# Trait std::iter::FromIterator

1.0.0 · source ·

    pub trait FromIterator<A>: Sized {
        // Required method
        fn from_iter<T>(iter: T) -> Self
           where T: IntoIterator<Item = A>;
    }

Expand description

Conversion from an `Iterator`.

By implementing `FromIterator` for a type, you define how it will be created from an iterator. This is common for types which describe a collection of some kind.

If you want to create a collection from the contents of an iterator, the `Iterator::collect()` method is preferred. However, when you need to specify the container type, `FromIterator::from_iter()` can be more readable than using a turbofish (e.g. `::<Vec<_>>()`). See the `Iterator::collect()` documentation for more examples of its use.

See also: `IntoIterator`.

## Examples

Basic usage:

    let five_fives = std::iter::repeat(5).take(5);

    let v = Vec::from_iter(five_fives);

    assert_eq!(v, vec![5, 5, 5, 5, 5]);

Run

Using `Iterator::collect()` to implicitly use `FromIterator`:

    let five_fives = std::iter::repeat(5).take(5);

    let v: Vec<i32> = five_fives.collect();

    assert_eq!(v, vec![5, 5, 5, 5, 5]);

Run

Using `FromIterator::from_iter()` as a more readable alternative to `Iterator::collect()`:

    use std::collections::VecDeque;
    let first = (0..10).collect::<VecDeque<i32>>();
    let second = VecDeque::from_iter(0..10);

    assert_eq!(first, second);

Run

Implementing `FromIterator` for your type:

    // A sample collection, that's just a wrapper over Vec<T>
    #[derive(Debug)]
    struct MyCollection(Vec<i32>);

    // Let's give it some methods so we can create one and add things
    // to it.
    impl MyCollection {
        fn new() -> MyCollection {
            MyCollection(Vec::new())
        }

        fn add(&mut self, elem: i32) {
            self.0.push(elem);
        }
    }

    // and we'll implement FromIterator
    impl FromIterator<i32> for MyCollection {
        fn from_iter<I: IntoIterator<Item=i32>>(iter: I) -> Self {
            let mut c = MyCollection::new();

            for i in iter {
                c.add(i);
            }

            c
        }
    }

    // Now we can make a new iterator...
    let iter = (0..5).into_iter();

    // ... and make a MyCollection out of it
    let c = MyCollection::from_iter(iter);

    assert_eq!(c.0, vec![0, 1, 2, 3, 4]);

    // collect works too!

    let iter = (0..5).into_iter();
    let c: MyCollection = iter.collect();

    assert_eq!(c.0, vec![0, 1, 2, 3, 4]);

Run

## Required Methods§

source

#### fn from_iter<T>(iter: T) -> Selfwhere T: IntoIterator<Item = A>,

Creates a value from an iterator.

See the module-level documentation for more.

##### Examples

    let five_fives = std::iter::repeat(5).take(5);

    let v = Vec::from_iter(five_fives);

    assert_eq!(v, vec![5, 5, 5, 5, 5]);

Run

## Object Safety§

This trait is not object safe.

## Implementors§

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### impl FromIterator<char> for String

1.23.0 · source§

### impl FromIterator<()> for ()

Collapses all unit items from an iterator into one.

This is more useful when combined with higher-level abstractions, like collecting to a `Result<(), E>` where you only care about errors:

    use std::io::*;
    let data = vec![1, 2, 3, 4, 5];
    let res: Result<()> = data.iter()
        .map(|x| writeln!(stdout(), "{x}"))
        .collect();
    assert!(res.is_ok());

Run
1.45.0 · source§

### impl FromIterator<Box<str>> for String

1.52.0 · source§

### impl FromIterator<OsString> for OsString

1.4.0 · source§

### impl FromIterator<String> for String

1.17.0 · source§

### impl<'a> FromIterator<&'a char> for String

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### impl<'a> FromIterator<&'a str> for String

1.52.0 · source§

### impl<'a> FromIterator<&'a OsStr> for OsString

1.19.0 · source§

### impl<'a> FromIterator<Cow<'a, str>> for String

1.52.0 · source§

### impl<'a> FromIterator<Cow<'a, OsStr>> for OsString

1.12.0 · source§

### impl<'a> FromIterator<char> for Cow<'a, str>

1.12.0 · source§

### impl<'a> FromIterator<String> for Cow<'a, str>

1.12.0 · source§

### impl<'a, 'b> FromIterator<&'b str> for Cow<'a, str>

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### impl<'a, T> FromIterator<T> for Cow<'a, [T]>where T: Clone,

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### impl<A, E, V> FromIterator<Result<A, E>> for Result<V, E>where V: FromIterator<A>,

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### impl<A, V> FromIterator<Option<A>> for Option<V>where V: FromIterator<A>,

1.32.0 · source§

### impl<I> FromIterator<I> for Box<[I]>

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### impl<K, V> FromIterator<(K, V)> for BTreeMap<K, V>where K: Ord,

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### impl<K, V, S> FromIterator<(K, V)> for HashMap<K, V, S>where K: Eq + Hash, S: BuildHasher + Default,

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### impl<P: AsRef<Path>> FromIterator<P> for PathBuf

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### impl<T> FromIterator<T> for Vec<T>

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### impl<T> FromIterator<T> for BTreeSet<T>where T: Ord,

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### impl<T> FromIterator<T> for BinaryHeap<T>where T: Ord,

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### impl<T> FromIterator<T> for LinkedList<T>

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

1.37.0 · source§

### impl<T> FromIterator<T> for Rc<[T]>

1.37.0 · source§

### impl<T> FromIterator<T> for Arc<[T]>

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### impl<T, S> FromIterator<T> for HashSet<T, S>where T: Eq + Hash, S: BuildHasher + Default,