# Function std::mem::forget

1.0.0 (const: 1.46.0) · source ·

    pub const fn forget<T>(t: T)

Expand description

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

The canonical safe use of `mem::forget` is to circumvent a value’s destructor implemented by the `Drop` trait. For example, this will leak a `File`, i.e. reclaim the space taken by the variable but never close the underlying system resource:

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

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

Run

This is useful when the ownership of the underlying resource was previously transferred to code outside of Rust, for example by transmitting the raw file descriptor to C code.

## Relationship with `ManuallyDrop`

While `mem::forget` can also be used to transfer memory ownership, doing so is error-prone. `ManuallyDrop` should be used instead. Consider, for example, this code:

    use std::mem;

    let mut v = vec![65, 122];
    // Build a `String` using the contents of `v`
    let s = unsafe { String::from_raw_parts(v.as_mut_ptr(), v.len(), v.capacity()) };
    // leak `v` because its memory is now managed by `s`
    mem::forget(v);  // ERROR - v is invalid and must not be passed to a function
    assert_eq!(s, "Az");
    // `s` is implicitly dropped and its memory deallocated.

Run

There are two issues with the above example:

- If more code were added between the construction of `String` and the invocation of `mem::forget()`, a panic within it would cause a double free because the same memory is handled by both `v` and `s`.
- After calling `v.as_mut_ptr()` and transmitting the ownership of the data to `s`, the `v` value is invalid. Even when a value is just moved to `mem::forget` (which won’t inspect it), some types have strict requirements on their values that make them invalid when dangling or no longer owned. Using invalid values in any way, including passing them to or returning them from functions, constitutes undefined behavior and may break the assumptions made by the compiler.

Switching to `ManuallyDrop` avoids both issues:

    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, len, cap) = (v.as_mut_ptr(), v.len(), v.capacity());
    // Finally, build a `String`.
    let s = unsafe { String::from_raw_parts(ptr, len, cap) };
    assert_eq!(s, "Az");
    // `s` is implicitly dropped and its memory deallocated.

Run

`ManuallyDrop` robustly prevents double-free because we disable `v`’s destructor before doing anything else. `mem::forget()` doesn’t allow this because it consumes its argument, forcing us to call it only after extracting anything we need from `v`. Even if a panic were introduced between construction of `ManuallyDrop` and building the string (which cannot happen in the code as shown), it would result in a leak and not a double free. In other words, `ManuallyDrop` errs on the side of leaking instead of erring on the side of (double-)dropping.

Also, `ManuallyDrop` prevents us from having to “touch” `v` after transferring the ownership to `s` — the final step of interacting with `v` to dispose of it without running its destructor is entirely avoided.