57 lines
1.8 KiB
Markdown
57 lines
1.8 KiB
Markdown
|
# The `Drop` trait
|
||
|
|
|
||
|
|
When we introduced [destructors](../../02_ticket_v1/11_destructor/README.md),
|
||
|
|
we mentioned that the `drop` function:
|
||
|
|
|
||
|
|
1. reclaims the memory occupied by the type (i.e. `std::mem::size_of` bytes)
|
||
|
|
2. cleans up any additional resources that the value might be managing (e.g. the heap buffer of a `String`)
|
||
|
|
|
||
|
|
Step 2. is where the `Drop` trait comes in.
|
||
|
|
|
||
|
|
```rust
|
||
|
|
pub trait Drop {
|
||
|
|
fn drop(&mut self);
|
||
|
|
}
|
||
|
|
```
|
||
|
|
|
||
|
|
The `Drop` trait is a mechanism for you to define _additional_ cleanup logic for your types,
|
||
|
|
beyond what the compiler does for you automatically.
|
||
|
|
Whatever you put in the `drop` method will be executed when the value goes out of scope.
|
||
|
|
|
||
|
|
## `Drop` and `Copy`
|
||
|
|
|
||
|
|
When talking about the `Copy` trait, we said that a type can't implement `Copy` if it
|
||
|
|
manages additional resources beyond the `std::mem::size_of` bytes that it occupies in memory.
|
||
|
|
|
||
|
|
You might wonder: how does the compiler know if a type manages additional resources?
|
||
|
|
That's right: `Drop` trait implementations!
|
||
|
|
If your type has an explicit `Drop` implementation, the compiler will assume
|
||
|
|
that your type has additional resources attached to it and won't allow you to implement `Copy`.
|
||
|
|
|
||
|
|
```rust
|
||
|
|
// This is a unit struct, i.e. a struct with no fields.
|
||
|
|
#[derive(Clone, Copy)]
|
||
|
|
struct MyType;
|
||
|
|
|
||
|
|
impl Drop for MyType {
|
||
|
|
fn drop(&mut self) {
|
||
|
|
// We don't need to do anything here,
|
||
|
|
// it's enough to have an "empty" Drop implementation
|
||
|
|
}
|
||
|
|
}
|
||
|
|
```
|
||
|
|
|
||
|
|
The compiler will complain with this error message:
|
||
|
|
|
||
|
|
```text
|
||
|
|
error[E0184]: the trait `Copy` cannot be implemented for this type; the type has a destructor
|
||
|
|
--> src/lib.rs:2:17
|
||
|
|
|
|
||
|
|
2 | #[derive(Clone, Copy)]
|
||
|
|
| ^^^^ `Copy` not allowed on types with destructors
|
||
|
|
```
|
||
|
|
|
||
|
|
## References
|
||
|
|
|
||
|
|
- The exercise for this section is located in `exercises/04_traits/12_drop`
|