# What `#[derive(Index)]` generates
Deriving `Index` only works for a single field of a struct.
The result is that you will index it's member directly.
With `#[index]` or `#[index(ignore)]` it's possible to indicate the field that
you want to derive `Index` for.
## Example usage
```rust
# use derive_more::Index;
#
#[derive(Index)]
struct MyVec(Vec<i32>);
// You can specify the field you want to derive Index for
#[derive(Index)]
struct Numbers {
#[index]
numbers: Vec<i32>,
useless: bool,
}
assert_eq!(5, MyVec(vec![5, 8])[0]);
assert_eq!(200, Numbers { numbers: vec![100, 200], useless: false }[1]);
```
## Structs
When deriving `Index` for a struct:
```rust
# use derive_more::Index;
#
#[derive(Index)]
struct Numbers {
#[index]
numbers: Vec<i32>,
useless: bool,
}
```
Code like this will be generated:
```rust
# struct Numbers {
# numbers: Vec<i32>,
# useless: bool,
# }
impl<__IdxT> derive_more::core::ops::Index<__IdxT> for Numbers
where
Vec<i32>: derive_more::core::ops::Index<__IdxT>,
{
type Output = <Vec<i32> as derive_more::core::ops::Index<__IdxT>>::Output;
#[inline]
fn index(&self, idx: __IdxT) -> &Self::Output {
<Vec<i32> as derive_more::core::ops::Index<__IdxT>>::index(&self.numbers, idx)
}
}
```
## Enums
Deriving `Index` is not supported for enums.