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# Using `#[derive(Deref)]`
Deriving `Deref` only works for a single field of a struct.
It's possible to use it in two ways:
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 `&` and `Box`.
With `#[deref]` or `#[deref(ignore)]` it's possible to indicate the field that
you want to derive `Deref` for.
## Example usage
```rust
# use derive_more::Deref;
#
#[derive(Deref)]
struct Num {
num: i32,
}
#[derive(Deref)]
#[deref(forward)]
struct MyBoxedInt(Box<i32>);
// You can specify the field you want to derive `Deref` for.
#[derive(Deref)]
struct CoolVec {
cool: bool,
#[deref]
vec: Vec<i32>,
}
let num = Num{num: 123};
let boxed = MyBoxedInt(Box::new(123));
let cool_vec = CoolVec{cool: true, vec: vec![123]};
assert_eq!(123, *num);
assert_eq!(123, *boxed);
assert_eq!(vec![123], *cool_vec);
```
## Structs
When deriving a non-forwarded `Deref` for a struct:
```rust
# use derive_more::Deref;
#
#[derive(Deref)]
struct CoolVec {
cool: bool,
#[deref]
vec: Vec<i32>,
}
```
Code like this will be generated:
```rust
# struct CoolVec {
# cool: bool,
# vec: Vec<i32>,
# }
impl derive_more::Deref for CoolVec {
type Target = Vec<i32>;
#[inline]
fn deref(&self) -> &Self::Target {
&self.vec
}
}
```
When deriving a forwarded `Deref` for a struct:
```rust
# use derive_more::Deref;
#
#[derive(Deref)]
#[deref(forward)]
struct MyBoxedInt(Box<i32>);
```
Code like this will be generated:
```rust
# struct MyBoxedInt(Box<i32>);
impl derive_more::Deref for MyBoxedInt {
type Target = <Box<i32> as derive_more::Deref>::Target;
#[inline]
fn deref(&self) -> &Self::Target {
<Box<i32> as derive_more::Deref>::deref(&self.0)
}
}
```
## Enums
Deriving `Deref` is not supported for enums.
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