std::mem::forget - Rust
Description: API documentation for the Rust `forget` fn in crate `std`.
Keywords: rust, rustlang, rust-lang, forget

# 1.0.0[−][src]Function std::mem::forget

    pub fn forget<T>(t: T)

Takes ownership and "forgets" about the value without running its destructor.

Any resources the value manages, such as heap memory or a file handle, will linger forever in an unreachable state. However, it does not guarantee that pointers to this memory will remain valid.

- If you want to leak memory, see `Box::leak`.
- If you want to obtain a raw pointer to the memory, see `Box::into_raw`.
- If you want to dispose of a value properly, running its destructor, see `mem::drop`.

# Safety

`forget` is not marked as `unsafe`, because Rust's safety guarantees do not include a guarantee that destructors will always run. For example, a program can create a reference cycle using `Rc`, or call `process::exit` to exit without running destructors. Thus, allowing `mem::forget` from safe code does not fundamentally change Rust's safety guarantees.

That said, leaking resources such as memory or I/O objects is usually undesirable. The need comes up in some specialized use cases for FFI or unsafe code, but even then, `ManuallyDrop` is typically preferred.

Because forgetting a value is allowed, any `unsafe` code you write must allow for this possibility. You cannot return a value and expect that the caller will necessarily run the value's destructor.

# Examples

Leak an I/O object, never closing the file:

    use std::mem;
    use std::fs::File;

    let file = File::open("foo.txt").unwrap();
    mem::forget(file);Run

The practical use cases for `forget` are rather specialized and mainly come up in unsafe or FFI code. However, `ManuallyDrop` is usually preferred for such cases, e.g.:

    use std::mem::ManuallyDrop;

    let v = vec![65, 122];
    // Before we disassemble `v` into its raw parts, make sure it
    // does not get dropped!
    let mut v = ManuallyDrop::new(v);
    // Now disassemble `v`. These operations cannot panic, so there cannot be a leak.
    let ptr = v.as_mut_ptr();
    let cap = v.capacity();
    // Finally, build a `String`.
    let s = unsafe { String::from_raw_parts(ptr, 2, cap) };
    assert_eq!(s, "Az");
    // `s` is implicitly dropped and its memory deallocated.Run

Using `ManuallyDrop` here has two advantages:

- We do not "touch" `v` after disassembling it. For some types, operations such as passing ownership (to a funcion like `mem::forget`) requires them to actually be fully owned right now; that is a promise we do not want to make here as we are in the process of transferring ownership to the new `String` we are building.
- In case of an unexpected panic, `ManuallyDrop` is not dropped, but if the panic occurs before `mem::forget` was called we might end up dropping invalid data, or double-dropping. In other words, `ManuallyDrop` errs on the side of leaking instead of erring on the side of dropping.