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% Drop
Now that we’ve discussed traits, let’s talk about a particular trait provided
by the Rust standard library, [`Drop`][drop]. The `Drop` trait provides a way
to run some code when a value goes out of scope. For example:
[drop]: ../std/ops/trait.Drop.html
```rust
struct HasDrop;
impl Drop for HasDrop {
fn drop(&mut self) {
println!("Dropping!");
}
}
fn main() {
let x = HasDrop;
// do stuff
} // x goes out of scope here
```
When `x` goes out of scope at the end of `main()`, the code for `Drop` will
run. `Drop` has one method, which is also called `drop()`. It takes a mutable
reference to `self`.
That’s it! The mechanics of `Drop` are very simple, but there are some
subtleties. For example, values are dropped in the opposite order they are
declared. Here’s another example:
```rust
struct Firework {
strength: i32,
}
impl Drop for Firework {
fn drop(&mut self) {
println!("BOOM times {}!!!", self.strength);
}
}
fn main() {
let firecracker = Firework { strength: 1 };
let tnt = Firework { strength: 100 };
}
```
This will output:
```text
BOOM times 100!!!
BOOM times 1!!!
```
The TNT goes off before the firecracker does, because it was declared
afterwards. Last in, first out.
So what is `Drop` good for? Generally, `Drop` is used to clean up any resources
associated with a `struct`. For example, the [`Arc<T>` type][arc] is a
reference-counted type. When `Drop` is called, it will decrement the reference
count, and if the total number of references is zero, will clean up the
underlying value.
[arc]: ../std/sync/struct.Arc.html
|
