src/doc/rust-by-example/src/trait/drop.md

   1 # Drop
   2
   3 The [`Drop`][Drop] trait only has one method: `drop`, which is called automatically
   4 when an object goes out of scope. The main use of the `Drop` trait is to free the
   5 resources that the implementor instance owns.
   6
   7 `Box`, `Vec`, `String`, `File`, and `Process` are some examples of types that
   8 implement the `Drop` trait to free resources. The `Drop` trait can also be
   9 manually implemented for any custom data type.
  10
  11 The following example adds a print to console to the `drop` function to announce
  12 when it is called.
  13
  14 ```rust,editable
  15 struct Droppable {
  16     name: &'static str,
  17 }
  18
  19 // This trivial implementation of `drop` adds a print to console.
  20 impl Drop for Droppable {
  21     fn drop(&mut self) {
  22         println!("> Dropping {}", self.name);
  23     }
  24 }
  25
  26 fn main() {
  27     let _a = Droppable { name: "a" };
  28
  29     // block A
  30     {
  31         let _b = Droppable { name: "b" };
  32
  33         // block B
  34         {
  35             let _c = Droppable { name: "c" };
  36             let _d = Droppable { name: "d" };
  37
  38             println!("Exiting block B");
  39         }
  40         println!("Just exited block B");
  41
  42         println!("Exiting block A");
  43     }
  44     println!("Just exited block A");
  45
  46     // Variable can be manually dropped using the `drop` function
  47     drop(_a);
  48     // TODO ^ Try commenting this line
  49
  50     println!("end of the main function");
  51
  52     // `_a` *won't* be `drop`ed again here, because it already has been
  53     // (manually) `drop`ed
  54 }
  55 ```
  56
  57 [Drop]: https://doc.rust-lang.org/std/ops/trait.Drop.html