Trait nom::lib::std::ops::Drop

#[lang = "drop"]
pub trait Drop {
    pub fn drop(&mut self);
}

[−] Expand description

Custom code within the destructor.

When a value is no longer needed, Rust will run a “destructor” on that value. The most common way that a value is no longer needed is when it goes out of scope. Destructors may still run in other circumstances, but we’re going to focus on scope for the examples here. To learn about some of those other cases, please see the reference section on destructors.

This destructor consists of two components:

• A call to Drop::drop for that value, if this special Drop trait is implemented for its type.
• The automatically generated “drop glue” which recursively calls the destructors of the all fields of this value.

As Rust automatically calls the destructors of all contained fields, you don’t have to implement Drop in most cases. But there are some cases where it is useful, for example for types which directly manage a resource. That resource may be memory, it may be a file descriptor, it may be a network socket. Once a value of that type is no longer going to be used, it should “clean up” its resource by freeing the memory or closing the file or socket. This is the job of a destructor, and therefore the job of Drop::drop.

Examples

To see destructors in action, let’s take a look at the following program:

struct HasDrop;

impl Drop for HasDrop {
    fn drop(&mut self) {
        println!("Dropping HasDrop!");
    }
}

struct HasTwoDrops {
    one: HasDrop,
    two: HasDrop,
}

impl Drop for HasTwoDrops {
    fn drop(&mut self) {
        println!("Dropping HasTwoDrops!");
    }
}

fn main() {
    let _x = HasTwoDrops { one: HasDrop, two: HasDrop };
    println!("Running!");
}

Rust will first call Drop::drop for _x and then for both _x.one and _x.two, meaning that running this will print

Running!
Dropping HasTwoDrops!
Dropping HasDrop!
Dropping HasDrop!

Even if we remove the implementation of Drop for HasTwoDrop, the destructors of its fields are still called. This would result in

Running!
Dropping HasDrop!
Dropping HasDrop!

You cannot call Drop::drop yourself

Because Drop::drop is used to clean up a value, it may be dangerous to use this value after the method has been called. As Drop::drop does not take ownership of its input, Rust prevents misuse by not allowing you to call Drop::drop directly.

In other words, if you tried to explicitly call Drop::drop in the above example, you’d get a compiler error.

If you’d like to explicitly call the destructor of a value, mem::drop can be used instead.

Drop order

Which of our two HasDrop drops first, though? For structs, it’s the same order that they’re declared: first one, then two. If you’d like to try this yourself, you can modify HasDrop above to contain some data, like an integer, and then use it in the println! inside of Drop. This behavior is guaranteed by the language.

Unlike for structs, local variables are dropped in reverse order:

struct Foo;

impl Drop for Foo {
    fn drop(&mut self) {
        println!("Dropping Foo!")
    }
}

struct Bar;

impl Drop for Bar {
    fn drop(&mut self) {
        println!("Dropping Bar!")
    }
}

fn main() {
    let _foo = Foo;
    let _bar = Bar;
}

This will print

Dropping Bar!
Dropping Foo!

Please see the reference for the full rules.

Copy and Drop are exclusive

You cannot implement both Copy and Drop on the same type. Types that are Copy get implicitly duplicated by the compiler, making it very hard to predict when, and how often destructors will be executed. As such, these types cannot have destructors.

Required methods

pub 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 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.

Note that even if this panics, the value is considered to be dropped; you must not cause drop to be called again. This is normally automatically handled by the compiler, but when using unsafe code, can sometimes occur unintentionally, particularly when using ptr::drop_in_place.

Implementations on Foreign Types

impl Drop for Waker[−]

[+] Show hidden undocumented items

pub fn drop(&mut self)

impl<'f> Drop for VaListImpl<'f>[−]

[+] Show hidden undocumented items

pub fn drop(&mut self)

impl<T, const N: usize> Drop for IntoIter<T, N>[−]

[+] Show hidden undocumented items

pub fn drop(&mut self)

Implementors

impl<T> Drop for Sender<T>

impl Drop for PingSource

impl Drop for Signals

impl<T> Drop for Sender<T>

impl<T> Drop for Receiver<T>

impl Drop for SelectedOperation<'_>

impl<T> Drop for Injector<T>

impl<T: ?Sized + Pointable> Drop for Owned<T>

impl Drop for LocalHandle

impl Drop for Guard

impl<T: ?Sized> Drop for ShardedLockWriteGuard<'_, T>

impl Drop for WaitGroup

impl<W: Write> Drop for DeflateEncoder<W>

impl<W: Write> Drop for ZlibEncoder<W>

impl<'a, R: Resources> Drop for AccessGuard<'a, R>

impl<W: Write> Drop for Encoder<W>

impl Drop for Library

impl<'a, R: RawMutex + 'a, T: ?Sized + 'a> Drop for MutexGuard<'a, R, T>

impl<'a, R: RawMutex + 'a, T: ?Sized + 'a> Drop for MappedMutexGuard<'a, R, T>

impl<'a, R: RawMutex + 'a, G: GetThreadId + 'a, T: ?Sized + 'a> Drop for ReentrantMutexGuard<'a, R, G, T>

impl<'a, R: RawMutex + 'a, G: GetThreadId + 'a, T: ?Sized + 'a> Drop for MappedReentrantMutexGuard<'a, R, G, T>

impl<'a, R: RawRwLock + 'a, T: ?Sized + 'a> Drop for RwLockReadGuard<'a, R, T>

impl<'a, R: RawRwLock + 'a, T: ?Sized + 'a> Drop for RwLockWriteGuard<'a, R, T>

impl<'a, R: RawRwLockUpgrade + 'a, T: ?Sized + 'a> Drop for RwLockUpgradableReadGuard<'a, R, T>

impl<'a, R: RawRwLock + 'a, T: ?Sized + 'a> Drop for MappedRwLockReadGuard<'a, R, T>

impl<'a, R: RawRwLock + 'a, T: ?Sized + 'a> Drop for MappedRwLockWriteGuard<'a, R, T>

impl Drop for Registration

impl Drop for Dir

impl<'d> Drop for Iter<'d>

impl Drop for InterfaceAddressIterator

impl Drop for PtyMaster

impl<'a> Drop for AioCb<'a>

impl Drop for SignalFd

impl Drop for TimerFd

impl<T> Drop for OnceBox<T>

impl<W: Write> Drop for Writer<W>

impl<'a, W: Write> Drop for StreamWriter<'a, W>

impl<'a, T: Ord + Send> Drop for Drain<'a, T>

impl<'a, T: Send> Drop for Drain<'a, T>

impl<'a> Drop for Drain<'a>

impl<'data, T: Send> Drop for Drain<'data, T>

impl Drop for ThreadPool

impl Drop for Pool

impl<'pool, 'scope> Drop for Scope<'pool, 'scope>

impl<T, F, S> Drop for ScopeGuard<T, F, S> where     F: FnOnce(T),     S: Strategy,

impl Drop for DynamicLibrary

impl<'a, T: 'a + Array> Drop for Drain<'a, T>

impl<A: Array> Drop for SmallVec<A>

impl<A: Array> Drop for IntoIter<A>

impl Drop for DataDevice

impl Drop for DataOffer

impl Drop for PrimarySelectionDevice

impl Drop for PrimarySelectionOffer

impl Drop for MemPool

impl Drop for ConceptFrame

impl<F: Frame> Drop for Window<F>

impl<'a> Drop for ParseBuffer<'a>

impl<'a, W: Write + Seek> Drop for DirectoryEncoder<'a, W>

impl<'a, W: Write + Seek, C: ColorType> Drop for ImageEncoder<'a, W, C>

impl Drop for ReadEventsGuard

impl Drop for Socket

impl Drop for UserData

impl<T: ?Sized> Drop for ThreadGuard<T>

impl Drop for WlEglSurface

impl Drop for Window