【LevelDB源码阅读】Slice

是什么

slice是对字符串的封装，包括一个字符指针和一个字符串长度，相当于c++中std::string。

为什么要用

• 开销小，直接操作指针避免不必要的数据拷贝

学到什么

• int memcmp(const void *str1, const void *str2, size_t n)：比较str1和str2两个内存块前n个字符
• operator!=通过operator==实现
• 使用=default来显示要求编译器生成合成版本成员函数
• 用const修饰不修改数据成员的函数和不修改的形参变量
• inline机制优化规模小、流程直接、频繁调用的函数，避免函数调用开销

源码分析

• 构造函数

  // 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)) {}

• 拷贝构造和拷贝赋值

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

• 查询

  // 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];
  }

• 修改

  // 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;
  }

• 输出

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

• 比较及其实现

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

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) {  // 字符串相等情况下长度不等
    if (size_ < b.size_)
      r = -1;
    else if (size_ > b.size_)
      r = +1;
  }
  return r;
}

• 其它有用函数

  // 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));
  }

• 重载操作符

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); }