# 1.28.0[−][src]Struct std::alloc::System

    pub struct System;

The default memory allocator provided by the operating system.

This is based on `malloc` on Unix platforms and `HeapAlloc` on Windows, plus related functions.

This type implements the `GlobalAlloc` trait and Rust programs by default work as if they had this definition:

    use std::alloc::System;

    #[global_allocator]
    static A: System = System;

    fn main() {
        let a = Box::new(4); // Allocates from the system allocator.
        println!("{}", a);
    }Run

You can also define your own wrapper around `System` if you'd like, such as keeping track of the number of all bytes allocated:

    use std::alloc::{System, GlobalAlloc, Layout};
    use std::sync::atomic::{AtomicUsize, ATOMIC_USIZE_INIT, Ordering::SeqCst};

    struct Counter;

    static ALLOCATED: AtomicUsize = ATOMIC_USIZE_INIT;

    unsafe impl GlobalAlloc for Counter {
        unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
            let ret = System.alloc(layout);
            if !ret.is_null() {
                ALLOCATED.fetch_add(layout.size(), SeqCst);
            }
            return ret
        }

        unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) {
            System.dealloc(ptr, layout);
            ALLOCATED.fetch_sub(layout.size(), SeqCst);
        }
    }

    #[global_allocator]
    static A: Counter = Counter;

    fn main() {
        println!("allocated bytes before main: {}", ALLOCATED.load(SeqCst));
    }Run

It can also be used directly to allocate memory independently of whatever global allocator has been selected for a Rust program. For example if a Rust program opts in to using jemalloc as the global allocator, `System` will still allocate memory using `malloc` and `HeapAlloc`.

## Trait Implementations

###

`impl Debug for System`

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####

`fn fmt(&self, f: &mut Formatter) -> Result`

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Formats the value using the given formatter. Read more

###

`impl Copy for System`

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`impl Clone for System`

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####

`fn clone(&self) -> System`

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Returns a copy of the value. Read more

####

`fn clone_from(&mut self, source: &Self)`

1.0.0
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Performs copy-assignment from `source`. Read more

###

`impl GlobalAlloc for System`

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####

`unsafe fn alloc(&self, layout: Layout) -> *mut u8`

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Allocate memory as described by the given `layout`. Read more

####

`unsafe fn alloc_zeroed(&self, layout: Layout) -> *mut u8`

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Behaves like `alloc`, but also ensures that the contents are set to zero before being returned. Read more

####

`unsafe fn dealloc(&self, ptr: *mut u8, _layout: Layout)`

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Deallocate the block of memory at the given `ptr` pointer with the given `layout`. Read more

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`unsafe fn realloc( &self, ptr: *mut u8, layout: Layout, new_size: usize ) -> *mut u8`

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Shrink or grow a block of memory to the given `new_size`. The block is described by the given `ptr` pointer and `layout`. Read more

###

`impl Alloc for System`

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####

`unsafe fn alloc(&mut self, layout: Layout) -> Result<NonNull<u8>, AllocErr>`

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🔬 This is a nightly-only experimental API. (`allocator_api` #32838)

Returns a pointer meeting the size and alignment guarantees of `layout`. Read more

####

`unsafe fn alloc_zeroed( &mut self, layout: Layout ) -> Result<NonNull<u8>, AllocErr>`

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🔬 This is a nightly-only experimental API. (`allocator_api` #32838)

Behaves like `alloc`, but also ensures that the contents are set to zero before being returned. Read more

####

`unsafe fn dealloc(&mut self, ptr: NonNull<u8>, layout: Layout)`

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🔬 This is a nightly-only experimental API. (`allocator_api` #32838)

Deallocate the memory referenced by `ptr`. Read more

####

`unsafe fn realloc( &mut self, ptr: NonNull<u8>, layout: Layout, new_size: usize ) -> Result<NonNull<u8>, AllocErr>`

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🔬 This is a nightly-only experimental API. (`allocator_api` #32838)

Returns a pointer suitable for holding data described by a new layout with `layout`’s alignment and a size given by `new_size`. To accomplish this, this may extend or shrink the allocation referenced by `ptr` to fit the new layout. Read more

####

`fn usable_size(&self, layout: &Layout) -> (usize, usize)`

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🔬 This is a nightly-only experimental API. (`allocator_api` #32838)

Returns bounds on the guaranteed usable size of a successful allocation created with the specified `layout`. Read more

####

`unsafe fn alloc_excess(&mut self, layout: Layout) -> Result<Excess, AllocErr>`

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🔬 This is a nightly-only experimental API. (`allocator_api` #32838)

Behaves like `alloc`, but also returns the whole size of the returned block. For some `layout` inputs, like arrays, this may include extra storage usable for additional data. Read more

####

`unsafe fn realloc_excess( &mut self, ptr: NonNull<u8>, layout: Layout, new_size: usize ) -> Result<Excess, AllocErr>`

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🔬 This is a nightly-only experimental API. (`allocator_api` #32838)

Behaves like `realloc`, but also returns the whole size of the returned block. For some `layout` inputs, like arrays, this may include extra storage usable for additional data. Read more

####

`unsafe fn grow_in_place( &mut self, ptr: NonNull<u8>, layout: Layout, new_size: usize ) -> Result<(), CannotReallocInPlace>`

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🔬 This is a nightly-only experimental API. (`allocator_api` #32838)

Attempts to extend the allocation referenced by `ptr` to fit `new_size`. Read more

####

`unsafe fn shrink_in_place( &mut self, ptr: NonNull<u8>, layout: Layout, new_size: usize ) -> Result<(), CannotReallocInPlace>`

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🔬 This is a nightly-only experimental API. (`allocator_api` #32838)

Attempts to shrink the allocation referenced by `ptr` to fit `new_size`. Read more

####

`fn alloc_one<T>(&mut self) -> Result<NonNull<T>, AllocErr>`

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🔬 This is a nightly-only experimental API. (`allocator_api` #32838)

Allocates a block suitable for holding an instance of `T`. Read more

####

`unsafe fn dealloc_one<T>(&mut self, ptr: NonNull<T>)`

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🔬 This is a nightly-only experimental API. (`allocator_api` #32838)

Deallocates a block suitable for holding an instance of `T`. Read more

####

`fn alloc_array<T>(&mut self, n: usize) -> Result<NonNull<T>, AllocErr>`

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🔬 This is a nightly-only experimental API. (`allocator_api` #32838)

Allocates a block suitable for holding `n` instances of `T`. Read more

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`unsafe fn realloc_array<T>( &mut self, ptr: NonNull<T>, n_old: usize, n_new: usize ) -> Result<NonNull<T>, AllocErr>`

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🔬 This is a nightly-only experimental API. (`allocator_api` #32838)

Reallocates a block previously suitable for holding `n_old` instances of `T`, returning a block suitable for holding `n_new` instances of `T`. Read more

####

`unsafe fn dealloc_array<T>( &mut self, ptr: NonNull<T>, n: usize ) -> Result<(), AllocErr>`

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🔬 This is a nightly-only experimental API. (`allocator_api` #32838)

Deallocates a block suitable for holding `n` instances of `T`. Read more

## Auto Trait Implementations

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`impl Send for System`

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`impl Sync for System`

## Blanket Implementations

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`impl<T, U> TryFrom for T where T: From<U>,`

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#### `type Error = !`

🔬 This is a nightly-only experimental API. (`try_from` #33417)

The type returned in the event of a conversion error.

####

`fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>`

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🔬 This is a nightly-only experimental API. (`try_from` #33417)

Performs the conversion.

###

`impl<T> From for T`

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`fn from(t: T) -> T`

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Performs the conversion.

###

`impl<T, U> TryInto for T where U: TryFrom<T>,`

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#### `type Error = <U as TryFrom<T>>::Error`

🔬 This is a nightly-only experimental API. (`try_from` #33417)

The type returned in the event of a conversion error.

####

`fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>`

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🔬 This is a nightly-only experimental API. (`try_from` #33417)

Performs the conversion.

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`impl<T, U> Into for T where U: From<T>,`

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`fn into(self) -> U`

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Performs the conversion.

###

`impl<T> Borrow for T where T: ?Sized,`

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####
ⓘImportant traits for &'_ mut I

### Important traits for &'_ mut I

`impl<'_, I> Iterator for &'_ mut I where
I: Iterator + ?Sized, type Item = <I as Iterator>::Item;impl<'a, R: Read + ?Sized> Read for &'a mut Rimpl<'a, W: Write + ?Sized> Write for &'a mut W`

`fn borrow(&self) -> &T`

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Immutably borrows from an owned value. Read more

###

`impl<T> BorrowMut for T where T: ?Sized,`

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####
ⓘImportant traits for &'_ mut I

### Important traits for &'_ mut I

`impl<'_, I> Iterator for &'_ mut I where
I: Iterator + ?Sized, type Item = <I as Iterator>::Item;impl<'a, R: Read + ?Sized> Read for &'a mut Rimpl<'a, W: Write + ?Sized> Write for &'a mut W`

`fn borrow_mut(&mut self) -> &mut T`

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Mutably borrows from an owned value. Read more

###

`impl<T> Any for T where T: 'static + ?Sized,`

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####

`fn get_type_id(&self) -> TypeId`

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🔬 This is a nightly-only experimental API. (`get_type_id` #27745)

this method will likely be replaced by an associated static

Gets the `TypeId` of `self`. Read more

###

`impl<T> ToOwned for T where T: Clone,`

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#### `type Owned = T`

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`fn to_owned(&self) -> T`

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Creates owned data from borrowed data, usually by cloning. Read more

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`fn clone_into(&self, target: &mut T)`

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🔬 This is a nightly-only experimental API. (`toowned_clone_into` #41263)

recently added

Uses borrowed data to replace owned data, usually by cloning. Read more

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