Region

A Region allocator allocates memory straight from one contiguous chunk. There is no deallocation, and once the region is full, allocation requests return null. Therefore, Regions are often used (a) in conjunction with more sophisticated allocators; or (b) for batch-style very fast allocations that deallocate everything at once.

The region only stores three pointers, corresponding to the current position in the store and the limits. One allocation entails rounding up the allocation size for alignment purposes, bumping the current pointer, and comparing it against the limit.

Region deallocates the chunk of memory during destruction.

The minAlign parameter establishes alignment. If minAlign > 1, the sizes of all allocation requests are rounded up to a multiple of minAlign. Applications aiming at maximum speed may want to choose minAlign = 1 and control alignment externally.

struct Region (

ParentAllocator

uint minAlign = platformAlignment

Flag!"growDownwards" growDownwards = No.growDownwards

) {

eponymoustemplate hasMember(T, string name) via import std.traits : hasMember;;

struct Ternary via import std.typecons : Ternary;;

static if(stateSize!ParentAllocator)

ParentAllocator parent;

this(ubyte[] store);

this(size_t n);

this(ParentAllocator parent, size_t n);

~this();

size_t goodAllocSize(size_t n);

alias alignment = minAlign;

void[] allocate(size_t n);

void[] alignedAllocate(size_t n, uint a);

void[] allocateAll();

static if(growDownwards == No.growDownwards)

bool expand(void[] b, size_t delta);

bool deallocate(void[] b);

bool deallocateAll();

Ternary owns(void[] b);

Ternary empty();

size_t available();

}

Constructors

this this(ubyte[] store)
this(size_t n)
this(ParentAllocator parent, size_t n): Constructs a region backed by a user-provided store. Assumes the memory was allocated with ParentAllocator.

Destructor

~this ~this(): If ParentAllocator defines deallocate, the region defines a destructor that uses ParentAllocator.deallocate to free the memory chunk.

Members

Aliases

alignment alias alignment = minAlign: Alignment offered.

Functions

alignedAllocate void[] alignedAllocate(size_t n, uint a): Allocates n bytes of memory aligned at alignment a.
allocate void[] allocate(size_t n): Allocates n bytes of memory. The shortest path involves an alignment adjustment (if alignment > 1), an increment, and a comparison.
allocateAll void[] allocateAll(): Allocates and returns all memory available to this region.
available size_t available(): Nonstandard property that returns bytes available for allocation.
deallocate bool deallocate(void[] b): Deallocates b. This works only if b was obtained as the last call to allocate; otherwise (i.e. another allocation has occurred since) it does nothing.
deallocateAll bool deallocateAll(): Deallocates all memory allocated by this region, which can be subsequently reused for new allocations.
empty Ternary empty(): Returns Ternary.yes if no memory has been allocated in this region, Ternary.no otherwise. (Never returns Ternary.unknown.)
expand bool expand(void[] b, size_t delta): Expands an allocated block in place. Expansion will succeed only if the block is the last allocated. Defined only if growDownwards is No.growDownwards.
goodAllocSize size_t goodAllocSize(size_t n): Rounds the given size to a multiple of the alignment
owns Ternary owns(void[] b): Queries whether b has been allocated with this region.

Variables

parent ParentAllocator parent;: The parent allocator. Depending on whether ParentAllocator holds state or not, this is a member variable or an alias for ParentAllocator.instance.

Examples

import std.algorithm.comparison : max;
import std.experimental.allocator.building_blocks.allocator_list
    : AllocatorList;
import std.experimental.allocator.mallocator : Mallocator;
import std.typecons : Ternary;
// Create a scalable list of regions. Each gets at least 1MB at a time by
// using malloc.
auto batchAllocator = AllocatorList!(
    (size_t n) => Region!Mallocator(max(n, 1024 * 1024))
)();
assert(batchAllocator.empty ==  Ternary.yes);
auto b = batchAllocator.allocate(101);
assert(b.length == 101);
assert(batchAllocator.empty ==  Ternary.no);
// This will cause a second allocation
b = batchAllocator.allocate(2 * 1024 * 1024);
assert(b.length == 2 * 1024 * 1024);
// Destructor will free the memory

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