# Using `#[derive(Deref)]`
Deriving `Deref` only works for a single field of a struct, or a single field of each variant of an enum.
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)]
enum Enum {
V1(i32),
V2 { num: i32 },
}
#[derive(Deref)]
#[deref(forward)]
struct MyBoxedInt(Box<i32>);
#[derive(Deref)]
#[deref(forward)]
enum MyBoxedIntEnum {
V1(Box<i32>),
V2 { num: Box<i32> },
}
// You can specify the field you want to derive `Deref` for.
#[derive(Deref)]
struct CoolVec {
cool: bool,
#[deref]
vec: Vec<i32>,
}
#[derive(Deref)]
enum CoolVecEnum {
V1(Vec<i32>),
V2 { 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);
let num_v2 = Enum::V2{num: 123};
let boxed_v1 = MyBoxedIntEnum::V1(Box::new(123));
let cool_vec_v2 = CoolVecEnum::V2{cool: true, vec: vec![123]};
assert_eq!(123, *num_v2);
assert_eq!(123, *boxed_v1);
assert_eq!(vec![123], *cool_vec_v2);
```
## 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::core::ops::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::core::ops::Deref for MyBoxedInt {
type Target = <Box<i32> as derive_more::core::ops::Deref>::Target;
#[inline]
fn deref(&self) -> &Self::Target {
<Box<i32> as derive_more::core::ops::Deref>::deref(&self.0)
}
}
```
## Enums
When deriving a non-forwarded `Deref` for an enum:
```rust
# use derive_more::Deref;
#
#[derive(Deref)]
enum CoolVecEnum {
V1(Vec<i32>),
V2 { cool: bool, #[deref] vec: Vec<i32> },
}
```
Code like this will be generated:
```rust
# enum CoolVecEnum {
# V1(Vec<i32>),
# V2 { cool: bool, vec: Vec<i32> },
# }
impl derive_more::with_trait::Deref for CoolVecEnum {
type Target = Vec<i32>;
#[inline]
fn deref(&self) -> &Self::Target {
match self {
CoolVecEnum::V1(__0) => __0,
CoolVecEnum::V2 { cool: _, vec: __0 } => __0,
}
}
}
```
When deriving a forwarded `Deref` for an enum:
```rust
# use derive_more::Deref;
#
#[derive(Deref)]
#[deref(forward)]
enum MyBoxedIntEnum {
V1(Box<i32>),
V2 { num: Box<i32> },
}
```
Code like this will be generated:
```rust
# enum MyBoxedIntEnum {
# V1(Box<i32>),
# V2 { num: Box<i32> },
# }
impl derive_more::with_trait::Deref for MyBoxedIntEnum {
type Target = <Box<i32> as derive_more::with_trait::Deref>::Target;
#[inline]
fn deref(&self) -> &Self::Target {
match self {
MyBoxedIntEnum::V1(__0) =>
<Box<i32> as derive_more::with_trait::Deref>::deref(__0),
MyBoxedIntEnum::V2 { num: __0 } =>
<Box<i32> as derive_more::with_trait::Deref>::deref(__0),
}
}
}
```