# Struct core::iter::Peekable [−] [src]

    #[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
    pub struct Peekable<I: Iterator> {
        // some fields omitted
    }

An iterator with a `peek()` that returns an optional reference to the next element.

This `struct` is created by the `peekable()` method on `Iterator`. See its documentation for more.

## Methods

### `impl<I: Iterator> Peekable<I>`

#### `fn peek(&mut self) -> Option<&I::Item>`

Returns a reference to the next() value without advancing the iterator.

The `peek()` method will return the value that a call to `next()` would return, but does not advance the iterator. Like `next()`, if there is a value, it's wrapped in a `Some(T)`, but if the iterator is over, it will return `None`.

Because `peek()` returns reference, and many iterators iterate over references, this leads to a possibly confusing situation where the return value is a double reference. You can see this effect in the examples below, with `&&i32`.

# Examples

Basic usage:

fn main() { let xs = [1, 2, 3]; let mut iter = xs.iter().peekable(); // peek() lets us see into the future assert_eq!(iter.peek(), Some(&&1)); assert_eq!(iter.next(), Some(&1)); assert_eq!(iter.next(), Some(&2)); // we can peek() multiple times, the iterator won't advance assert_eq!(iter.peek(), Some(&&3)); assert_eq!(iter.peek(), Some(&&3)); assert_eq!(iter.next(), Some(&3)); // after the iterator is finished, so is peek() assert_eq!(iter.peek(), None); assert_eq!(iter.next(), None); }

    let xs = [1, 2, 3];

    let mut iter = xs.iter().peekable();

    // peek() lets us see into the future
    assert_eq!(iter.peek(), Some(&&1));
    assert_eq!(iter.next(), Some(&1));

    assert_eq!(iter.next(), Some(&2));

    // we can peek() multiple times, the iterator won't advance
    assert_eq!(iter.peek(), Some(&&3));
    assert_eq!(iter.peek(), Some(&&3));

    assert_eq!(iter.next(), Some(&3));

    // after the iterator is finished, so is peek()
    assert_eq!(iter.peek(), None);
    assert_eq!(iter.next(), None);

#### `fn is_empty(&mut self) -> bool`

Unstable (`peekable_is_empty` #27701)

Checks if the iterator has finished iterating.

Returns `true` if there are no more elements in the iterator, and `false` if there are.

# Examples

Basic usage:

#![feature(peekable_is_empty)] fn main() { let xs = [1, 2, 3]; let mut iter = xs.iter().peekable(); // there are still elements to iterate over assert_eq!(iter.is_empty(), false); // let's consume the iterator iter.next(); iter.next(); iter.next(); assert_eq!(iter.is_empty(), true); }

    #![feature(peekable_is_empty)]

    let xs = [1, 2, 3];

    let mut iter = xs.iter().peekable();

    // there are still elements to iterate over
    assert_eq!(iter.is_empty(), false);

    // let's consume the iterator
    iter.next();
    iter.next();
    iter.next();

    assert_eq!(iter.is_empty(), true);

## Trait Implementations

### `impl<I: Iterator> Iterator for Peekable<I>`

#### `type Item = I::Item`

#### `fn next(&mut self) -> Option<I::Item>`

#### `fn count(self) -> usize`

#### `fn nth(&mut self, n: usize) -> Option<I::Item>`

#### `fn last(self) -> Option<I::Item>`

#### `fn size_hint(&self) -> (usize, Option<usize>)`

#### `fn chain<U>(self, other: U) -> Chain<Self, U::IntoIter> where Self: Sized, U: IntoIterator<Item=Self::Item>`

#### `fn zip<U>(self, other: U) -> Zip<Self, U::IntoIter> where Self: Sized, U: IntoIterator`

#### `fn map<B, F>(self, f: F) -> Map<Self, F> where Self: Sized, F: FnMut(Self::Item) -> B`

#### `fn filter<P>(self, predicate: P) -> Filter<Self, P> where Self: Sized, P: FnMut(&Self::Item) -> bool`

#### `fn filter_map<B, F>(self, f: F) -> FilterMap<Self, F> where Self: Sized, F: FnMut(Self::Item) -> Option<B>`

#### `fn enumerate(self) -> Enumerate<Self> where Self: Sized`

#### `fn peekable(self) -> Peekable<Self> where Self: Sized`

#### `fn skip_while<P>(self, predicate: P) -> SkipWhile<Self, P> where Self: Sized, P: FnMut(&Self::Item) -> bool`

#### `fn take_while<P>(self, predicate: P) -> TakeWhile<Self, P> where Self: Sized, P: FnMut(&Self::Item) -> bool`

#### `fn skip(self, n: usize) -> Skip<Self> where Self: Sized`

#### `fn take(self, n: usize) -> Take<Self> where Self: Sized`

#### `fn scan<St, B, F>(self, initial_state: St, f: F) -> Scan<Self, St, F> where Self: Sized, F: FnMut(&mut St, Self::Item) -> Option<B>`

#### `fn flat_map<U, F>(self, f: F) -> FlatMap<Self, U, F> where Self: Sized, U: IntoIterator, F: FnMut(Self::Item) -> U`

#### `fn fuse(self) -> Fuse<Self> where Self: Sized`

#### `fn inspect<F>(self, f: F) -> Inspect<Self, F> where Self: Sized, F: FnMut(&Self::Item)`

#### `fn by_ref(&mut self) -> &mut Self where Self: Sized`

#### `fn collect<B: FromIterator<Self::Item>>(self) -> B where Self: Sized`

#### `fn partition<B, F>(self, f: F) -> (B, B) where Self: Sized, B: Default + Extend<Self::Item>, F: FnMut(&Self::Item) -> bool`

#### `fn fold<B, F>(self, init: B, f: F) -> B where Self: Sized, F: FnMut(B, Self::Item) -> B`

#### `fn all<F>(&mut self, f: F) -> bool where Self: Sized, F: FnMut(Self::Item) -> bool`

#### `fn any<F>(&mut self, f: F) -> bool where Self: Sized, F: FnMut(Self::Item) -> bool`

#### `fn find<P>(&mut self, predicate: P) -> Option<Self::Item> where Self: Sized, P: FnMut(&Self::Item) -> bool`

#### `fn position<P>(&mut self, predicate: P) -> Option<usize> where Self: Sized, P: FnMut(Self::Item) -> bool`

#### `fn rposition<P>(&mut self, predicate: P) -> Option<usize> where P: FnMut(Self::Item) -> bool, Self: Sized + ExactSizeIterator + DoubleEndedIterator`

#### `fn max(self) -> Option<Self::Item> where Self: Sized, Self::Item: Ord`

#### `fn min(self) -> Option<Self::Item> where Self: Sized, Self::Item: Ord`

#### `fn max_by<B: Ord, F>(self, f: F) -> Option<Self::Item> where Self: Sized, F: FnMut(&Self::Item) -> B`

#### `fn max_by_key<B: Ord, F>(self, f: F) -> Option<Self::Item> where Self: Sized, F: FnMut(&Self::Item) -> B`

#### `fn min_by<B: Ord, F>(self, f: F) -> Option<Self::Item> where Self: Sized, F: FnMut(&Self::Item) -> B`

#### `fn min_by_key<B: Ord, F>(self, f: F) -> Option<Self::Item> where Self: Sized, F: FnMut(&Self::Item) -> B`

#### `fn rev(self) -> Rev<Self> where Self: Sized + DoubleEndedIterator`

#### `fn unzip<A, B, FromA, FromB>(self) -> (FromA, FromB) where FromA: Default + Extend<A>, FromB: Default + Extend<B>, Self: Sized + Iterator<Item=(A, B)>`

#### `fn cloned<'a, T: 'a>(self) -> Cloned<Self> where Self: Sized + Iterator<Item=&'a T>, T: Clone`

#### `fn cycle(self) -> Cycle<Self> where Self: Sized + Clone`

#### `fn sum<S>(self) -> S where S: Add<Self::Item, Output=S> + Zero, Self: Sized`

#### `fn product<P>(self) -> P where P: Mul<Self::Item, Output=P> + One, Self: Sized`

#### `fn cmp<I>(self, other: I) -> Ordering where I: IntoIterator<Item=Self::Item>, Self::Item: Ord, Self: Sized`

#### `fn partial_cmp<I>(self, other: I) -> Option<Ordering> where I: IntoIterator, Self::Item: PartialOrd<I::Item>, Self: Sized`

#### `fn eq<I>(self, other: I) -> bool where I: IntoIterator, Self::Item: PartialEq<I::Item>, Self: Sized`

#### `fn ne<I>(self, other: I) -> bool where I: IntoIterator, Self::Item: PartialEq<I::Item>, Self: Sized`

#### `fn lt<I>(self, other: I) -> bool where I: IntoIterator, Self::Item: PartialOrd<I::Item>, Self: Sized`

#### `fn le<I>(self, other: I) -> bool where I: IntoIterator, Self::Item: PartialOrd<I::Item>, Self: Sized`

#### `fn gt<I>(self, other: I) -> bool where I: IntoIterator, Self::Item: PartialOrd<I::Item>, Self: Sized`

#### `fn ge<I>(self, other: I) -> bool where I: IntoIterator, Self::Item: PartialOrd<I::Item>, Self: Sized`

### `impl<I: ExactSizeIterator> ExactSizeIterator for Peekable<I>`

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

### Derived Implementations

### `impl<I: Clone + Iterator> Clone for Peekable<I> where I::Item: Clone`

#### `fn clone(&self) -> Peekable<I>`

#### `fn clone_from(&mut self, source: &Self)`

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