100 exercises to learn Rust

book/src/06_ticket_management/13_index.md

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+# Indexing
+
+`TicketStore::get` returns an `Option<&Ticket>` for a given `TicketId`.  
+We've seen before how to access elements of arrays and vectors using Rust's 
+indexing syntax:
+
+```rust
+let v = vec![0, 1, 2];
+assert_eq!(v[0], 0);
+```
+
+How can we provide the same experience for `TicketStore`?  
+You guessed right: we need to implement a trait, `Index`!
+
+## `Index`
+
+The `Index` trait is defined in Rust's standard library:
+
+```rust
+// Slightly simplified
+pub trait Index<Idx>
+{
+    type Output;
+
+    // Required method
+    fn index(&self, index: Idx) -> &Self::Output;
+}
+```
+
+It has:
+
+- One generic parameter, `Idx`, to represent the index type
+- One associated type, `Output`, to represent the type we retrieved using the index
+
+Notice how the `index` method doesn't return an `Option`. The assumption is that
+`index` will panic if you try to access an element that's not there, as it happens
+for array and vec indexing.