API Reference — `pma/resources/ArenaMemoryResource`

`ArenaMemoryResource`

Allocate memory from a growing pool of preallocated regions, avoiding per-request calls to the system allocator. Use this when many objects share the same lifetime and you want to free them all at once by destroying the arena.

When to use this

Reach for ArenaMemoryResource when you have a burst of allocations that all become invalid at the same point — for example, per-frame scratch buffers, per-request parsing state, or loading-time temporaries. Individual deallocate calls are no-ops; reclamation happens only when the arena is destroyed. Use a general-purpose MemoryResource instead when objects have independent, unpredictable lifetimes.

Example

// Upstream allocator (e.g. the default system allocator)
pma::MemoryResource* upstream = pma::getAllocator();

// Arena with 64 KiB initial region, 256 KiB subsequent regions, 2x growth
pma::ArenaMemoryResource arena{65536u, 262144u, 2.0f, upstream};

// Serve individual allocations — no system-allocator calls until a new region is needed
void* buf = arena.allocate(1024u, alignof(std::max_align_t));

// No-op: individual frees do nothing; all memory reclaimed when arena destructs
arena.deallocate(buf, 1024u, alignof(std::max_align_t));

// Fixed-size regions (no growth): all regions are 128 KiB
pma::ArenaMemoryResource fixed{131072u, upstream};

Parameters

The class provides three constructor overloads; all require an upstream allocator.

Name	Type	Description
`initialSize`	`std::size_t`	optional (overload 1 only) — size in bytes of the first region; subsequent regions use `regionSize`.
`regionSize`	`std::size_t`	required — size in bytes of each additionally allocated region (and the initial region when `initialSize` is absent).
`growthFactor`	`float`	optional (overloads 1–2) — multiplier applied to each subsequent region relative to the previous one. `1.0` means all regions are the same size. Omit to default to `1.0` (no growth).
`upstream`	`MemoryResource*`	required — backing allocator used to obtain each region from the OS or a parent pool.

Watch out for

deallocate is a no-op. Calling it does not free any memory. All memory held by the arena is released only when the ArenaMemoryResource object is destroyed or goes out of scope. Do not rely on individual frees for resource management.
Copy construction and copy assignment are deleted. ArenaMemoryResource is move-only — transfer ownership with std::move rather than copying.

Constraints

Region allocation sequence follows the geometric series: {initialSize, regionSize, regionSize × growthFactor, regionSize × growthFactor², …}. When initialSize is omitted, the first region is also regionSize.
The upstream pointer must remain valid for the entire lifetime of the ArenaMemoryResource.

API Reference — pma/resources/ArenaMemoryResource

ArenaMemoryResource