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std.experimental.allocator.showcase

Collection of typical and useful prebuilt allocators using the given components. User code would typically import this module and use its facilities, or import individual heap building blocks and assemble them.
template StackFront(size_t stackSize, Allocator = GCAllocator)
Allocator that uses stack allocation for up to stackSize bytes and then falls back to Allocator. Defined as: 
alias StackFront(size_t stackSize, Allocator) =
    FallbackAllocator!(
        InSituRegion!(stackSize, Allocator.alignment,
            hasMember!(Allocator, "deallocate")
                ? Yes.defineDeallocate
                : No.defineDeallocate),
        Allocator);

Choosing stackSize is as always a compromise. Too small a size exhausts the stack storage after a few allocations, after which there are no gains over the backup allocator. Too large a size increases the stack consumed by the thread and may end up worse off because it explores cold portions of the stack.
Examples: 
StackFront!4096 a;
auto b = a.allocate(4000);
assert(b.length == 4000);
auto c = a.allocate(4000);
assert(c.length == 4000);
a.deallocate(b);
a.deallocate(c);
auto mmapRegionList(size_t bytesPerRegion);
Creates a scalable AllocatorList of Regions, each having at least bytesPerRegion bytes. Allocation is very fast. This allocator does not offer deallocate but does free all regions in its destructor. It is recommended for short-lived batch applications that count on never running out of memory.
Examples: 
auto alloc = mmapRegionList(1024 * 1024);
const b = alloc.allocate(100);
assert(b.length == 100);