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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.
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.
static ref Prefix prefix(void[] b);

static ref Suffix suffix(void[] b);
Affix access functions offering mutable references to the affixes of a block previously allocated with this allocator. b may not be null. They are defined if and only if the corresponding affix is not void.

Precondition: b !is null