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std.experimental.allocator.building_blocks.fallback_allocator.FallbackAllocator/fallbackAllocator - multiple declarations

Function fallbackAllocator

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.

FallbackAllocator!(Primary,Fallback) fallbackAllocator(Primary, Fallback) (
  auto ref Primary p,
  auto ref Fallback f
);

Example

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

Struct FallbackAllocator

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.

struct FallbackAllocator(Primary, Fallback) ;

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.

Fields

NameTypeDescription
fallback FallbackThe fallback allocator.
instance FallbackAllocatorIf both Primary and Fallback are stateless, FallbackAllocator defines a static instance called instance.
primary PrimaryThe primary allocator.

Methods

NameDescription
alignedAllocate FallbackAllocator offers alignedAllocate iff at least one of the allocators also offers it. It attempts to allocate using either or both.
allocate Allocates memory trying the primary allocator first. If it returns null, the fallback allocator is tried.
deallocate deallocate is defined if and only if at least one of the allocators define deallocate. 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 own b, then the request is forwarded to fallback.deallocate if it is defined, or is a no-op otherwise.
empty empty is defined if both allocators also define it.
expand expand is defined if and only if at least one of the allocators defines expand. 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 own b, then the request is forwarded to fallback.expand if it is defined, or fails (returning false) otherwise.
owns owns is defined if and only if both allocators define owns. Returns primary.owns(b) | fallback.owns(b).
reallocate 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.
resolveInternalPointer resolveInternalPointer is defined if and only if both allocators define it.

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