PoolAllocator

A pool allocator is a memory allocator that serves allocations from a pre-allocated pool of memory, usually consisting of many fixed-size blocks. It is designed to make frequent allocations and deallocations of objects of the same size fast and predictable, while reducing external fragmentation compared to general-purpose allocators.

On creation, a pool allocator reserves one or more blocks of memory and divides them into uniform blocks. It maintains a free list, bitmap, or similar structure to track available blocks. To allocate, it returns a free block; to deallocate, it returns the block to the pool. Some implementations use per-thread pools to minimize contention.

Variants include fixed-size block pools, object pools, and slab allocators. In practice, many libraries expose a pool_allocator interface that can be used with standard containers or custom allocators. In C++ contexts, types named PoolAllocator or pool_allocator are commonly provided to reuse pooled memory with containers. Notable examples include Boost.Pool and Linux kernel slab allocators. The concept is also used in real-time and embedded systems.

Advantages include fast, deterministic allocation times, improved cache locality for objects of the same size, and reduced fragmentation within the pool. Limitations include wasted space if pools are undersized or over-provisioned, difficulty handling varying object sizes, and added complexity in memory management, especially for large or diverse data lifetimes.

In software design, pool allocators are chosen when predictable latency and performance are more important than raw memory efficiency. They pair well with custom allocators and memory-resource abstractions, and are commonly used for game engines, graphics, networking, and real-time applications.