# 1.0.0[−][src]Trait std::ops::FnMut

    #[lang = "fn_mut"]
    #[must_use = "closures are lazy and do nothing unless called"]
    pub trait FnMut<Args>: FnOnce<Args> {
        extern "rust-call" fn call_mut(&mut self, args: Args) -> Self::Output;
    }

The version of the call operator that takes a mutable receiver.

Instances of `FnMut` can be called repeatedly and may mutate state.

`FnMut` is implemented automatically by closures which take mutable references to captured variables, as well as all types that implement `Fn`, e.g., (safe) function pointers (since `FnMut` is a supertrait of `Fn`). Additionally, for any type `F` that implements `FnMut`, `&mut F` implements `FnMut`, too.

Since `FnOnce` is a supertrait of `FnMut`, any instance of `FnMut` can be used where a `FnOnce` is expected, and since `Fn` is a subtrait of `FnMut`, any instance of `Fn` can be used where `FnMut` is expected.

Use `FnMut` as a bound when you want to accept a parameter of function-like type and need to call it repeatedly, while allowing it to mutate state. If you don't want the parameter to mutate state, use `Fn` as a bound; if you don't need to call it repeatedly, use `FnOnce`.

See the chapter on closures in The Rust Programming Language for some more information on this topic.

Also of note is the special syntax for `Fn` traits (e.g. `Fn(usize, bool) -> usize`). Those interested in the technical details of this can refer to the relevant section in the Rustonomicon.

# Examples

## Calling a mutably capturing closure

    let mut x = 5;
    {
        let mut square_x = || x *= x;
        square_x();
    }
    assert_eq!(x, 25);Run

## Using a `FnMut` parameter

    fn do_twice<F>(mut func: F)
        where F: FnMut()
    {
        func();
        func();
    }

    let mut x: usize = 1;
    {
        let add_two_to_x = || x += 2;
        do_twice(add_two_to_x);
    }

    assert_eq!(x, 5);Run

## Required methods

### `extern "rust-call" fn call_mut(&mut self, args: Args) -> Self::Output`

🔬 This is a nightly-only experimental API. (`fn_traits` #29625)

Performs the call operation.

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## Implementors

### `impl<'_, A, F> FnMut<A> for &'_ F where
F: Fn<A> + ?Sized,` [src]

#### `extern "rust-call" fn call_mut(&mut self, args: A) -> <F as FnOnce<A>>::Output`[src]

🔬 This is a nightly-only experimental API. (`fn_traits` #29625)

### `impl<'_, A, F> FnMut<A> for &'_ mut F where
F: FnMut<A> + ?Sized,` [src]

#### `extern "rust-call" fn call_mut(&mut self, args: A) -> <F as FnOnce<A>>::Output`[src]

🔬 This is a nightly-only experimental API. (`fn_traits` #29625)

### `impl<A, F> FnMut<A> for Box<F> where
F: FnMut<A> + ?Sized,` [src]

####
ⓘImportant traits for Box<I>

### Important traits for Box<I>

`impl<I> Iterator for Box<I> where
I: Iterator + ?Sized, type Item = <I as Iterator>::Item;impl<F> Future for Box<F> where
F: Unpin + Future + ?Sized, type Output = <F as Future>::Output;impl<R: Read + ?Sized> Read for Box<R>impl<W: Write + ?Sized> Write for Box<W>`
`extern "rust-call" fn call_mut(
&mut self,
args: A
) -> <Box<F> as FnOnce<A>>::Output`[src]

🔬 This is a nightly-only experimental API. (`fn_traits` #29625)
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