FnMut in core::ops - Rust
Description: The version of the call operator that takes a mutable receiver.

# Trait FnMut

1.6.0 · Source

    pub trait FnMut<Args: Tuple>: FnOnce<Args> {
        // Required method
        extern "rust-call" fn call_mut(
            &mut self,
            args: Args,
        ) -> Self::Output;
    }

Expand description

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);

### §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);

## Required Methods§

Source

#### 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.

## Implementors§

1.0.0 · Source§

### impl<A: Tuple, F> FnMut<A> for &F
where F: Fn<A> + ?Sized,

1.0.0 · Source§

### impl<A: Tuple, F> FnMut<A> for &mut F
where F: FnMut<A> + ?Sized,

Source§

### impl<F, Args> FnMut<Args> for Exclusive<F>
where F: FnMut<Args>, Args: Tuple,