Trait caffe2_imports::Drop

1.0.0 · source ·

pub trait Drop {
    // Required method
    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 all the 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§

source

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.

Implementors§

1.36.0 · source§

Trait caffe2_imports::Drop

Required Methods§

fn drop(&mut self)

Implementors§

impl Drop for Waker

impl Drop for Algorithm

impl Drop for Arrays

impl Drop for AsyncArray

impl Drop for AsyncPromise

impl Drop for Buffer

impl Drop for BufferPool

impl Drop for CommandLineParser

impl Drop for Context

impl Drop for Context_UserContext

impl Drop for Detail_CheckContext

impl Drop for Device

impl Drop for DeviceInfo

impl Drop for Event

impl Drop for Exception

impl Drop for FileNode

impl Drop for FileNodeIterator

impl Drop for FileStorage

impl Drop for GpuData

impl Drop for GpuMat

impl Drop for GpuMatND

impl Drop for Hamming

impl Drop for HostMem

impl Drop for Image2D

impl Drop for Kernel

impl Drop for KernelArg

impl Drop for KeyPoint

impl Drop for LDA

impl Drop for LogTag

impl Drop for Mat

impl Drop for MatConstIterator

impl Drop for MatExpr

impl Drop for MatSize

impl Drop for MatStep

impl Drop for Matx_AddOp

impl Drop for Matx_DivOp

impl Drop for Matx_MatMulOp

impl Drop for Matx_MulOp

impl Drop for Matx_ScaleOp

impl Drop for Matx_SubOp

impl Drop for Matx_TOp

impl Drop for NodeData

impl Drop for OpenCLExecutionContext

impl Drop for OriginalClassName

impl Drop for PCA

impl Drop for Platform

impl Drop for PlatformInfo

impl Drop for Program

impl Drop for ProgramSource

impl Drop for Queue

impl Drop for RNG

impl Drop for RNG_MT19937

impl Drop for Range

impl Drop for RotatedRect

impl Drop for SVD

impl Drop for SparseMat

impl Drop for SparseMatConstIterator

impl Drop for SparseMatIterator

impl Drop for SparseMat_Hdr

impl Drop for SparseMat_Node

impl Drop for Stream

impl Drop for TargetArchs

impl Drop for Texture2D

impl Drop for TickMeter

impl Drop for Timer

impl Drop for UMat

impl Drop for UMatData

impl Drop for WriteStructContext

impl Drop for _InputArray

impl Drop for _InputOutputArray

impl Drop for _OutputArray

impl Drop for CString

impl Drop for alloc::string::Drain<'_>

impl Drop for OwnedFd

impl Drop for ConvolutionDescriptor

impl Drop for Cudnn