Trait Allocator

pub unsafe trait Allocator {
    // Required methods
    fn allocate(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError>;
    unsafe fn deallocate(&self, ptr: NonNull<u8>, layout: Layout);

    // Provided methods
    fn allocate_zeroed(
        &self,
        layout: Layout,
    ) -> Result<NonNull<[u8]>, AllocError> { ... }
    unsafe fn grow(
        &self,
        ptr: NonNull<u8>,
        old_layout: Layout,
        new_layout: Layout,
    ) -> Result<NonNull<[u8]>, AllocError> { ... }
    unsafe fn grow_zeroed(
        &self,
        ptr: NonNull<u8>,
        old_layout: Layout,
        new_layout: Layout,
    ) -> Result<NonNull<[u8]>, AllocError> { ... }
    unsafe fn shrink(
        &self,
        ptr: NonNull<u8>,
        old_layout: Layout,
        new_layout: Layout,
    ) -> Result<NonNull<[u8]>, AllocError> { ... }
    fn by_ref(&self) -> &Self
       where Self: Sized { ... }
}

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

An implementation of Allocator can allocate, grow, shrink, and deallocate arbitrary blocks of data described via Layout.

Allocator is designed to be implemented on ZSTs, references, or smart pointers. An allocator for MyAlloc([u8; N]) cannot be moved, without updating the pointers to the allocated memory.

In contrast to GlobalAlloc, Allocator allows zero-sized allocations. If an underlying allocator does not support this (like jemalloc) or responds by returning a null pointer (such as libc::malloc), this must be caught by the implementation.

Currently allocated memory

Some of the methods require that a memory block is currently allocated by an allocator. This means that:

• the starting address for that memory block was previously returned by allocate, grow, or shrink, and
• the memory block has not subsequently been deallocated.

A memory block is deallocated by a call to deallocate, or by a call to grow or shrink that returns Ok. A call to grow or shrink that returns Err, does not deallocate the memory block passed to it.

Memory fitting

Some of the methods require that a layout fit a memory block or vice versa. This means that the following conditions must hold:

• the memory block must be currently allocated with alignment of layout.align(), and
• layout.size() must fall in the range min ..= max, where:
  • min is the size of the layout used to allocate the block, and
  • max is the actual size returned from allocate, grow, or shrink.

Safety

Memory blocks that are currently allocated by an allocator, must point to valid memory, and retain their validity while until either:

• the memory block is deallocated, or
• the allocator is dropped.

Copying, cloning, or moving the allocator must not invalidate memory blocks returned from it. A copied or cloned allocator must behave like the original allocator.

A memory block which is currently allocated may be passed to any method of the allocator that accepts such an argument.

Required Methods

fn allocate(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError>

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

Attempts to allocate a block of memory.

On success, returns a NonNull<[u8]> meeting the size and alignment guarantees of layout.

The returned block may have a larger size than specified by layout.size(), and may or may not have its contents initialized.

The returned block of memory remains valid as long as it is [currently allocated] and the shorter of:

• the borrow-checker lifetime of the allocator type itself.
• as long as at the allocator and all its clones has not been dropped.

Errors

Returning Err indicates that either memory is exhausted or layout does not meet allocator’s size or alignment constraints.

Implementations are encouraged to return Err on memory exhaustion rather than panicking or aborting, but this is not a strict requirement. (Specifically: it is legal to implement this trait atop an underlying native allocation library that aborts on memory exhaustion.)

Clients wishing to abort computation in response to an allocation error are encouraged to call the handle_alloc_error function, rather than directly invoking panic! or similar.

unsafe fn deallocate(&self, ptr: NonNull<u8>, layout: Layout)

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

Deallocates the memory referenced by ptr.

Safety

• ptr must denote a block of memory currently allocated via this allocator, and
• layout must fit that block of memory.

Provided Methods

fn allocate_zeroed(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError>

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

Behaves like allocate, but also ensures that the returned memory is zero-initialized.

Errors

Returning Err indicates that either memory is exhausted or layout does not meet allocator’s size or alignment constraints.

Implementations are encouraged to return Err on memory exhaustion rather than panicking or aborting, but this is not a strict requirement. (Specifically: it is legal to implement this trait atop an underlying native allocation library that aborts on memory exhaustion.)

Clients wishing to abort computation in response to an allocation error are encouraged to call the handle_alloc_error function, rather than directly invoking panic! or similar.

unsafe fn grow( &self, ptr: NonNull<u8>, old_layout: Layout, new_layout: Layout, ) -> Result<NonNull<[u8]>, AllocError>

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

Attempts to extend the memory block.

Returns a new NonNull<[u8]> containing a pointer and the actual size of the allocated memory. The pointer is suitable for holding data described by new_layout. To accomplish this, the allocator may extend the allocation referenced by ptr to fit the new layout.

If this returns Ok, then ownership of the memory block referenced by ptr has been transferred to this allocator. Any access to the old ptr is Undefined Behavior, even if the allocation was grown in-place. The newly returned pointer is the only valid pointer for accessing this memory now.

If this method returns Err, then ownership of the memory block has not been transferred to this allocator, and the contents of the memory block are unaltered.

Safety

• ptr must denote a block of memory currently allocated via this allocator.
• old_layout must fit that block of memory (The new_layout argument need not fit it.).
• new_layout.size() must be greater than or equal to old_layout.size().

Note that new_layout.align() need not be the same as old_layout.align().

Errors

Returns Err if the new layout does not meet the allocator’s size and alignment constraints of the allocator, or if growing otherwise fails.

Implementations are encouraged to return Err on memory exhaustion rather than panicking or aborting, but this is not a strict requirement. (Specifically: it is legal to implement this trait atop an underlying native allocation library that aborts on memory exhaustion.)

Clients wishing to abort computation in response to an allocation error are encouraged to call the handle_alloc_error function, rather than directly invoking panic! or similar.

unsafe fn grow_zeroed( &self, ptr: NonNull<u8>, old_layout: Layout, new_layout: Layout, ) -> Result<NonNull<[u8]>, AllocError>

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

Behaves like grow, but also ensures that the new contents are set to zero before being returned.

The memory block will contain the following contents after a successful call to grow_zeroed:

• Bytes 0..old_layout.size() are preserved from the original allocation.
• Bytes old_layout.size()..old_size will either be preserved or zeroed, depending on the allocator implementation. old_size refers to the size of the memory block prior to the grow_zeroed call, which may be larger than the size that was originally requested when it was allocated.
• Bytes old_size..new_size are zeroed. new_size refers to the size of the memory block returned by the grow_zeroed call.

Safety

• ptr must denote a block of memory currently allocated via this allocator.
• old_layout must fit that block of memory (The new_layout argument need not fit it.).
• new_layout.size() must be greater than or equal to old_layout.size().

Note that new_layout.align() need not be the same as old_layout.align().

Errors

Returns Err if the new layout does not meet the allocator’s size and alignment constraints of the allocator, or if growing otherwise fails.

Implementations are encouraged to return Err on memory exhaustion rather than panicking or aborting, but this is not a strict requirement. (Specifically: it is legal to implement this trait atop an underlying native allocation library that aborts on memory exhaustion.)

Clients wishing to abort computation in response to an allocation error are encouraged to call the handle_alloc_error function, rather than directly invoking panic! or similar.

unsafe fn shrink( &self, ptr: NonNull<u8>, old_layout: Layout, new_layout: Layout, ) -> Result<NonNull<[u8]>, AllocError>

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

Attempts to shrink the memory block.

Returns a new NonNull<[u8]> containing a pointer and the actual size of the allocated memory. The pointer is suitable for holding data described by new_layout. To accomplish this, the allocator may shrink the allocation referenced by ptr to fit the new layout.

If this returns Ok, then ownership of the memory block referenced by ptr has been transferred to this allocator. Any access to the old ptr is Undefined Behavior, even if the allocation was shrunk in-place. The newly returned pointer is the only valid pointer for accessing this memory now.

If this method returns Err, then ownership of the memory block has not been transferred to this allocator, and the contents of the memory block are unaltered.

Safety

• ptr must denote a block of memory currently allocated via this allocator.
• old_layout must fit that block of memory (The new_layout argument need not fit it.).
• new_layout.size() must be smaller than or equal to old_layout.size().

Note that new_layout.align() need not be the same as old_layout.align().

Errors

Returns Err if the new layout does not meet the allocator’s size and alignment constraints of the allocator, or if shrinking otherwise fails.

Implementations are encouraged to return Err on memory exhaustion rather than panicking or aborting, but this is not a strict requirement. (Specifically: it is legal to implement this trait atop an underlying native allocation library that aborts on memory exhaustion.)

Clients wishing to abort computation in response to an allocation error are encouraged to call the handle_alloc_error function, rather than directly invoking panic! or similar.

fn by_ref(&self) -> &Self

where Self: Sized,

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

Creates a “by reference” adapter for this instance of Allocator.

The returned adapter also implements Allocator and will simply borrow this.

Implementors

impl Allocator for Global

impl Allocator for System

impl<A> Allocator for &A

where A: Allocator + ?Sized,