Expand description
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(whereTis neither a reference nor a raw pointer) is equivalent to*DerefMut::deref_mut(&mut x). - Values of type
&mut Tare coerced to values of type&mut U Timplicitly implements all the (mutable) methods of the typeU.
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) -> &Self::Target {
&self.value
}
}
impl<T> DerefMut for DerefMutExample<T> {
fn deref_mut(&mut self) -> &mut Self::Target {
&mut self.value
}
}
let mut x = DerefMutExample { value: 'a' };
*x = 'b';
assert_eq!('b', *x);