boxed.rs.html -- source
Description: Source to the Rust file `/usr/obj/ports/rust-1.20.0/rustc-1.20.0-src/src/liballoc/boxed.rs`.
Keywords: rust, rustlang, rust-lang

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    // Copyright 2012-2015 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    //! A pointer type for heap allocation.
    //!
    //! `Box<T>`, casually referred to as a 'box', provides the simplest form of
    //! heap allocation in Rust. Boxes provide ownership for this allocation, and
    //! drop their contents when they go out of scope.
    //!
    //! # Examples
    //!
    //! Creating a box:
    //!
    //! ```
    //! let x = Box::new(5);
    //! ```
    //!
    //! Creating a recursive data structure:
    //!
    //! ```
    //! #[derive(Debug)]
    //! enum List<T> {
    //!     Cons(T, Box<List<T>>),
    //!     Nil,
    //! }
    //!
    //! fn main() {
    //!     let list: List<i32> = List::Cons(1, Box::new(List::Cons(2, Box::new(List::Nil))));
    //!     println!("{:?}", list);
    //! }
    //! ```
    //!
    //! This will print `Cons(1, Cons(2, Nil))`.
    //!
    //! Recursive structures must be boxed, because if the definition of `Cons`
    //! looked like this:
    //!
    //! ```compile_fail,E0072
    //! # enum List<T> {
    //! Cons(T, List<T>),
    //! # }
    //! ```
    //!
    //! It wouldn't work. This is because the size of a `List` depends on how many
    //! elements are in the list, and so we don't know how much memory to allocate
    //! for a `Cons`. By introducing a `Box`, which has a defined size, we know how
    //! big `Cons` needs to be.

    #![stable(feature = "rust1", since = "1.0.0")]

    use heap::{Heap, Layout, Alloc};
    use raw_vec::RawVec;

    use core::any::Any;
    use core::borrow;
    use core::cmp::Ordering;
    use core::fmt;
    use core::hash::{self, Hash};
    use core::iter::FusedIterator;
    use core::marker::{self, Unsize};
    use core::mem;
    use core::ops::{CoerceUnsized, Deref, DerefMut};
    use core::ops::{BoxPlace, Boxed, InPlace, Place, Placer};
    use core::ptr::{self, Unique};
    use core::convert::From;
    use str::from_boxed_utf8_unchecked;

    /// A value that represents the heap. This is the default place that the `box`
    /// keyword allocates into when no place is supplied.
    ///
    /// The following two examples are equivalent:
    ///
    /// ```
    /// #![feature(box_heap)]
    ///
    /// #![feature(box_syntax, placement_in_syntax)]
    /// use std::boxed::HEAP;
    ///
    /// fn main() {
    ///     let foo: Box<i32> = in HEAP { 5 };
    ///     let foo = box 5;
    /// }
    /// ```
    #[unstable(feature = "box_heap",
               reason = "may be renamed; uncertain about custom allocator design",
               issue = "27779")]
    pub const HEAP: ExchangeHeapSingleton = ExchangeHeapSingleton { _force_singleton: () };

    /// This the singleton type used solely for `boxed::HEAP`.
    #[unstable(feature = "box_heap",
               reason = "may be renamed; uncertain about custom allocator design",
               issue = "27779")]
    #[allow(missing_debug_implementations)]
    #[derive(Copy, Clone)]
    pub struct ExchangeHeapSingleton {
        _force_singleton: (),
    }

    /// A pointer type for heap allocation.
    ///
    /// See the [module-level documentation](../../std/boxed/index.html) for more.
    #[lang = "owned_box"]
    #[fundamental]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Box<T: ?Sized>(Unique<T>);

    /// `IntermediateBox` represents uninitialized backing storage for `Box`.
    ///
    /// FIXME (pnkfelix): Ideally we would just reuse `Box<T>` instead of
    /// introducing a separate `IntermediateBox<T>`; but then you hit
    /// issues when you e.g. attempt to destructure an instance of `Box`,
    /// since it is a lang item and so it gets special handling by the
    /// compiler.  Easier just to make this parallel type for now.
    ///
    /// FIXME (pnkfelix): Currently the `box` protocol only supports
    /// creating instances of sized types. This IntermediateBox is
    /// designed to be forward-compatible with a future protocol that
    /// supports creating instances of unsized types; that is why the type
    /// parameter has the `?Sized` generalization marker, and is also why
    /// this carries an explicit size. However, it probably does not need
    /// to carry the explicit alignment; that is just a work-around for
    /// the fact that the `align_of` intrinsic currently requires the
    /// input type to be Sized (which I do not think is strictly
    /// necessary).
    #[unstable(feature = "placement_in",
               reason = "placement box design is still being worked out.",
               issue = "27779")]
    #[allow(missing_debug_implementations)]
    pub struct IntermediateBox<T: ?Sized> {
        ptr: *mut u8,
        layout: Layout,
        marker: marker::PhantomData<*mut T>,
    }

    #[unstable(feature = "placement_in",
               reason = "placement box design is still being worked out.",
               issue = "27779")]
    impl<T> Place<T> for IntermediateBox<T> {
        fn pointer(&mut self) -> *mut T {
            self.ptr as *mut T
        }
    }

    unsafe fn finalize<T>(b: IntermediateBox<T>) -> Box<T> {
        let p = b.ptr as *mut T;
        mem::forget(b);
        mem::transmute(p)
    }

    fn make_place<T>() -> IntermediateBox<T> {
        let layout = Layout::new::<T>();

        let p = if layout.size() == 0 {
            mem::align_of::<T>() as *mut u8
        } else {
            unsafe {
                Heap.alloc(layout.clone()).unwrap_or_else(|err| {
                    Heap.oom(err)
                })
            }
        };

        IntermediateBox {
            ptr: p,
            layout: layout,
            marker: marker::PhantomData,
        }
    }

    #[unstable(feature = "placement_in",
               reason = "placement box design is still being worked out.",
               issue = "27779")]
    impl<T> BoxPlace<T> for IntermediateBox<T> {
        fn make_place() -> IntermediateBox<T> {
            make_place()
        }
    }

    #[unstable(feature = "placement_in",
               reason = "placement box design is still being worked out.",
               issue = "27779")]
    impl<T> InPlace<T> for IntermediateBox<T> {
        type Owner = Box<T>;
        unsafe fn finalize(self) -> Box<T> {
            finalize(self)
        }
    }

    #[unstable(feature = "placement_new_protocol", issue = "27779")]
    impl<T> Boxed for Box<T> {
        type Data = T;
        type Place = IntermediateBox<T>;
        unsafe fn finalize(b: IntermediateBox<T>) -> Box<T> {
            finalize(b)
        }
    }

    #[unstable(feature = "placement_in",
               reason = "placement box design is still being worked out.",
               issue = "27779")]
    impl<T> Placer<T> for ExchangeHeapSingleton {
        type Place = IntermediateBox<T>;

        fn make_place(self) -> IntermediateBox<T> {
            make_place()
        }
    }

    #[unstable(feature = "placement_in",
               reason = "placement box design is still being worked out.",
               issue = "27779")]
    impl<T: ?Sized> Drop for IntermediateBox<T> {
        fn drop(&mut self) {
            if self.layout.size() > 0 {
                unsafe {
                    Heap.dealloc(self.ptr, self.layout.clone())
                }
            }
        }
    }

    impl<T> Box<T> {
        /// Allocates memory on the heap and then places `x` into it.
        ///
        /// This doesn't actually allocate if `T` is zero-sized.
        ///
        /// # Examples
        ///
        /// ```
        /// let five = Box::new(5);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[inline(always)]
        pub fn new(x: T) -> Box<T> {
            box x
        }
    }

    impl<T: ?Sized> Box<T> {
        /// Constructs a box from a raw pointer.
        ///
        /// After calling this function, the raw pointer is owned by the
        /// resulting `Box`. Specifically, the `Box` destructor will call
        /// the destructor of `T` and free the allocated memory. Since the
        /// way `Box` allocates and releases memory is unspecified, the
        /// only valid pointer to pass to this function is the one taken
        /// from another `Box` via the [`Box::into_raw`] function.
        ///
        /// This function is unsafe because improper use may lead to
        /// memory problems. For example, a double-free may occur if the
        /// function is called twice on the same raw pointer.
        ///
        /// [`Box::into_raw`]: struct.Box.html#method.into_raw
        ///
        /// # Examples
        ///
        /// ```
        /// let x = Box::new(5);
        /// let ptr = Box::into_raw(x);
        /// let x = unsafe { Box::from_raw(ptr) };
        /// ```
        #[stable(feature = "box_raw", since = "1.4.0")]
        #[inline]
        pub unsafe fn from_raw(raw: *mut T) -> Self {
            mem::transmute(raw)
        }

        /// Consumes the `Box`, returning the wrapped raw pointer.
        ///
        /// After calling this function, the caller is responsible for the
        /// memory previously managed by the `Box`. In particular, the
        /// caller should properly destroy `T` and release the memory. The
        /// proper way to do so is to convert the raw pointer back into a
        /// `Box` with the [`Box::from_raw`] function.
        ///
        /// Note: this is an associated function, which means that you have
        /// to call it as `Box::into_raw(b)` instead of `b.into_raw()`. This
        /// is so that there is no conflict with a method on the inner type.
        ///
        /// [`Box::from_raw`]: struct.Box.html#method.from_raw
        ///
        /// # Examples
        ///
        /// ```
        /// let x = Box::new(5);
        /// let ptr = Box::into_raw(x);
        /// ```
        #[stable(feature = "box_raw", since = "1.4.0")]
        #[inline]
        pub fn into_raw(b: Box<T>) -> *mut T {
            unsafe { mem::transmute(b) }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    unsafe impl<#[may_dangle] T: ?Sized> Drop for Box<T> {
        fn drop(&mut self) {
            // FIXME: Do nothing, drop is currently performed by compiler.
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: Default> Default for Box<T> {
        /// Creates a `Box<T>`, with the `Default` value for T.
        fn default() -> Box<T> {
            box Default::default()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> Default for Box<[T]> {
        fn default() -> Box<[T]> {
            Box::<[T; 0]>::new([])
        }
    }

    #[stable(feature = "default_box_extra", since = "1.17.0")]
    impl Default for Box<str> {
        fn default() -> Box<str> {
            unsafe { from_boxed_utf8_unchecked(Default::default()) }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: Clone> Clone for Box<T> {
        /// Returns a new box with a `clone()` of this box's contents.
        ///
        /// # Examples
        ///
        /// ```
        /// let x = Box::new(5);
        /// let y = x.clone();
        /// ```
        #[rustfmt_skip]
        #[inline]
        fn clone(&self) -> Box<T> {
            box { (**self).clone() }
        }
        /// Copies `source`'s contents into `self` without creating a new allocation.
        ///
        /// # Examples
        ///
        /// ```
        /// let x = Box::new(5);
        /// let mut y = Box::new(10);
        ///
        /// y.clone_from(&x);
        ///
        /// assert_eq!(*y, 5);
        /// ```
        #[inline]
        fn clone_from(&mut self, source: &Box<T>) {
            (**self).clone_from(&(**source));
        }
    }

    #[stable(feature = "box_slice_clone", since = "1.3.0")]
    impl Clone for Box<str> {
        fn clone(&self) -> Self {
            let len = self.len();
            let buf = RawVec::with_capacity(len);
            unsafe {
                ptr::copy_nonoverlapping(self.as_ptr(), buf.ptr(), len);
                from_boxed_utf8_unchecked(buf.into_box())
            }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized + PartialEq> PartialEq for Box<T> {
        #[inline]
        fn eq(&self, other: &Box<T>) -> bool {
            PartialEq::eq(&**self, &**other)
        }
        #[inline]
        fn ne(&self, other: &Box<T>) -> bool {
            PartialEq::ne(&**self, &**other)
        }
    }
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized + PartialOrd> PartialOrd for Box<T> {
        #[inline]
        fn partial_cmp(&self, other: &Box<T>) -> Option<Ordering> {
            PartialOrd::partial_cmp(&**self, &**other)
        }
        #[inline]
        fn lt(&self, other: &Box<T>) -> bool {
            PartialOrd::lt(&**self, &**other)
        }
        #[inline]
        fn le(&self, other: &Box<T>) -> bool {
            PartialOrd::le(&**self, &**other)
        }
        #[inline]
        fn ge(&self, other: &Box<T>) -> bool {
            PartialOrd::ge(&**self, &**other)
        }
        #[inline]
        fn gt(&self, other: &Box<T>) -> bool {
            PartialOrd::gt(&**self, &**other)
        }
    }
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized + Ord> Ord for Box<T> {
        #[inline]
        fn cmp(&self, other: &Box<T>) -> Ordering {
            Ord::cmp(&**self, &**other)
        }
    }
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized + Eq> Eq for Box<T> {}

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized + Hash> Hash for Box<T> {
        fn hash<H: hash::Hasher>(&self, state: &mut H) {
            (**self).hash(state);
        }
    }

    #[stable(feature = "from_for_ptrs", since = "1.6.0")]
    impl<T> From<T> for Box<T> {
        fn from(t: T) -> Self {
            Box::new(t)
        }
    }

    #[stable(feature = "box_from_slice", since = "1.17.0")]
    impl<'a, T: Copy> From<&'a [T]> for Box<[T]> {
        fn from(slice: &'a [T]) -> Box<[T]> {
            let mut boxed = unsafe { RawVec::with_capacity(slice.len()).into_box() };
            boxed.copy_from_slice(slice);
            boxed
        }
    }

    #[stable(feature = "box_from_slice", since = "1.17.0")]
    impl<'a> From<&'a str> for Box<str> {
        fn from(s: &'a str) -> Box<str> {
            unsafe { from_boxed_utf8_unchecked(Box::from(s.as_bytes())) }
        }
    }

    #[stable(feature = "boxed_str_conv", since = "1.19.0")]
    impl From<Box<str>> for Box<[u8]> {
        fn from(s: Box<str>) -> Self {
            unsafe {
                mem::transmute(s)
            }
        }
    }

    impl Box<Any> {
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        /// Attempt to downcast the box to a concrete type.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::any::Any;
        ///
        /// fn print_if_string(value: Box<Any>) {
        ///     if let Ok(string) = value.downcast::<String>() {
        ///         println!("String ({}): {}", string.len(), string);
        ///     }
        /// }
        ///
        /// fn main() {
        ///     let my_string = "Hello World".to_string();
        ///     print_if_string(Box::new(my_string));
        ///     print_if_string(Box::new(0i8));
        /// }
        /// ```
        pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<Any>> {
            if self.is::<T>() {
                unsafe {
                    let raw: *mut Any = Box::into_raw(self);
                    Ok(Box::from_raw(raw as *mut T))
                }
            } else {
                Err(self)
            }
        }
    }

    impl Box<Any + Send> {
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        /// Attempt to downcast the box to a concrete type.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::any::Any;
        ///
        /// fn print_if_string(value: Box<Any + Send>) {
        ///     if let Ok(string) = value.downcast::<String>() {
        ///         println!("String ({}): {}", string.len(), string);
        ///     }
        /// }
        ///
        /// fn main() {
        ///     let my_string = "Hello World".to_string();
        ///     print_if_string(Box::new(my_string));
        ///     print_if_string(Box::new(0i8));
        /// }
        /// ```
        pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<Any + Send>> {
            <Box<Any>>::downcast(self).map_err(|s| unsafe {
                // reapply the Send marker
                mem::transmute::<Box<Any>, Box<Any + Send>>(s)
            })
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: fmt::Display + ?Sized> fmt::Display for Box<T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            fmt::Display::fmt(&**self, f)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: fmt::Debug + ?Sized> fmt::Debug for Box<T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            fmt::Debug::fmt(&**self, f)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized> fmt::Pointer for Box<T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            // It's not possible to extract the inner Uniq directly from the Box,
            // instead we cast it to a *const which aliases the Unique
            let ptr: *const T = &**self;
            fmt::Pointer::fmt(&ptr, f)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized> Deref for Box<T> {
        type Target = T;

        fn deref(&self) -> &T {
            &**self
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized> DerefMut for Box<T> {
        fn deref_mut(&mut self) -> &mut T {
            &mut **self
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<I: Iterator + ?Sized> Iterator for Box<I> {
        type Item = I::Item;
        fn next(&mut self) -> Option<I::Item> {
            (**self).next()
        }
        fn size_hint(&self) -> (usize, Option<usize>) {
            (**self).size_hint()
        }
        fn nth(&mut self, n: usize) -> Option<I::Item> {
            (**self).nth(n)
        }
    }
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<I: DoubleEndedIterator + ?Sized> DoubleEndedIterator for Box<I> {
        fn next_back(&mut self) -> Option<I::Item> {
            (**self).next_back()
        }
    }
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<I: ExactSizeIterator + ?Sized> ExactSizeIterator for Box<I> {
        fn len(&self) -> usize {
            (**self).len()
        }
        fn is_empty(&self) -> bool {
            (**self).is_empty()
        }
    }

    #[unstable(feature = "fused", issue = "35602")]
    impl<I: FusedIterator + ?Sized> FusedIterator for Box<I> {}

    /// `FnBox` is a version of the `FnOnce` intended for use with boxed
    /// closure objects. The idea is that where one would normally store a
    /// `Box<FnOnce()>` in a data structure, you should use
    /// `Box<FnBox()>`. The two traits behave essentially the same, except
    /// that a `FnBox` closure can only be called if it is boxed. (Note
    /// that `FnBox` may be deprecated in the future if `Box<FnOnce()>`
    /// closures become directly usable.)
    ///
    /// ### Example
    ///
    /// Here is a snippet of code which creates a hashmap full of boxed
    /// once closures and then removes them one by one, calling each
    /// closure as it is removed. Note that the type of the closures
    /// stored in the map is `Box<FnBox() -> i32>` and not `Box<FnOnce()
    /// -> i32>`.
    ///
    /// ```
    /// #![feature(fnbox)]
    ///
    /// use std::boxed::FnBox;
    /// use std::collections::HashMap;
    ///
    /// fn make_map() -> HashMap<i32, Box<FnBox() -> i32>> {
    ///     let mut map: HashMap<i32, Box<FnBox() -> i32>> = HashMap::new();
    ///     map.insert(1, Box::new(|| 22));
    ///     map.insert(2, Box::new(|| 44));
    ///     map
    /// }
    ///
    /// fn main() {
    ///     let mut map = make_map();
    ///     for i in &[1, 2] {
    ///         let f = map.remove(&i).unwrap();
    ///         assert_eq!(f(), i * 22);
    ///     }
    /// }
    /// ```
    #[rustc_paren_sugar]
    #[unstable(feature = "fnbox",
               reason = "will be deprecated if and when `Box<FnOnce>` becomes usable", issue = "28796")]
    pub trait FnBox<A> {
        type Output;

        fn call_box(self: Box<Self>, args: A) -> Self::Output;
    }

    #[unstable(feature = "fnbox",
               reason = "will be deprecated if and when `Box<FnOnce>` becomes usable", issue = "28796")]
    impl<A, F> FnBox<A> for F
        where F: FnOnce<A>
    {
        type Output = F::Output;

        fn call_box(self: Box<F>, args: A) -> F::Output {
            self.call_once(args)
        }
    }

    #[unstable(feature = "fnbox",
               reason = "will be deprecated if and when `Box<FnOnce>` becomes usable", issue = "28796")]
    impl<'a, A, R> FnOnce<A> for Box<FnBox<A, Output = R> + 'a> {
        type Output = R;

        extern "rust-call" fn call_once(self, args: A) -> R {
            self.call_box(args)
        }
    }

    #[unstable(feature = "fnbox",
               reason = "will be deprecated if and when `Box<FnOnce>` becomes usable", issue = "28796")]
    impl<'a, A, R> FnOnce<A> for Box<FnBox<A, Output = R> + Send + 'a> {
        type Output = R;

        extern "rust-call" fn call_once(self, args: A) -> R {
            self.call_box(args)
        }
    }

    #[unstable(feature = "coerce_unsized", issue = "27732")]
    impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Box<U>> for Box<T> {}

    #[stable(feature = "box_slice_clone", since = "1.3.0")]
    impl<T: Clone> Clone for Box<[T]> {
        fn clone(&self) -> Self {
            let mut new = BoxBuilder {
                data: RawVec::with_capacity(self.len()),
                len: 0,
            };

            let mut target = new.data.ptr();

            for item in self.iter() {
                unsafe {
                    ptr::write(target, item.clone());
                    target = target.offset(1);
                };

                new.len += 1;
            }

            return unsafe { new.into_box() };

            // Helper type for responding to panics correctly.
            struct BoxBuilder<T> {
                data: RawVec<T>,
                len: usize,
            }

            impl<T> BoxBuilder<T> {
                unsafe fn into_box(self) -> Box<[T]> {
                    let raw = ptr::read(&self.data);
                    mem::forget(self);
                    raw.into_box()
                }
            }

            impl<T> Drop for BoxBuilder<T> {
                fn drop(&mut self) {
                    let mut data = self.data.ptr();
                    let max = unsafe { data.offset(self.len as isize) };

                    while data != max {
                        unsafe {
                            ptr::read(data);
                            data = data.offset(1);
                        }
                    }
                }
            }
        }
    }

    #[stable(feature = "box_borrow", since = "1.1.0")]
    impl<T: ?Sized> borrow::Borrow<T> for Box<T> {
        fn borrow(&self) -> &T {
            &**self
        }
    }

    #[stable(feature = "box_borrow", since = "1.1.0")]
    impl<T: ?Sized> borrow::BorrowMut<T> for Box<T> {
        fn borrow_mut(&mut self) -> &mut T {
            &mut **self
        }
    }

    #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
    impl<T: ?Sized> AsRef<T> for Box<T> {
        fn as_ref(&self) -> &T {
            &**self
        }
    }

    #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
    impl<T: ?Sized> AsMut<T> for Box<T> {
        fn as_mut(&mut self) -> &mut T {
            &mut **self
        }
    }

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