MemoryPool.cpp

#ifndef MEMORY_POOL_HPP
#define MEMORY_POOL_HPP

#include <climits>
#include <cstddef>

template <typename T, size_t BlockSize = 4096>
class MemoryPool
{
  public:
    // 使用 typedef 简化类型书写
    typedef T*              pointer;

    // 定义 rebind<U>::other 接口
    template <typename U> struct rebind {
      typedef MemoryPool<U> other;
    };

    // 默认构造
    // C++11 使用了 noexcept 来显式的声明此函数不会抛出异常
    MemoryPool() noexcept {
      currentBlock_ = nullptr;
      currentSlot_ = nullptr;
      lastSlot_ = nullptr;
      freeSlots_ = nullptr;
    }

    // 销毁一个现有的内存池
    ~MemoryPool() noexcept {
      // 循环销毁内存池中分配的内存区块
      slot_pointer_ curr = currentBlock_;
      while (curr != nullptr) {
        slot_pointer_ prev = curr->next;
        operator delete(reinterpret_cast<void*>(curr));
        curr = prev;
      }
    }

    // 同一时间只能分配一个对象, n 和 hint 会被忽略
    pointer allocate(size_t n = 1, const T* hint = 0) {
      if (freeSlots_ != nullptr) {
        pointer result = reinterpret_cast<pointer>(freeSlots_);
        freeSlots_ = freeSlots_->next;
        return result;
      }
      else {
        if (currentSlot_ >= lastSlot_) {
          // 分配一个内存区块
          data_pointer_ newBlock = reinterpret_cast<data_pointer_>(operator new(BlockSize));
          reinterpret_cast<slot_pointer_>(newBlock)->next = currentBlock_;
          currentBlock_ = reinterpret_cast<slot_pointer_>(newBlock);
          data_pointer_ body = newBlock + sizeof(slot_pointer_);
          uintptr_t result = reinterpret_cast<uintptr_t>(body);
          size_t bodyPadding = (alignof(slot_type_) - result) % alignof(slot_type_);
          currentSlot_ = reinterpret_cast<slot_pointer_>(body + bodyPadding);
          lastSlot_ = reinterpret_cast<slot_pointer_>(newBlock + BlockSize - sizeof(slot_type_));
        }
        return reinterpret_cast<pointer>(currentSlot_++);
      }
    }

    // 销毁指针 p 指向的内存区块
    void deallocate(pointer p, size_t n = 1) {
      if (p != nullptr) {
        reinterpret_cast<slot_pointer_>(p)->next = freeSlots_;
        freeSlots_ = reinterpret_cast<slot_pointer_>(p);
      }
    }

    // 调用构造函数, 使用 std::forward 转发变参模板
    template <typename U, typename... Args>
    void construct(U* p, Args&&... args) {
      new (p) U (std::forward<Args>(args)...);
    }

    // 销毁内存池中的对象, 即调用对象的析构函数
    template <typename U>
    void destroy(U* p) {
      p->~U();
    }

  private:
    // 用于存储内存池中的对象槽
    union Slot_ {
      T element;
      Slot_* next;
    };

    // 数据指针
    typedef char* data_pointer_;
    // 对象槽
    typedef Slot_ slot_type_;
    // 对象槽指针
    typedef Slot_* slot_pointer_;

    // 指向当前内存区块
    slot_pointer_ currentBlock_;
    // 指向当前内存区块的一个对象槽
    slot_pointer_ currentSlot_;
    // 指向当前内存区块的最后一个对象槽
    slot_pointer_ lastSlot_;
    // 指向当前内存区块中的空闲对象槽
    slot_pointer_ freeSlots_;
    // 检查定义的内存池大小是否过小
    static_assert(BlockSize >= 2 * sizeof(slot_type_), "BlockSize too small.");
};

#endif // MEMORY_POOL_HPP