FreeListAllocator\

A fixed-size object pool that provides O(1) allocate/deallocate using an intrusive freelist — zero heap calls after construction.

template <typename T, std::size_t Capacity>
class FreeListAllocator {
public:
  FreeListAllocator();                              // pre-links freelist

  template <typename... Args>
  T* allocate(Args&&... args);                      // placement-new, or nullptr if full

  void deallocate(T* ptr);                          // placement-delete, push index back
};

Purpose

Serve latency-critical code that repeatedly allocates small, uniform objects.
Eliminate fragmentation and new/delete overhead by reusing a static array.

Key Characteristics

Feature	Detail
Capacity	Compile-time constant → no dynamic growth.
Freelist	`_next[]` array stores next-free indices.
Storage	`alignas(T) std::byte _storage[Cap][sizeof T]`.
Threading	Not thread-safe; guard externally if needed.

Example

FreeListAllocator<OrderEvent, 1024> pool;

auto* e1 = pool.allocate(args…);  // fast, from pre-allocated block
...
pool.deallocate(e1);              // returns slot to freelist

Notes

allocate() returns nullptr when the pool is exhausted — check the result.
Destructor of T is invoked on deallocation via std::destroy_at.