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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_ts
); - Allocates memory trying the primary allocator first. If it returns null, the fallback allocator is tried.
- void[]
alignedAllocate
(size_ts
, uinta
); - 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_tdelta
); expand
is defined if and only if at least one of the allocators definesexpand
. 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 ownb
, then the request is forwarded to fallback.expand
if it is defined, or fails (returning false) otherwise.- bool
reallocate
(ref void[]b
, size_tnewSize
); 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 ownb
, 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 defineowns
. Returns primary.owns(b) | fallback.owns(b).- Ternary
resolveInternalPointer
(const void*p
, ref void[]result
); 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 definedeallocate
. 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 ownb
, 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 Primaryp
, auto ref Fallbackf
); - 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); writeln(b1.length); // 1020 writeln(a.primary.owns(b1)); // Ternary.yes auto b2 = a.allocate(10); writeln(b2.length); // 10 writeln(a.primary.owns(b2)); // Ternary.no
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