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# std.experimental.allocator.building_blocks.free_list

struct FreeList(ParentAllocator, size_t minSize, size_t maxSize = minSize, Flag!"adaptive" adaptive = No.adaptive);
  Free list allocator, stackable on top of another allocator. Allocation requests between min and max bytes are rounded up to max and served from a singly-linked list of buffers deallocated in the past. All other allocations are directed to ParentAllocator. Due to the simplicity of free list management, allocations from the free list are fast.
  One instantiation is of particular interest: FreeList!(0, unbounded) puts every deallocation in the freelist, and subsequently serves any allocation from the freelist (if not empty). There is no checking of size matching, which would be incorrect for a freestanding allocator but is both correct and fast when an owning allocator on top of the free list allocator (such as Segregator) is already in charge of handling size checking.

  The following methods are defined if ParentAllocator defines them, and forward to it: expand, owns, reallocate.
  const @property size_t min();
    Returns the smallest allocation size eligible for allocation from the freelist. (If minSize != chooseAtRuntime, this is simply an alias for minSize.)
  @property void min(size_t low);
    If FreeList has been instantiated with minSize == chooseAtRuntime, then the min property is writable. Setting it must precede any allocation.
    Parameters:

    size_t low | new value for min

    Precondition: low <= max, or maxSize == chooseAtRuntime and max has not yet been initialized. Also, no allocation has been yet done with this allocator.

    Postcondition: min == low

  const @property size_t max();
    Returns the largest allocation size eligible for allocation from the freelist. (If maxSize != chooseAtRuntime, this is simply an alias for maxSize.) All allocation requests for sizes greater than or equal to min and less than or equal to max are rounded to max and forwarded to the parent allocator. When the block fitting the same constraint gets deallocated, it is put in the freelist with the allocated size assumed to be max.
  @property void max(size_t high);
    If FreeList has been instantiated with maxSize == chooseAtRuntime, then the max property is writable. Setting it must precede any allocation.
    Parameters:

    size_t high | new value for max

    Precondition: high >= min, or minSize == chooseAtRuntime and min has not yet been initialized. Also high >= (void*).sizeof. Also, no allocation has been yet done with this allocator.

    Postcondition: max == high

    Examples:

        FreeList!(Mallocator, chooseAtRuntime, chooseAtRuntime) a;
        a.min = 64;
        a.max = 128;
        assert(a.min == 64);
        assert(a.max == 128);

  ParentAllocator parent;
    The parent allocator. Depending on whether ParentAllocator holds state or not, this is a member variable or an alias for ParentAllocator.instance.
  alias alignment = ParentAllocator.alignment;
    Alignment offered.
  size_t goodAllocSize(size_t bytes);
    If maxSize == unbounded, returns parent.goodAllocSize(bytes). Otherwise, returns max for sizes in the interval [min, max], and parent.goodAllocSize(bytes) otherwise.

    Precondition: If set at runtime, min and/or max must be initialized appropriately.

    Postcondition: result >= bytes

  void[] allocate(size_t n);
    Allocates memory either off of the free list or from the parent allocator. If n is within [min, max] or if the free list is unchecked (minSize == 0 && maxSize == size_t.max), then the free list is consulted first. If not empty (hit), the block at the front of the free list is removed from the list and returned. Otherwise (miss), a new block of max bytes is allocated, truncated to n bytes, and returned.
    Parameters:

    size_t n | number of bytes to allocate

    Returns:
    The allocated block, or null.

    Precondition: If set at runtime, min and/or max must be initialized appropriately.

    Postcondition: result.length == bytes || result is null

  bool deallocate(void[] block);
    If block.length is within [min, max] or if the free list is unchecked (minSize == 0 && maxSize == size_t.max), then inserts the block at the front of the free list. For all others, forwards to parent.deallocate if Parent.deallocate is defined.
    Parameters:

    void[] block | Block to deallocate.

    Precondition: If set at runtime, min and/or max must be initialized appropriately. The block must have been allocated with this freelist, and no dynamic changing of min or max is allowed to occur between allocation and deallocation.

  bool deallocateAll();
    Defined only if ParentAllocator defines deallocateAll. If so, forwards to it and resets the freelist.
  void minimize();
    Nonstandard function that minimizes the memory usage of the freelist by freeing each element in turn. Defined only if ParentAllocator defines deallocate.
struct ContiguousFreeList(ParentAllocator, size_t minSize, size_t maxSize = minSize);
  Free list built on top of exactly one contiguous block of memory. The block is assumed to have been allocated with ParentAllocator, and is released in ContiguousFreeList's destructor (unless ParentAllocator is NullAllocator).
  ContiguousFreeList has most advantages of FreeList but fewer disadvantages. It has better cache locality because items are closer to one another. It imposes less fragmentation on its parent allocator.

  The disadvantages of ContiguousFreeList over FreeList are its pay upfront model (as opposed to FreeList's pay-as-you-go approach), and a hard limit on the number of nodes in the list. Thus, a large number of long- lived objects may occupy the entire block, making it unavailable for serving allocations from the free list. However, an absolute cap on the free list size may be beneficial.

  The options minSize == unbounded and maxSize == unbounded are not available for ContiguousFreeList.
  Examples:

      import std.experimental.allocator.gc_allocator : GCAllocator;
      import std.experimental.allocator.building_blocks.allocator_list
          : AllocatorList;

      alias ScalableFreeList = AllocatorList!((n) =>
          ContiguousFreeList!(GCAllocator, 0, unbounded)(4096)
      );

  SParent parent;
    The parent allocator. Depending on whether ParentAllocator holds state or not, this is a member variable or an alias for ParentAllocator.instance.
  enum uint alignment;
    Alignment offered.
  this(void[] buffer);
  this(ParentAllocator parent, void[] buffer);
  this(size_t bytes);
  this(ParentAllocator parent, size_t bytes);
  this(size_t bytes, size_t max);
  this(ParentAllocator parent, size_t bytes, size_t max);
  this(size_t bytes, size_t min, size_t max);
  this(ParentAllocator parent, size_t bytes, size_t min, size_t max);
    Constructors setting up the memory structured as a free list.
    Parameters:

    void[] buffer | Buffer to structure as a free list. If ParentAllocator is not NullAllocator, the buffer is assumed to be allocated by parent and will be freed in the destructor.
    ParentAllocator parent | Parent allocator. For construction from stateless allocators, use their instance static member.
    size_t bytes | Bytes (not items) to be allocated for the free list. Memory will be allocated during construction and deallocated in the destructor.
    size_t max | Maximum size eligible for freelisting. Construction with this parameter is defined only if maxSize == chooseAtRuntime or maxSize == unbounded.
    size_t min | Minimum size eligible for freelisting. Construction with this parameter is defined only if minSize == chooseAtRuntime. If this condition is met and no min parameter is present, min is initialized with max.

  size_t goodAllocSize(size_t n);
    If n is eligible for freelisting, returns max. Otherwise, returns parent.goodAllocSize(n).

    Precondition: If set at runtime, min and/or max must be initialized appropriately.

    Postcondition: result >= bytes

  void[] allocate(size_t n);
    Allocate n bytes of memory. If n is eligible for freelist and the freelist is not empty, pops the memory off the free list. In all other cases, uses the parent allocator.
  Ternary owns(void[] b);
    Defined if ParentAllocator defines it. Checks whether the block belongs to this allocator.
  bool deallocate(void[] b);
    Deallocates b. If it's of eligible size, it's put on the free list. Otherwise, it's returned to parent.

    Precondition: b has been allocated with this allocator, or is null.

  bool deallocateAll();
    Deallocates everything from the parent.
  Ternary empty();
    Returns Ternary.yes if no memory is currently allocated with this allocator, Ternary.no otherwise. This method never returns Ternary.unknown.
struct SharedFreeList(ParentAllocator, size_t minSize, size_t maxSize = minSize, size_t approxMaxNodes = unbounded);
  FreeList shared across threads. Allocation and deallocation are lock-free. The parameters have the same semantics as for FreeList.
  expand is defined to forward to (it must be also shared).
  @property size_t min();
  @property void min(size_t newMinSize);
  @property size_t max();
  @property void max(size_t newMaxSize);
  void setBounds(size_t newMin, size_t newMax);
    Properties for getting (and possibly setting) the bounds. Setting bounds is allowed only once , and before any allocation takes place. Otherwise, the primitives have the same semantics as those of FreeList.
    Examples:

        SharedFreeList!(Mallocator, chooseAtRuntime, chooseAtRuntime) a;
        // Set the maxSize first so setting the minSize doesn't throw
        a.max = 128;
        a.min = 64;
        a.setBounds(64, 128); // equivalent
        assert(a.max == 128);
        assert(a.min == 64);

  shared const @property size_t approxMaxLength();
  shared @property void approxMaxLength(size_t x);
    Properties for getting (and possibly setting) the approximate maximum length of a shared freelist.
    Examples:

        SharedFreeList!(Mallocator, 50, 50, chooseAtRuntime) a;
        // Set the maxSize first so setting the minSize doesn't throw
        a.approxMaxLength = 128;
        assert(a.approxMaxLength  == 128);
        a.approxMaxLength = 1024;
        assert(a.approxMaxLength  == 1024);
        a.approxMaxLength = 1;
        assert(a.approxMaxLength  == 1);

  shared ParentAllocator parent;
    The parent allocator. Depending on whether ParentAllocator holds state or not, this is a member variable or an alias for ParentAllocator.instance.
  enum uint alignment;
  shared size_t goodAllocSize(size_t bytes);
  shared const Ternary owns(void[] b);
  bool reallocate(void[] b, size_t s);
  shared void[] allocate(size_t bytes);
  shared bool deallocate(void[] b);
  shared bool deallocateAll();
    Standard primitives.

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