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# std.experimental.allocator.building_blocks.fallback_allocator

struct FallbackAllocator(Primary, Fallback);
  FallbackAllocator is the allocator equivalent of an "or" operator in algebra. An allocation request is first attempted with the Primary allocator. If that returns null, the request is forwarded to the Fallback allocator. All other requests are dispatched appropriately to one of the two allocators.
  In order to work, FallbackAllocator requires that Primary defines the owns method. This is needed in order to decide which allocator was responsible for a given allocation.
  FallbackAllocator is useful for fast, special-purpose allocators backed up by general-purpose allocators. The example below features a stack region backed up by the GCAllocator.
  Primary primary;
    The primary allocator.
  Fallback fallback;
    The fallback allocator.
  static FallbackAllocator instance;
    If both Primary and Fallback are stateless, FallbackAllocator defines a static instance called instance.
  enum uint alignment;
    The alignment offered is the minimum of the two allocators' alignment.
  void[] allocate(size_t s);
    Allocates memory trying the primary allocator first. If it returns null, the fallback allocator is tried.
  void[] alignedAllocate(size_t s, uint a);
    FallbackAllocator offers alignedAllocate iff at least one of the allocators also offers it. It attempts to allocate using either or both.
  bool expand(ref void[] b, size_t delta);
    expand is defined if and only if at least one of the allocators defines expand. It works as follows. If primary.owns(b), then the request is forwarded to primary.expand if it is defined, or fails (returning false) otherwise. If primary does not own b, then the request is forwarded to fallback.expand if it is defined, or fails (returning false) otherwise.
  bool reallocate(ref void[] b, size_t newSize);
    reallocate works as follows. If primary.owns(b), then primary.reallocate(b, newSize) is attempted. If it fails, an attempt is made to move the allocation from primary to fallback.
    If primary does not own b, then fallback.reallocate(b, newSize) is attempted. If that fails, an attempt is made to move the allocation from fallback to primary.
  Ternary owns(void[] b);
    owns is defined if and only if both allocators define owns. Returns primary.owns(b) | fallback.owns(b).
  void[] resolveInternalPointer(void* p);
    resolveInternalPointer is defined if and only if both allocators define it.
  bool deallocate(void[] b);
    deallocate is defined if and only if at least one of the allocators define deallocate. It works as follows. If primary.owns(b), then the request is forwarded to primary.deallocate if it is defined, or is a no-op otherwise. If primary does not own b, then the request is forwarded to fallback.deallocate if it is defined, or is a no-op otherwise.
  Ternary empty();
    empty is defined if both allocators also define it.
    Returns:
    primary.empty & fallback.empty
FallbackAllocator!(Primary, Fallback) fallbackAllocator(Primary, Fallback)(auto ref Primary p, auto ref Fallback f);
  Convenience function that uses type deduction to return the appropriate FallbackAllocator instance. To initialize with allocators that don't have state, use their it static member.
  Examples:

      import std.experimental.allocator.building_blocks.region : Region;
      import std.experimental.allocator.gc_allocator : GCAllocator;
      import std.typecons : Ternary;
      auto a = fallbackAllocator(Region!GCAllocator(1024), GCAllocator.instance);
      auto b1 = a.allocate(1020);
      assert(b1.length == 1020);
      assert(a.primary.owns(b1) == Ternary.yes);
      auto b2 = a.allocate(10);
      assert(b2.length == 10);
      assert(a.primary.owns(b2) == Ternary.no);

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