ObjectPool.hh

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#ifndef OBJECTPOOL_HH
#define OBJECTPOOL_HH
 
#include <array>
#include <cstdint>
#include <memory>
#include <vector>
 
// ObjectPool
//
// This is a bit like a traditional pool-allocator, but with these differences:
// - Instead of only allocating and deallocating memory, this class also
//   constructs/destructs the objects contained in the pool. (But see the
//   remark below about destroying the ObjectPool itself).
// - The objects in this pool are not accessed via a pointer but instead via an
//   index. That is inserting a object into this pool returns an index. And that
//   index can later be used to retrieve the actual object or to remove the
//   object again. (As an optimization the insert operation additionally also
//   returns a pointer to the object).
//
// The choice of identifying objects via a (32-bit) index rather than via a
// pointer involves some trade-offs:
// - On a 64-bit system, storing (many) indices only takes half as much space
//   as storing pointers.
// - Accessing an object via an index is a bit slower than via a pointer.
// - Removing an object (via an index) is about the same speed as accessing the
//   object. Removing via pointer is not possible.
//
// Objects in this pool have stable addresses. IOW once you retrieve a pointer
// to an object that pointer remains valid. Also when more elements are inserted
// later. Even if (internally) the pool needs to allocate more memory.
//
// Memory is allocated in chunks (in the current implementation a chunk has
// room for 256 objects). Typically two objects that are inserted one after the
// other will be next to each other in memory (good for cache-locality). Though
// this is not a guarantee. When objects are removed, those free locations are
// re-filled first before allocating new memory.
//
// Inserting an object is done via the emplace() function. This will in-place
// constructs the object in the pool. So you cannot request uninitialized
// memory from the pool. Removing an object from the pool will call the
// destructor on that object. However when the ObjectPool itself is destructed
// it will NOT destruct all objects that are still present in the pool.
// Depending on the type of object and the state of your program this may or
// may not be a problem. E.g. maybe the object has a trivial destructor, or
// maybe it's fine to leak memory on exit.
//
// It's not possible to loop over all the objects that are present in the pool.
 
template<typename T>
class ObjectPool
{
public:
        using Index = uint32_t;
 
        struct EmplaceResult {
                Index idx;
                T* ptr;
        };
 
private:
        union Element {
                Element() {}
                ~Element() {}
 
                Index next;
                T t;
        };
        using Element256 = std::array<Element, 256>;
 
public:
        template<typename... Args>
        [[nodiscard]] EmplaceResult emplace(Args&& ...args) {
                Index idx;
                if (free != Index(-1)) {
                        idx = free;
                        free = get(idx).next;
                } else {
                        if (cntr == 0) {
                                pool.push_back(std::make_unique<Element256>());
                        }
                        idx = Index(((pool.size() - 1) << 8) + cntr);
                        ++cntr;
                }
                T* ptr = &get(idx).t;
                new(ptr) T(std::forward<Args>(args)...);
                return {idx, ptr};
        }
 
        [[nodiscard]] const T& operator[](Index idx) const { return get(idx).t; }
        [[nodiscard]]       T& operator[](Index idx)       { return get(idx).t; }
 
        void remove(Index idx) {
                auto& elem = get(idx);
                elem.t.~T();
                elem.next = free;
                free = idx;
        }
 
        // for unittest
        [[nodiscard]] Index capacity() const {
                return 256 * pool.size();
        }
 
private:
        [[nodiscard]] const Element& get(Index idx) const { return (*pool[idx >> 8])[idx & 255]; }
        [[nodiscard]]       Element& get(Index idx)       { return (*pool[idx >> 8])[idx & 255]; }
 
private:
        std::vector<std::unique_ptr<Element256>> pool;
        Index free = Index(-1);
        uint8_t cntr = 0;
};
 
#endif