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The book was located under 'src/doc/trpl' because originally, it was going to be hosted under that URL. Late in the game, before 1.0, we decided that /book was a better one, so we changed the output, but not the input. This causes confusion for no good reason. So we'll change the source directory to look like the output directory, like for every other thing in src/doc.
67 lines
1.6 KiB
Markdown
67 lines
1.6 KiB
Markdown
% Drop
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Now that we’ve discussed traits, let’s talk about a particular trait provided
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by the Rust standard library, [`Drop`][drop]. The `Drop` trait provides a way
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to run some code when a value goes out of scope. For example:
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[drop]: ../std/ops/trait.Drop.html
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```rust
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struct HasDrop;
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impl Drop for HasDrop {
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fn drop(&mut self) {
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println!("Dropping!");
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}
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}
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fn main() {
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let x = HasDrop;
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// do stuff
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} // x goes out of scope here
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```
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When `x` goes out of scope at the end of `main()`, the code for `Drop` will
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run. `Drop` has one method, which is also called `drop()`. It takes a mutable
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reference to `self`.
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That’s it! The mechanics of `Drop` are very simple, but there are some
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subtleties. For example, values are dropped in the opposite order they are
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declared. Here’s another example:
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```rust
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struct Firework {
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strength: i32,
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}
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impl Drop for Firework {
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fn drop(&mut self) {
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println!("BOOM times {}!!!", self.strength);
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}
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}
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fn main() {
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let firecracker = Firework { strength: 1 };
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let tnt = Firework { strength: 100 };
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}
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```
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This will output:
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```text
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BOOM times 100!!!
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BOOM times 1!!!
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```
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The TNT goes off before the firecracker does, because it was declared
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afterwards. Last in, first out.
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So what is `Drop` good for? Generally, `Drop` is used to clean up any resources
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associated with a `struct`. For example, the [`Arc<T>` type][arc] is a
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reference-counted type. When `Drop` is called, it will decrement the reference
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count, and if the total number of references is zero, will clean up the
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underlying value.
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[arc]: ../std/sync/struct.Arc.html
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