Trait otter_base::prelude::IndexMut 1.0.0[−][src]
pub trait IndexMut<Idx>: Index<Idx> where
Idx: ?Sized, { fn index_mut(&mut self, index: Idx) -> &mut Self::Output; }
Expand description
Used for indexing operations (container[index]
) in mutable contexts.
container[index]
is actually syntactic sugar for
*container.index_mut(index)
, but only when used as a mutable value. If
an immutable value is requested, the Index
trait is used instead. This
allows nice things such as v[index] = value
.
Examples
A very simple implementation of a Balance
struct that has two sides, where
each can be indexed mutably and immutably.
use std::ops::{Index, IndexMut}; #[derive(Debug)] enum Side { Left, Right, } #[derive(Debug, PartialEq)] enum Weight { Kilogram(f32), Pound(f32), } struct Balance { pub left: Weight, pub right: Weight, } impl Index<Side> for Balance { type Output = Weight; fn index(&self, index: Side) -> &Self::Output { println!("Accessing {:?}-side of balance immutably", index); match index { Side::Left => &self.left, Side::Right => &self.right, } } } impl IndexMut<Side> for Balance { fn index_mut(&mut self, index: Side) -> &mut Self::Output { println!("Accessing {:?}-side of balance mutably", index); match index { Side::Left => &mut self.left, Side::Right => &mut self.right, } } } let mut balance = Balance { right: Weight::Kilogram(2.5), left: Weight::Pound(1.5), }; // In this case, `balance[Side::Right]` is sugar for // `*balance.index(Side::Right)`, since we are only *reading* // `balance[Side::Right]`, not writing it. assert_eq!(balance[Side::Right], Weight::Kilogram(2.5)); // However, in this case `balance[Side::Left]` is sugar for // `*balance.index_mut(Side::Left)`, since we are writing // `balance[Side::Left]`. balance[Side::Left] = Weight::Kilogram(3.0);
Required methods
Implementations on Foreign Types
Mutably access an element of this map. Panics if the given key is not present in the map.
map["key"] = json!("value");
Write into a serde_json::Value
using the syntax value[0] = ...
or
value["k"] = ...
.
If the index is a number, the value must be an array of length bigger than the index. Indexing into a value that is not an array or an array that is too small will panic.
If the index is a string, the value must be an object or null which is treated like an empty object. If the key is not already present in the object, it will be inserted with a value of null. Indexing into a value that is neither an object nor null will panic.
Examples
let mut data = json!({ "x": 0 }); // replace an existing key data["x"] = json!(1); // insert a new key data["y"] = json!([false, false, false]); // replace an array value data["y"][0] = json!(true); // inserted a deeply nested key data["a"]["b"]["c"]["d"] = json!(true); println!("{}", data);