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# std.experimental.allocator.building_blocks.region

Source std/experimental/allocator/building_blocks/region.d

struct `Region`(ParentAllocator = NullAllocator, uint minAlign = platformAlignment, Flag!"growDownwards" growDownwards = No.growDownwards);
  A `Region` allocator allocates memory straight from one contiguous chunk. There is no deallocation, and once the region is full, allocation requests return null. Therefore, `Region`s are often used (a) in conjunction with more sophisticated allocators; or (b) for batch-style very fast allocations that deallocate everything at once.
  The region only stores three pointers, corresponding to the current position in the store and the limits. One allocation entails rounding up the allocation size for alignment purposes, bumping the current pointer, and comparing it against the limit.
  If ParentAllocator is different from NullAllocator, `Region` deallocates the chunk of memory during destruction.
  The minAlign parameter establishes alignment. If minAlign > 1, the sizes of all allocation requests are rounded up to a multiple of minAlign. Applications aiming at maximum speed may want to choose minAlign = 1 and control alignment externally.
  Examples:

      import std.algorithm.comparison : max;
      import std.experimental.allocator.building_blocks.allocator_list
          : AllocatorList;
      import std.experimental.allocator.mallocator : Mallocator;
      import std.typecons : Ternary;
      // Create a scalable list of regions. Each gets at least 1MB at a time by
      // using malloc.
      auto batchAllocator = AllocatorList!(
          (size_t n) => Region!Mallocator(max(n, 1024 * 1024))
      )();
      writeln(batchAllocator.empty); // Ternary.yes
      auto b = batchAllocator.allocate(101);
      writeln(b.length); // 101
      writeln(batchAllocator.empty); // Ternary.no
      // This will cause a second allocation
      b = batchAllocator.allocate(2 * 1024 * 1024);
      writeln(b.length); // 2 * 1024 * 1024
      // Destructor will free the memory

  ParentAllocator `parent`;
    The parent allocator. Depending on whether ParentAllocator holds state or not, this is a member variable or an alias for ParentAllocator.instance.
  pure nothrow @nogc this(ubyte[] `store`);
  this(size_t `n`);
  this(ParentAllocator `parent`, size_t `n`);
    Constructs a region backed by a user-provided store. Assumes the memory was allocated with ParentAllocator (if different from NullAllocator).
    Parameters:

    ubyte[] `store` | User-provided store backing up the region. If ParentAllocator is different from NullAllocator, memory is assumed to have been allocated with ParentAllocator.
    size_t `n` | Bytes to allocate using ParentAllocator. This constructor is only defined If ParentAllocator is different from NullAllocator. If `parent`.allocate(`n`) returns null, the region will be initialized as empty (correctly initialized but unable to allocate).

  const pure nothrow @nogc @safe size_t `goodAllocSize`(size_t `n`);
    Rounds the given size to a multiple of the alignment
  alias `alignment` = minAlign;
    Alignment offered.
  pure nothrow @nogc @trusted void[] `allocate`(size_t `n`);
    Allocates `n` bytes of memory. The shortest path involves an alignment adjustment (if alignment > 1), an increment, and a comparison.
    Parameters:

    size_t `n` | number of bytes to allocate

    Returns:
    A properly-aligned buffer of size `n` or null if request could not be satisfied.
  pure nothrow @nogc @trusted void[] `alignedAllocate`(size_t `n`, uint `a`);
    Allocates `n` bytes of memory aligned at alignment `a`.
    Parameters:

    size_t `n` | number of bytes to allocate
    uint `a` | alignment for the allocated block

    Returns:
    Either a suitable block of `n` bytes aligned at `a`, or null.
  pure nothrow @nogc @trusted void[] `allocateAll`();
    Allocates and returns all memory available to this region.
  pure nothrow @nogc @safe bool `expand`(ref void[] `b`, size_t `delta`);
    Expands an allocated block in place. Expansion will succeed only if the block is the last allocated. Defined only if growDownwards is No.growDownwards.
  pure nothrow @nogc bool `deallocate`(void[] `b`);
    Deallocates `b`. This works only if `b` was obtained as the last call to allocate; otherwise (i.e. another allocation has occurred since) it does nothing.
    Parameters:

    void[] `b` | Block previously obtained by a call to allocate against this allocator (null is allowed).

  pure nothrow @nogc bool `deallocateAll`();
    Deallocates all memory allocated by this region, which can be subsequently reused for new allocations.
  const pure nothrow @nogc @trusted Ternary `owns`(const void[] `b`);
    Queries whether `b` has been allocated with this region.
    Parameters:

    void[] `b` | Arbitrary block of memory (null is allowed; `owns`(null) returns false).

    Returns:
    true if `b` has been allocated with this region, false otherwise.
  const pure nothrow @nogc @safe Ternary `empty`();
    Returns Ternary.yes if no memory has been allocated in this region, Ternary.no otherwise. (Never returns Ternary.unknown.)
  const pure nothrow @nogc @safe size_t `available`();
    Nonstandard property that returns bytes available for allocation.
struct `InSituRegion`(size_t size, size_t minAlign = platformAlignment);
  `InSituRegion` is a convenient region that carries its storage within itself (in the form of a statically-sized array).
  The first template argument is the size of the region and the second is the needed alignment. Depending on the alignment requested and platform details, the actual available storage may be smaller than the compile-time parameter. To make sure that at least n bytes are available in the region, use InSituRegion!(n + a - 1, a).
  Given that the most frequent use of `InSituRegion` is as a stack allocator, it allocates starting at the end on systems where stack grows downwards, such that hot memory is used first.
  Examples:

      // 128KB region, allocated to x86's cache line
      InSituRegion!(128 * 1024, 16) r1;
      auto a1 = r1.allocate(101);
      writeln(a1.length); // 101

      // 128KB region, with fallback to the garbage collector.
      import std.experimental.allocator.building_blocks.fallback_allocator
          : FallbackAllocator;
      import std.experimental.allocator.building_blocks.free_list
          : FreeList;
      import std.experimental.allocator.building_blocks.bitmapped_block
          : BitmappedBlock;
      import std.experimental.allocator.gc_allocator : GCAllocator;
      FallbackAllocator!(InSituRegion!(128 * 1024), GCAllocator) r2;
      const a2 = r2.allocate(102);
      writeln(a2.length); // 102

      // Reap with GC fallback.
      InSituRegion!(128 * 1024, 8) tmp3;
      FallbackAllocator!(BitmappedBlock!(64, 8), GCAllocator) r3;
      r3.primary = BitmappedBlock!(64, 8)(cast(ubyte[]) (tmp3.allocateAll()));
      const a3 = r3.allocate(103);
      writeln(a3.length); // 103

      // Reap/GC with a freelist for small objects up to 16 bytes.
      InSituRegion!(128 * 1024, 64) tmp4;
      FreeList!(FallbackAllocator!(BitmappedBlock!(64, 64), GCAllocator), 0, 16) r4;
      r4.parent.primary = BitmappedBlock!(64, 64)(cast(ubyte[]) (tmp4.allocateAll()));
      const a4 = r4.allocate(104);
      writeln(a4.length); // 104

  alias `alignment` = minAlign;
    An alias for minAlign, which must be a valid alignment (nonzero power of 2). The start of the region and all allocation requests will be rounded up to a multiple of the alignment.

        InSituRegion!(4096) a1;
        assert(a1.alignment == platformAlignment);
        InSituRegion!(4096, 64) a2;
        assert(a2.alignment == 64);

  void[] `allocate`(size_t `n`);
    Allocates bytes and returns them, or null if the region cannot accommodate the request. For efficiency reasons, if bytes == 0 the function returns an empty non-null slice.
  void[] `alignedAllocate`(size_t `n`, uint `a`);
    As above, but the memory allocated is aligned at `a` bytes.
  bool `deallocate`(void[] `b`);
    Deallocates `b`. This works only if `b` was obtained as the last call to allocate; otherwise (i.e. another allocation has occurred since) it does nothing. This semantics is tricky and therefore `deallocate` is defined only if Region is instantiated with Yes.defineDeallocate as the third template argument.
    Parameters:

    void[] `b` | Block previously obtained by a call to allocate against this allocator (null is allowed).

  pure nothrow @nogc @safe Ternary `owns`(const void[] `b`);
    Returns Ternary.yes if `b` is the result of a previous allocation, Ternary.no otherwise.
  bool `expand`(ref void[] `b`, size_t `delta`);
    Expands an allocated block in place. Expansion will succeed only if the block is the last allocated.
  bool `deallocateAll`();
    Deallocates all memory allocated with this allocator.
  void[] `allocateAll`();
    Allocates all memory available with this allocator.
  size_t `available`();
    Nonstandard function that returns the bytes available for allocation.
struct `SbrkRegion`(uint minAlign = platformAlignment);
  Allocator backed by sbrk for Posix systems. Due to the fact that sbrk is not thread-safe by design, `SbrkRegion` uses a mutex internally. This implies that uncontrolled calls to brk and sbrk may affect the workings of SbrkRegion adversely.
  static shared SbrkRegion `instance`;
    Instance shared by all callers.
  enum uint `alignment`;
    Standard allocator primitives.
  shared const pure nothrow @nogc @safe size_t `goodAllocSize`(size_t `n`);
  shared nothrow @nogc @trusted void[] `allocate`(size_t `bytes`);
  shared nothrow @nogc @trusted void[] `alignedAllocate`(size_t `bytes`, uint `a`);
    Rounds the given size to a multiple of thew alignment
  shared nothrow @nogc @trusted bool `expand`(ref void[] `b`, size_t `delta`);
  shared pure nothrow @nogc @trusted Ternary `owns`(const void[] `b`);
    The `expand` method may only succeed if the argument is the last block allocated. In that case, `expand` attempts to push the break pointer to the right.
  shared nothrow @nogc bool `deallocate`(void[] `b`);
    The `deallocate` method only works (and returns true) on systems that support reducing the break address (i.e. accept calls to sbrk with negative offsets). OSX does not accept such. In addition the argument must be the last block allocated.
  shared nothrow @nogc bool `deallocateAll`();
    The `deallocateAll` method only works (and returns true) on systems that support reducing the break address (i.e. accept calls to sbrk with negative offsets). OSX does not accept such.
  shared pure nothrow @nogc @safe Ternary `empty`();
    Standard allocator API.
struct `SharedRegion`(ParentAllocator = NullAllocator, uint minAlign = platformAlignment, Flag!"growDownwards" growDownwards = No.growDownwards);
  The threadsafe version of the Region allocator. Allocations and deallocations are lock-free based using core.atomic.cas.
  ParentAllocator `parent`;
    The parent allocator. Depending on whether ParentAllocator holds state or not, this is a member variable or an alias for ParentAllocator.instance.
  pure nothrow @nogc this(ubyte[] `store`);
  this(size_t `n`);
    Constructs a region backed by a user-provided store. Assumes the memory was allocated with ParentAllocator (if different from NullAllocator).
    Parameters:

    ubyte[] `store` | User-provided store backing up the region. If ParentAllocator is different from NullAllocator, memory is assumed to have been allocated with ParentAllocator.
    size_t `n` | Bytes to allocate using ParentAllocator. This constructor is only defined If ParentAllocator is different from NullAllocator. If parent.allocate(`n`) returns null, the region will be initialized as empty (correctly initialized but unable to allocate).

  const pure nothrow @nogc @safe size_t `goodAllocSize`(size_t `n`);
    Rounds the given size to a multiple of the alignment
  alias `alignment` = minAlign;
    Alignment offered.
  pure nothrow @nogc @trusted void[] `allocate`(size_t `n`);
    Allocates `n` bytes of memory. The allocation is served by atomically incrementing a pointer which keeps track of the current used space.
    Parameters:

    size_t `n` | number of bytes to allocate

    Returns:
    A properly-aligned buffer of size `n`, or null if request could not be satisfied.
  pure nothrow @nogc bool `deallocate`(void[] `b`);
    Deallocates `b`. This works only if `b` was obtained as the last call to allocate; otherwise (i.e. another allocation has occurred since) it does nothing.
    Parameters:

    void[] `b` | Block previously obtained by a call to allocate against this allocator (null is allowed).

  pure nothrow @nogc bool `deallocateAll`();
    Deallocates all memory allocated by this region, which can be subsequently reused for new allocations.
  pure nothrow @nogc @trusted void[] `alignedAllocate`(size_t `n`, uint `a`);
    Allocates `n` bytes of memory aligned at alignment `a`.
    Parameters:

    size_t `n` | number of bytes to allocate
    uint `a` | alignment for the allocated block

    Returns:
    Either a suitable block of `n` bytes aligned at `a`, or null.
  const pure nothrow @nogc @trusted Ternary `owns`(const void[] `b`);
    Queries whether `b` has been allocated with this region.
    Parameters:

    void[] `b` | Arbitrary block of memory (null is allowed; `owns`(null) returns false).

    Returns:
    true if `b` has been allocated with this region, false otherwise.
  const pure nothrow @nogc @safe Ternary `empty`();
    Returns Ternary.yes if no memory has been allocated in this region, Ternary.no otherwise. (Never returns Ternary.unknown.)

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