【leveldb】Slice（四）

leveldb內部沒有使用string，而是使用了一個Slice數據結構來指向一塊數據，每次給參數都給Slice而不用給數據的拷貝，這樣就省去內存的拷貝。leveldb所涉及到KV操作的接口都是用的Slice。
注意事項：

1. 如果是多線程調用同一個Slice的const方法，則不用關注多線程同步問題，如果是非const方法則由調用則保證多線程之間的同步；
2. 使用Slice之前，必須保證Slice指向的內存數據未被釋放。

其內部參數有兩個：

 private:
  const char* data_; //指向數據地址
  size_t size_;      //數據大小

源碼

源碼比較簡單，這裏就不說明了。

class LEVELDB_EXPORT Slice {
 public:
  // Create an empty slice.
  Slice() : data_(""), size_(0) {}

  // Create a slice that refers to d[0,n-1].
  Slice(const char* d, size_t n) : data_(d), size_(n) {}

  // Create a slice that refers to the contents of "s"
  Slice(const std::string& s) : data_(s.data()), size_(s.size()) {}

  // Create a slice that refers to s[0,strlen(s)-1]
  Slice(const char* s) : data_(s), size_(strlen(s)) {}

  // Intentionally copyable.
  Slice(const Slice&) = default;
  Slice& operator=(const Slice&) = default;

  // Return a pointer to the beginning of the referenced data
  const char* data() const { return data_; }

  // Return the length (in bytes) of the referenced data
  size_t size() const { return size_; }

  // Return true iff the length of the referenced data is zero
  bool empty() const { return size_ == 0; }

  // Return the ith byte in the referenced data.
  // REQUIRES: n < size()
  char operator[](size_t n) const {
    assert(n < size());
    return data_[n];
  }

  // Change this slice to refer to an empty array
  void clear() {
    data_ = "";
    size_ = 0;
  }

  // Drop the first "n" bytes from this slice.
  void remove_prefix(size_t n) {
    assert(n <= size());
    data_ += n;
    size_ -= n;
  }

  // Return a string that contains the copy of the referenced data.
  std::string ToString() const { return std::string(data_, size_); }

  // Three-way comparison.  Returns value:
  //   <  0 iff "*this" <  "b",
  //   == 0 iff "*this" == "b",
  //   >  0 iff "*this" >  "b"
  int compare(const Slice& b) const;

  // Return true iff "x" is a prefix of "*this"
  bool starts_with(const Slice& x) const {
    return ((size_ >= x.size_) && (memcmp(data_, x.data_, x.size_) == 0));
  }

 private:
  const char* data_;
  size_t size_;
};

inline bool operator==(const Slice& x, const Slice& y) {
  return ((x.size() == y.size()) &&
          (memcmp(x.data(), y.data(), x.size()) == 0));
}

inline bool operator!=(const Slice& x, const Slice& y) { return !(x == y); }

inline int Slice::compare(const Slice& b) const {
  const size_t min_len = (size_ < b.size_) ? size_ : b.size_;
  int r = memcmp(data_, b.data_, min_len);
  if (r == 0) {
    if (size_ < b.size_)
      r = -1;
    else if (size_ > b.size_)
      r = +1;
  }
  return r;
}

}  // namespace leveldb