1.0.0[−][src]Trait core::ops::Drop
Used to run some code when a value goes out of scope. This is sometimes called a 'destructor'.
When a value goes out of scope, it will have its drop
method called if
its type implements Drop
. Then, any fields the value contains will also
be dropped recursively.
Because of this recursive dropping, you do not need to implement this trait unless your type needs its own destructor logic.
Refer to the chapter on Drop
in The Rust Programming Language
for some more elaboration.
Examples
Implementing Drop
The drop
method is called when _x
goes out of scope, and therefore
main
prints Dropping!
.
struct HasDrop; impl Drop for HasDrop { fn drop(&mut self) { println!("Dropping!"); } } fn main() { let _x = HasDrop; }Run
Dropping is done recursively
When outer
goes out of scope, the drop
method will be called first for
Outer
, then for Inner
. Therefore, main
prints Dropping Outer!
and
then Dropping Inner!
.
struct Inner; struct Outer(Inner); impl Drop for Inner { fn drop(&mut self) { println!("Dropping Inner!"); } } impl Drop for Outer { fn drop(&mut self) { println!("Dropping Outer!"); } } fn main() { let _x = Outer(Inner); }Run
Variables are dropped in reverse order of declaration
_first
is declared first and _second
is declared second, so main
will
print Declared second!
and then Declared first!
.
struct PrintOnDrop(&'static str); impl Drop for PrintOnDrop { fn drop(&mut self) { println!("{}", self.0); } } fn main() { let _first = PrintOnDrop("Declared first!"); let _second = PrintOnDrop("Declared second!"); }Run
Required methods
fn drop(&mut self)
Executes the destructor for this type.
This method is called implicitly when the value goes out of scope,
and cannot be called explicitly (this is compiler error E0040).
However, the std::mem::drop
function in the prelude can be
used to call the argument's Drop
implementation.
When this method has been called, self
has not yet been deallocated.
That only happens after the method is over.
If this wasn't the case, self
would be a dangling reference.
Panics
Given that a panic!
will call drop
as it unwinds, any panic!
in a drop
implementation will likely abort.