Level DB --- Iterator

Iterator是Level DB中的一个基类，它定义了迭代器的基础的操作，同时对内存资源进行了维护。

虚函数

Iterator类中的虚函数操作如下：

cpp 复制代码

  virtual ~Iterator();

  // An iterator is either positioned at a key/value pair, or
  // not valid.  This method returns true iff the iterator is valid.
  virtual bool Valid() const = 0;

  // Position at the first key in the source.  The iterator is Valid()
  // after this call iff the source is not empty.
  virtual void SeekToFirst() = 0;

  // Position at the last key in the source.  The iterator is
  // Valid() after this call iff the source is not empty.
  virtual void SeekToLast() = 0;

  // Position at the first key in the source that is at or past target.
  // The iterator is Valid() after this call iff the source contains
  // an entry that comes at or past target.
  virtual void Seek(const Slice& target) = 0;

  // Moves to the next entry in the source.  After this call, Valid() is
  // true iff the iterator was not positioned at the last entry in the source.
  // REQUIRES: Valid()
  virtual void Next() = 0;

  // Moves to the previous entry in the source.  After this call, Valid() is
  // true iff the iterator was not positioned at the first entry in source.
  // REQUIRES: Valid()
  virtual void Prev() = 0;

  // Return the key for the current entry.  The underlying storage for
  // the returned slice is valid only until the next modification of
  // the iterator.
  // REQUIRES: Valid()
  virtual Slice key() const = 0;

  // Return the value for the current entry.  The underlying storage for
  // the returned slice is valid only until the next modification of
  // the iterator.
  // REQUIRES: Valid()
  virtual Slice value() const = 0;

  // If an error has occurred, return it.  Else return an ok status.
  virtual Status status() const = 0;

注释给了详细的虚函数操作介绍，继承类需要实现这些虚函数。

RegisterCleanup

include/leveldb/iterator.h中声明了：

cpp 复制代码

  // Clients are allowed to register function/arg1/arg2 triples that
  // will be invoked when this iterator is destroyed.
  //
  // Note that unlike all of the preceding methods, this method is
  // not abstract and therefore clients should not override it.
  using CleanupFunction = void (*)(void* arg1, void* arg2);
  void RegisterCleanup(CleanupFunction function, void* arg1, void* arg2);

/table/iterator.cc中实现了RegisterCleanup

cpp 复制代码

void Iterator::RegisterCleanup(CleanupFunction func, void* arg1, void* arg2) {
  assert(func != nullptr);
  CleanupNode* node;
  if (cleanup_head_.IsEmpty()) {
    node = &cleanup_head_;
  } else {
    node = new CleanupNode();
    node->next = cleanup_head_.next;
    cleanup_head_.next = node;
  }
  node->function = func;
  node->arg1 = arg1;
  node->arg2 = arg2;
}

Iterator中不光实现了迭代器的功能，同时它维护了一个资源的list（即cleanup_head_），在迭代器析构的时候，资源会被释放，而释放的方式是注册的clean up函数。