std::ops::DerefMut - Rust
Description: API documentation for the Rust `DerefMut` trait in crate `std`.
Keywords: rust, rustlang, rust-lang, DerefMut

# Trait std::ops::DerefMut 1.0.0 [−] [src]

    #[lang = "deref_mut"]
    pub trait DerefMut: Deref {
        fn deref_mut(&mut self) -> &mut Self::Target;
    }

Used for mutable dereferencing operations, like in `*v = 1;`.

In addition to being used for explicit dereferencing operations with the (unary) `*` operator in mutable contexts, `DerefMut` is also used implicitly by the compiler in many circumstances. This mechanism is called '`Deref` coercion'. In immutable contexts, `Deref` is used.

Implementing `DerefMut` for smart pointers makes mutating the data behind them convenient, which is why they implement `DerefMut`. On the other hand, the rules regarding `Deref` and `DerefMut` were designed specifically to accommodate smart pointers. Because of this, `DerefMut` should only be implemented for smart pointers to avoid confusion.

For similar reasons, this trait should never fail. Failure during dereferencing can be extremely confusing when `DerefMut` is invoked implicitly.

# More on `Deref` coercion

If `T` implements `DerefMut<Target = U>`, and `x` is a value of type `T`, then:

- In mutable contexts, `*x` on non-pointer types is equivalent to `*Deref::deref(&x)`.
- Values of type `&mut T` are coerced to values of type `&mut U`
- `T` implicitly implements all the (mutable) methods of the type `U`.

For more details, visit the chapter in The Rust Programming Language as well as the reference sections on the dereference operator, method resolution and type coercions.

# Examples

A struct with a single field which is modifiable by dereferencing the struct.

    use std::ops::{Deref, DerefMut};

    struct DerefMutExample<T> {
        value: T
    }

    impl<T> Deref for DerefMutExample<T> {
        type Target = T;

        fn deref(&self) -> &T {
            &self.value
        }
    }

    impl<T> DerefMut for DerefMutExample<T> {
        fn deref_mut(&mut self) -> &mut T {
            &mut self.value
        }
    }

    let mut x = DerefMutExample { value: 'a' };
    *x = 'b';
    assert_eq!('b', *x);Run

## Required Methods

### `fn deref_mut(&mut self) -> &mut Self::Target`

Mutably dereferences the value.

## Implementors

- [src]
  `impl<T> DerefMut for Vec<T>`
- [src]
  `impl<T> DerefMut for Box<T> where
  T: ?Sized,`
- [src]
  `impl DerefMut for String`
- [src]
  `impl<'a, T> DerefMut for PeekMut<'a, T> where
  T: Ord,`
- [src]
  `impl<T> DerefMut for ManuallyDrop<T>`
- [src]
  `impl<'a, T> DerefMut for &'a mut T where
  T: ?Sized,`
- [src]
  `impl<'b, T> DerefMut for RefMut<'b, T> where
  T: ?Sized,`
- [src]
  `impl<T> DerefMut for AssertUnwindSafe<T>`
- [src]
  `impl<'mutex, T: ?Sized> DerefMut for MutexGuard<'mutex, T>`
- [src]
  `impl<'rwlock, T: ?Sized> DerefMut for RwLockWriteGuard<'rwlock, T>`

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