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# std.experimental.allocator.building_blocks.affix_allocator

struct `AffixAllocator`(Allocator, Prefix, Suffix = void);
  Allocator that adds some extra data before (of type Prefix) and/or after (of type Suffix) any allocation made with its parent allocator. This is useful for uses where additional allocation-related information is needed, such as mutexes, reference counts, or walls for debugging memory corruption errors.
  If Prefix is not void, Allocator must guarantee an alignment at least as large as Prefix.alignof.
  Suffixes are slower to get at because of alignment rounding, so prefixes should be preferred. However, small prefixes blunt the alignment so if a large alignment with a small affix is needed, suffixes should be chosen.
  The following methods are defined if Allocator defines them, and forward to it: deallocateAll, empty, owns.
  Examples:

      import std.experimental.allocator.mallocator : Mallocator;
      // One word before and after each allocation.
      alias A = AffixAllocator!(Mallocator, size_t, size_t);
      auto b = A.instance.allocate(11);
      A.instance.prefix(b) = 0xCAFE_BABE;
      A.instance.suffix(b) = 0xDEAD_BEEF;
      assert(A.instance.prefix(b) == 0xCAFE_BABE
          && A.instance.suffix(b) == 0xDEAD_BEEF);

  enum uint `alignment`;
    If Prefix is void, the `alignment` is that of the parent. Otherwise, the `alignment` is the same as the Prefix's `alignment`.
  Allocator `parent`;
    If the `parent` allocator Allocator is stateful, an instance of it is stored as a member. Otherwise, AffixAllocator uses Allocator.instance. In either case, the name parent is uniformly used for accessing the `parent` allocator.
  size_t `goodAllocSize`(size_t);
  void[] `allocate`(size_t);
  Ternary `owns`(void[]);
  bool `expand`(ref void[] `b`, size_t `delta`);
  bool `reallocate`(ref void[] `b`, size_t `s`);
  bool `deallocate`(void[] `b`);
  bool `deallocateAll`();
  Ternary `empty`();
    Standard allocator methods. Each is defined if and only if the parent allocator defines the homonym method (except for `goodAllocSize`, which may use the global default). Also, the methods will be shared if the parent allocator defines them as such.
  static AffixAllocator `instance`;
    The `instance` singleton is defined if and only if the parent allocator has no state and defines its own it object.
  ref auto `prefix`(T)(T[] `b`);
  ref auto `suffix`(T)(T[] `b`);
    Affix access functions offering references to the affixes of a block `b` previously allocated with this allocator. `b` may not be `null`. They are defined if and only if the corresponding affix is not void.
    The qualifiers of the affix are not always the same as the qualifiers of the argument. This is because the affixes are not part of the data itself, but instead are just associated with the data and known to the allocator. The table below documents the type of preffix(`b`) and affix(`b`) depending on the type of `b`.

    Result of `prefix`/`suffix` depending on argument (U is any unqualified type, Affix is Prefix or Suffix)
    Argument Type | Return | Comments
    --- | --- | ---
    shared(U)[] | ref shared Affix | Data is shared across threads and the affix follows suit.
    immutable(U)[] | ref shared Affix | Although the data is immutable, the allocator "knows" the underlying memory is mutable, so immutable is elided for the affix which is independent from the data itself. However, the result is shared because immutable is implicitly shareable so multiple threads may access and manipulate the affix for the same data.
    const(shared(U))[] | ref shared Affix | The data is always shareable across threads. Even if the data is const, the affix is modifiable by the same reasoning as for immutable.
    const(U)[] | ref const Affix | The input may have originated from U[] or immutable(U)[], so it may be actually shared or not. Returning an unqualified affix may result in race conditions, whereas returning a shared affix may result in inadvertent sharing of mutable thread-local data across multiple threads. So the returned type is conservatively ref const.
    U[] | ref Affix | Unqualified data has unqualified affixes.

    Precondition: `b` !is null and `b` must have been allocated with this allocator.

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