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# What `#[derive(DerefMut)]` generates
Deriving `Deref` only works for a single field of a struct.
Furthermore it requires that the type also implements `Deref`, so usually
`Deref` should also be derived.
The resulting implementation of `Deref` will allow you to mutably dereference
the struct its member directly.
1. Dereferencing to the field, i.e. like if your type was a reference type.
2. Doing a dereference on the field, for when the field itself is a reference
type like `&mut` and `Box`.
With `#[deref_mut]` or `#[deref_mut(ignore)]` it's possible to indicate the
field that you want to derive `DerefMut` for.
## Example usage
```rust
# use derive_more::{Deref, DerefMut};
#
#[derive(Deref, DerefMut)]
struct Num {
num: i32,
}
#[derive(Deref, DerefMut)]
#[deref(forward)]
#[deref_mut(forward)]
struct MyBoxedInt(Box<i32>);
// You can specify the field you want to derive DerefMut for
#[derive(Deref, DerefMut)]
struct CoolVec {
cool: bool,
#[deref]
#[deref_mut]
vec: Vec<i32>,
}
let mut num = Num{num: 123};
let mut boxed = MyBoxedInt(Box::new(123));
let mut cool_vec = CoolVec{cool: true, vec: vec![123]};
*num += 123;
assert_eq!(246, *num);
*boxed += 1000;
assert_eq!(1123, *boxed);
cool_vec.push(456);
assert_eq!(vec![123, 456], *cool_vec);
```
## Structs
When deriving a non-forwarded `Deref` for a struct:
```rust
# use derive_more::{Deref, DerefMut};
#
#[derive(Deref, DerefMut)]
struct CoolVec {
cool: bool,
#[deref]
#[deref_mut]
vec: Vec<i32>,
}
```
Code like this will be generated:
```rust
# struct CoolVec {
# cool: bool,
# vec: Vec<i32>,
# }
# impl ::core::ops::Deref for CoolVec {
# type Target = Vec<i32>;
# #[inline]
# fn deref(&self) -> &Self::Target {
# &self.vec
# }
# }
impl ::core::ops::DerefMut for CoolVec {
#[inline]
fn deref_mut(&mut self) -> &mut Self::Target {
&mut self.vec
}
}
```
When deriving `DerefMut` for a tuple struct with one field:
```rust
# use derive_more::{Deref, DerefMut};
#
#[derive(Deref, DerefMut)]
#[deref(forward)]
#[deref_mut(forward)]
struct MyBoxedInt(Box<i32>);
```
When deriving a forwarded `DerefMut` for a struct:
```rust
# struct MyBoxedInt(Box<i32>);
# impl ::core::ops::Deref for MyBoxedInt {
# type Target = <Box<i32> as ::core::ops::Deref>::Target;
# #[inline]
# fn deref(&self) -> &Self::Target {
# <Box<i32> as ::core::ops::Deref>::deref(&self.0)
# }
# }
impl ::core::ops::DerefMut for MyBoxedInt {
#[inline]
fn deref_mut(&mut self) -> &mut Self::Target {
<Box<i32> as ::core::ops::DerefMut>::deref_mut(&mut self.0)
}
}
```
## Enums
Deriving `DerefMut` is not supported for enums.