assert_eq!(buf, [5, 10, 15, 0, 0]);

    buf.resize_with(2, || unreachable!());
    assert_eq!(buf, [5, 10]);

    let mut state = 100;
    buf.resize_with(5, || { state += 1; state });
    assert_eq!(buf, [5, 10, 101, 102, 103]);

Run
1.48.0 · source

#### pub fn make_contiguous(&mut self) -> &mut [T]

Rearranges the internal storage of this deque so it is one contiguous slice, which is then returned.

This method does not allocate and does not change the order of the inserted elements. As it returns a mutable slice, this can be used to sort a deque.

Once the internal storage is contiguous, the `as_slices` and `as_mut_slices` methods will return the entire contents of the deque in a single slice.

##### Examples

Sorting the content of a deque.

    use std::collections::VecDeque;

    let mut buf = VecDeque::with_capacity(15);

    buf.push_back(2);
    buf.push_back(1);
    buf.push_front(3);

    // sorting the deque
    buf.make_contiguous().sort();
    assert_eq!(buf.as_slices(), (&[1, 2, 3] as &[_], &[] as &[_]));

    // sorting it in reverse order
    buf.make_contiguous().sort_by(|a, b| b.cmp(a));
    assert_eq!(buf.as_slices(), (&[3, 2, 1] as &[_], &[] as &[_]));

Run

Getting immutable access to the contiguous slice.

    use std::collections::VecDeque;

    let mut buf = VecDeque::new();

    buf.push_back(2);
    buf.push_back(1);
    buf.push_front(3);

    buf.make_contiguous();
    if let (slice, &[]) = buf.as_slices() {
        // we can now be sure that `slice` contains all elements of the deque,
        // while still having immutable access to `buf`.
        assert_eq!(buf.len(), slice.len());
        assert_eq!(slice, &[3, 2, 1] as &[_]);
    }

Run
1.36.0 · source

#### pub fn rotate_left(&mut self, mid: usize)

Rotates the double-ended queue `mid` places to the left.

Equivalently,

- Rotates item `mid` into the first position.
- Pops the first `mid` items and pushes them to the end.
- Rotates `len() - mid` places to the right.

##### Panics

If `mid` is greater than `len()`. Note that `mid == len()` does not panic and is a no-op rotation.

##### Complexity

Takes `*O*(min(mid, len() - mid))` time and no extra space.

##### Examples

    use std::collections::VecDeque;

    let mut buf: VecDeque<_> = (0..10).collect();

    buf.rotate_left(3);
    assert_eq!(buf, [3, 4, 5, 6, 7, 8, 9, 0, 1, 2]);

    for i in 1..10 {
        assert_eq!(i * 3 % 10, buf[0]);
        buf.rotate_left(3);
    }
    assert_eq!(buf, [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]);

Run
1.36.0 · source

#### pub fn rotate_right(&mut self, k: usize)

Rotates the double-ended queue `k` places to the right.

Equivalently,

- Rotates the first item into position `k`.
- Pops the last `k` items and pushes them to the front.
- Rotates `len() - k` places to the left.

##### Panics

If `k` is greater than `len()`. Note that `k == len()` does not panic and is a no-op rotation.

##### Complexity

Takes `*O*(min(k, len() - k))` time and no extra space.

##### Examples

    use std::collections::VecDeque;

    let mut buf: VecDeque<_> = (0..10).collect();

    buf.rotate_right(3);
    assert_eq!(buf, [7, 8, 9, 0, 1, 2, 3, 4, 5, 6]);

    for i in 1..10 {
        assert_eq!(0, buf[i * 3 % 10]);
        buf.rotate_right(3);
    }
    assert_eq!(buf, [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]);

Run
1.54.0 · source

#### pub fn binary_search(&self, x: &T) -> Result<usize, usize>where
T: Ord,

Binary searches this `VecDeque` for a given element. This behaves similarly to `contains` if this `VecDeque` is sorted.

If the value is found then `Result::Ok` is returned, containing the index of the matching element. If there are multiple matches, then any one of the matches could be returned. If the value is not found then `Result::Err` is returned, containing the index where a matching element could be inserted while maintaining sorted order.

See also `binary_search_by`, `binary_search_by_key`, and `partition_point`.

##### Examples

Looks up a series of four elements. The first is found, with a uniquely determined position; the second and third are not found; the fourth could match any position in `[1, 4]`.

    use std::collections::VecDeque;

    let deque: VecDeque<_> = [0, 1, 1, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55].into();

    assert_eq!(deque.binary_search(&13),  Ok(9));
    assert_eq!(deque.binary_search(&4),   Err(7));
    assert_eq!(deque.binary_search(&100), Err(13));
    let r = deque.binary_search(&1);
    assert!(matches!(r, Ok(1..=4)));

Run

If you want to insert an item to a sorted deque, while maintaining sort order, consider using `partition_point`:

    use std::collections::VecDeque;

    let mut deque: VecDeque<_> = [0, 1, 1, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55].into();
    let num = 42;
    let idx = deque.partition_point(|&x| x < num);
    // The above is equivalent to `let idx = deque.binary_search(&num).unwrap_or_else(|x| x);`
    deque.insert(idx, num);
    assert_eq!(deque, &[0, 1, 1, 1, 1, 2, 3, 5, 8, 13, 21, 34, 42, 55]);

Run
1.54.0 · source

#### pub fn binary_search_by<'a, F>(&'a self, f: F) -> Result<usize, usize>where
F: FnMut(&'a T) -> Ordering,

Binary searches this `VecDeque` with a comparator function. This behaves similarly to `contains` if this `VecDeque` is sorted.

The comparator function should implement an order consistent with the sort order of the deque, returning an order code that indicates whether its argument is `Less`, `Equal` or `Greater` than the desired target.

If the value is found then `Result::Ok` is returned, containing the index of the matching element. If there are multiple matches, then any one of the matches could be returned. If the value is not found then `Result::Err` is returned, containing the index where a matching element could be inserted while maintaining sorted order.

See also `binary_search`, `binary_search_by_key`, and `partition_point`.

##### Examples

Looks up a series of four elements. The first is found, with a uniquely determined position; the second and third are not found; the fourth could match any position in `[1, 4]`.

    use std::collections::VecDeque;

    let deque: VecDeque<_> = [0, 1, 1, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55].into();

    assert_eq!(deque.binary_search_by(|x| x.cmp(&13)),  Ok(9));
    assert_eq!(deque.binary_search_by(|x| x.cmp(&4)),   Err(7));
    assert_eq!(deque.binary_search_by(|x| x.cmp(&100)), Err(13));
    let r = deque.binary_search_by(|x| x.cmp(&1));
    assert!(matches!(r, Ok(1..=4)));

Run
1.54.0 · source

#### pub fn binary_search_by_key<'a, B, F>(
&'a self,
b: &B,
f: F
) -> Result<usize, usize>where
F: FnMut(&'a T) -> B,
B: Ord,

Binary searches this `VecDeque` with a key extraction function. This behaves similarly to `contains` if this `VecDeque` is sorted.

Assumes that the deque is sorted by the key, for instance with `make_contiguous().sort_by_key()` using the same key extraction function.

If the value is found then `Result::Ok` is returned, containing the index of the matching element. If there are multiple matches, then any one of the matches could be returned. If the value is not found then `Result::Err` is returned, containing the index where a matching element could be inserted while maintaining sorted order.

See also `binary_search`, `binary_search_by`, and `partition_point`.

##### Examples

Looks up a series of four elements in a slice of pairs sorted by their second elements. The first is found, with a uniquely determined position; the second and third are not found; the fourth could match any position in `[1, 4]`.

    use std::collections::VecDeque;

    let deque: VecDeque<_> = [(0, 0), (2, 1), (4, 1), (5, 1),
             (3, 1), (1, 2), (2, 3), (4, 5), (5, 8), (3, 13),
             (1, 21), (2, 34), (4, 55)].into();

    assert_eq!(deque.binary_search_by_key(&13, |&(a, b)| b),  Ok(9));
    assert_eq!(deque.binary_search_by_key(&4, |&(a, b)| b),   Err(7));
    assert_eq!(deque.binary_search_by_key(&100, |&(a, b)| b), Err(13));
    let r = deque.binary_search_by_key(&1, |&(a, b)| b);
    assert!(matches!(r, Ok(1..=4)));

Run
1.54.0 · source

#### pub fn partition_point<P>(&self, pred: P) -> usize where
P: FnMut(&T) -> bool,

Returns the index of the partition point according to the given predicate (the index of the first element of the second partition).

The deque is assumed to be partitioned according to the given predicate. This means that all elements for which the predicate returns true are at the start of the deque and all elements for which the predicate returns false are at the end. For example, [7, 15, 3, 5, 4, 12, 6] is a partitioned under the predicate x % 2 != 0 (all odd numbers are at the start, all even at the end).

If the deque is not partitioned, the returned result is unspecified and meaningless, as this method performs a kind of binary search.

See also `binary_search`, `binary_search_by`, and `binary_search_by_key`.

##### Examples

    use std::collections::VecDeque;

    let deque: VecDeque<_> = [1, 2, 3, 3, 5, 6, 7].into();
    let i = deque.partition_point(|&x| x < 5);

    assert_eq!(i, 4);
    assert!(deque.iter().take(i).all(|&x| x < 5));
    assert!(deque.iter().skip(i).all(|&x| !(x < 5)));

Run

If you want to insert an item to a sorted deque, while maintaining sort order:

    use std::collections::VecDeque;

    let mut deque: VecDeque<_> = [0, 1, 1, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55].into();
    let num = 42;
    let idx = deque.partition_point(|&x| x < num);
    deque.insert(idx, num);
    assert_eq!(deque, &[0, 1, 1, 1, 1, 2, 3, 5, 8, 13, 21, 34, 42, 55]);

Run
source

### impl<T: Clone, A: Allocator> VecDeque<T, A>

1.16.0 · source

#### pub fn resize(&mut self, new_len: usize, value: T)

Modifies the deque in-place so that `len()` is equal to new_len, either by removing excess elements from the back or by appending clones of `value` to the back.

##### Examples

    use std::collections::VecDeque;

    let mut buf = VecDeque::new();
    buf.push_back(5);
    buf.push_back(10);
    buf.push_back(15);
    assert_eq!(buf, [5, 10, 15]);

    buf.resize(2, 0);
    assert_eq!(buf, [5, 10]);

    buf.resize(5, 20);
    assert_eq!(buf, [5, 10, 20, 20, 20]);

Run

## Trait Implementations

source

### impl<T: Clone, A: Allocator + Clone> Clone for VecDeque<T, A>

source

#### fn clone(&self) -> Self

Returns a copy of the value. Read more
source

#### fn clone_from(&mut self, other: &Self)

Performs copy-assignment from `source`. Read more
source

### impl<T: Debug, A: Allocator> Debug for VecDeque<T, A>

source

#### fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter. Read more
source

### impl<T> Default for VecDeque<T>

source

#### fn default() -> VecDeque<T>

Creates an empty deque.

source

### impl<T, A: Allocator> Drop for VecDeque<T, A>

source

#### fn drop(&mut self)

Executes the destructor for this type. Read more
1.2.0 · source

### impl<'a, T: 'a + Copy, A: Allocator> Extend<&'a T> for VecDeque<T, A>

source

#### fn extend<I: IntoIterator<Item = &'a T>>(&mut self, iter: I)

Extends a collection with the contents of an iterator. Read more
source

#### fn extend_one(&mut self, elem: &T)

🔬This is a nightly-only experimental API. (`extend_one` #72631)
Extends a collection with exactly one element.
source

#### fn extend_reserve(&mut self, additional: usize)

🔬This is a nightly-only experimental API. (`extend_one` #72631)
Reserves capacity in a collection for the given number of additional elements. Read more
source

### impl<T, A: Allocator> Extend<T> for VecDeque<T, A>

source

#### fn extend<I: IntoIterator<Item = T>>(&mut self, iter: I)

Extends a collection with the contents of an iterator. Read more
source

#### fn extend_one(&mut self, elem: T)

🔬This is a nightly-only experimental API. (`extend_one` #72631)
Extends a collection with exactly one element.
source

#### fn extend_reserve(&mut self, additional: usize)

🔬This is a nightly-only experimental API. (`extend_one` #72631)
Reserves capacity in a collection for the given number of additional elements. Read more
1.56.0 · source

### impl<T, const N: usize> From<[T; N]> for VecDeque<T>

source

#### fn from(arr: [T; N]) -> Self

Converts a `[T; N]` into a `VecDeque<T>`.

    use std::collections::VecDeque;

    let deq1 = VecDeque::from([1, 2, 3, 4]);
    let deq2: VecDeque<_> = [1, 2, 3, 4].into();
    assert_eq!(deq1, deq2);

Run
1.10.0 · source

### impl<T, A: Allocator> From<Vec<T, A>> for VecDeque<T, A>

source

#### fn from(other: Vec<T, A>) -> Self

Turn a `Vec<T>` into a `VecDeque<T>`.

This avoids reallocating where possible, but the conditions for that are strict, and subject to change, and so shouldn’t be relied upon unless the `Vec<T>` came from `From<VecDeque<T>>` and hasn’t been reallocated.

1.10.0 · source

### impl<T, A: Allocator> From<VecDeque<T, A>> for Vec<T, A>

source

#### fn from(other: VecDeque<T, A>) -> Self

Turn a `VecDeque<T>` into a `Vec<T>`.

This never needs to re-allocate, but does need to do O(n) data movement if the circular buffer doesn’t happen to be at the beginning of the allocation.

##### Examples

    use std::collections::VecDeque;

    // This one is *O*(1).
    let deque: VecDeque<_> = (1..5).collect();
    let ptr = deque.as_slices().0.as_ptr();
    let vec = Vec::from(deque);
    assert_eq!(vec, [1, 2, 3, 4]);
    assert_eq!(vec.as_ptr(), ptr);

    // This one needs data rearranging.
    let mut deque: VecDeque<_> = (1..5).collect();
    deque.push_front(9);
    deque.push_front(8);
    let ptr = deque.as_slices().1.as_ptr();
    let vec = Vec::from(deque);
    assert_eq!(vec, [8, 9, 1, 2, 3, 4]);
    assert_eq!(vec.as_ptr(), ptr);

Run
source

### impl<T> FromIterator<T> for VecDeque<T>

source

#### fn from_iter<I: IntoIterator<Item = T>>(iter: I) -> VecDeque<T>

Creates a value from an iterator. Read more
source

### impl<T: Hash, A: Allocator> Hash for VecDeque<T, A>

source

#### fn hash<H: Hasher>(&self, state: &mut H)

Feeds this value into the given `Hasher`. Read more
1.3.0 · source

#### fn hash_slice<H>(data: &[Self], state: &mut H)where
H: Hasher,

Feeds a slice of this type into the given `Hasher`. Read more
source

### impl<T, A: Allocator> Index<usize> for VecDeque<T, A>

#### type Output = T

The returned type after indexing.
source

#### fn index(&self, index: usize) -> &T

Performs the indexing (`container[index]`) operation. Read more
source

### impl<T, A: Allocator> IndexMut<usize> for VecDeque<T, A>

source

#### fn index_mut(&mut self, index: usize) -> &mut T

Performs the mutable indexing (`container[index]`) operation. Read more
source

### impl<'a, T, A: Allocator> IntoIterator for &'a VecDeque<T, A>

#### type Item = &'a T

The type of the elements being iterated over.

#### type IntoIter = Iter<'a, T>

Which kind of iterator are we turning this into?
source

#### fn into_iter(self) -> Iter<'a, T>ⓘNotable traits for Iter<'a, T>`impl<'a, T> Iterator for Iter<'a, T> type Item = &'a T;`

Creates an iterator from a value. Read more
source

### impl<'a, T, A: Allocator> IntoIterator for &'a mut VecDeque<T, A>

#### type Item = &'a mut T

The type of the elements being iterated over.

#### type IntoIter = IterMut<'a, T>

Which kind of iterator are we turning this into?
source

#### fn into_iter(self) -> IterMut<'a, T>ⓘNotable traits for IterMut<'a, T>`impl<'a, T> Iterator for IterMut<'a, T> type Item = &'a mut T;`

Creates an iterator from a value. Read more
source

### impl<T, A: Allocator> IntoIterator for VecDeque<T, A>

source

#### fn into_iter(self) -> IntoIter<T, A>ⓘNotable traits for IntoIter<T, A>`impl<T, A: Allocator> Iterator for IntoIter<T, A> type Item = T;`

Consumes the deque into a front-to-back iterator yielding elements by value.

#### type Item = T

The type of the elements being iterated over.

#### type IntoIter = IntoIter<T, A>

Which kind of iterator are we turning this into?
source

### impl<T: Ord, A: Allocator> Ord for VecDeque<T, A>

source

#### fn cmp(&self, other: &Self) -> Ordering

This method returns an `Ordering` between `self` and `other`. Read more
1.21.0 · source

#### fn max(self, other: Self) -> Self

Compares and returns the maximum of two values. Read more
1.21.0 · source

#### fn min(self, other: Self) -> Self

Compares and returns the minimum of two values. Read more
1.50.0 · source

#### fn clamp(self, min: Self, max: Self) -> Selfwhere
Self: PartialOrd<Self>,

Restrict a value to a certain interval. Read more
1.17.0 · source

### impl<T, U, A: Allocator, const N: usize> PartialEq<&[U; N]> for VecDeque<T, A>where
T: PartialEq<U>,

source

#### fn eq(&self, other: &&[U; N]) -> bool

This method tests for `self` and `other` values to be equal, and is used by `==`. Read more
source

#### fn ne(&self, other: &Rhs) -> bool

This method tests for `!=`. The default implementation is almost always sufficient, and should not be overridden without very good reason. Read more
1.17.0 · source

### impl<T, U, A: Allocator> PartialEq<&[U]> for VecDeque<T, A>where
T: PartialEq<U>,

source

#### fn eq(&self, other: &&[U]) -> bool

This method tests for `self` and `other` values to be equal, and is used by `==`. Read more
source

#### fn ne(&self, other: &Rhs) -> bool

This method tests for `!=`. The default implementation is almost always sufficient, and should not be overridden without very good reason. Read more
1.17.0 · source

### impl<T, U, A: Allocator, const N: usize> PartialEq<&mut [U; N]> for VecDeque<T, A>where
T: PartialEq<U>,

source

#### fn eq(&self, other: &&mut [U; N]) -> bool

This method tests for `self` and `other` values to be equal, and is used by `==`. Read more
source

#### fn ne(&self, other: &Rhs) -> bool

This method tests for `!=`. The default implementation is almost always sufficient, and should not be overridden without very good reason. Read more
1.17.0 · source

### impl<T, U, A: Allocator> PartialEq<&mut [U]> for VecDeque<T, A>where
T: PartialEq<U>,

source

#### fn eq(&self, other: &&mut [U]) -> bool

This method tests for `self` and `other` values to be equal, and is used by `==`. Read more
source

#### fn ne(&self, other: &Rhs) -> bool

This method tests for `!=`. The default implementation is almost always sufficient, and should not be overridden without very good reason. Read more
1.17.0 · source

### impl<T, U, A: Allocator, const N: usize> PartialEq<[U; N]> for VecDeque<T, A>where
T: PartialEq<U>,

source

#### fn eq(&self, other: &[U; N]) -> bool

This method tests for `self` and `other` values to be equal, and is used by `==`. Read more
source

#### fn ne(&self, other: &Rhs) -> bool

This method tests for `!=`. The default implementation is almost always sufficient, and should not be overridden without very good reason. Read more
1.17.0 · source

### impl<T, U, A: Allocator> PartialEq<Vec<U, A>> for VecDeque<T, A>where
T: PartialEq<U>,

source

#### fn eq(&self, other: &Vec<U, A>) -> bool

This method tests for `self` and `other` values to be equal, and is used by `==`. Read more
source

#### fn ne(&self, other: &Rhs) -> bool

This method tests for `!=`. The default implementation is almost always sufficient, and should not be overridden without very good reason. Read more
source

### impl<T: PartialEq, A: Allocator> PartialEq<VecDeque<T, A>> for VecDeque<T, A>

source

#### fn eq(&self, other: &Self) -> bool

This method tests for `self` and `other` values to be equal, and is used by `==`. Read more
source

#### fn ne(&self, other: &Rhs) -> bool

This method tests for `!=`. The default implementation is almost always sufficient, and should not be overridden without very good reason. Read more
source

### impl<T: PartialOrd, A: Allocator> PartialOrd<VecDeque<T, A>> for VecDeque<T, A>

source

#### fn partial_cmp(&self, other: &Self) -> Option<Ordering>

This method returns an ordering between `self` and `other` values if one exists. Read more
source

#### fn lt(&self, other: &Rhs) -> bool

This method tests less than (for `self` and `other`) and is used by the `<` operator. Read more
source

#### fn le(&self, other: &Rhs) -> bool

This method tests less than or equal to (for `self` and `other`) and is used by the `<=` operator. Read more
source

#### fn gt(&self, other: &Rhs) -> bool

This method tests greater than (for `self` and `other`) and is used by the `>` operator. Read more
source

#### fn ge(&self, other: &Rhs) -> bool

This method tests greater than or equal to (for `self` and `other`) and is used by the `>=` operator. Read more
source

### impl<T: Eq, A: Allocator> Eq for VecDeque<T, A>

## Auto Trait Implementations

### impl<T, A> RefUnwindSafe for VecDeque<T, A>where
A: RefUnwindSafe,
T: RefUnwindSafe,

### impl<T, A> Send for VecDeque<T, A>where
A: Send,
T: Send,

### impl<T, A> Sync for VecDeque<T, A>where
A: Sync,
T: Sync,

### impl<T, A> Unpin for VecDeque<T, A>where
A: Unpin,
T: Unpin,

### impl<T, A> UnwindSafe for VecDeque<T, A>where
A: UnwindSafe,
T: UnwindSafe,

## Blanket Implementations

source

### impl<T> Any for Twhere
T: 'static + ?Sized,

source

#### fn type_id(&self) -> TypeId

Gets the `TypeId` of `self`. Read more
source

### impl<T> Borrow<T> for Twhere
T: ?Sized,

const: unstable · source

#### fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
source

### impl<T> BorrowMut<T> for Twhere
T: ?Sized,

const: unstable · source

#### fn borrow_mut(&mut self) -> &mut T

Mutably borrows from an owned value. Read more
source

### impl<T> From<T> for T

const: unstable · source

#### fn from(t: T) -> T

Returns the argument unchanged.

source

### impl<T, U> Into<U> for Twhere
U: From<T>,

const: unstable · source

#### fn into(self) -> U

Calls `U::from(self)`.

That is, this conversion is whatever the implementation of `From<T> for U` chooses to do.

source

### impl<T> ToOwned for Twhere
T: Clone,

#### type Owned = T

The resulting type after obtaining ownership.
source

#### fn to_owned(&self) -> T

Creates owned data from borrowed data, usually by cloning. Read more
source

#### fn clone_into(&self, target: &mut T)

Uses borrowed data to replace owned data, usually by cloning. Read more
source

### impl<T, U> TryFrom<U> for Twhere
U: Into<T>,

#### type Error = Infallible

The type returned in the event of a conversion error.
const: unstable · source

#### fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

Performs the conversion.
source

### impl<T, U> TryInto<U> for Twhere
U: TryFrom<T>,

#### type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.
const: unstable · source

#### fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.