leveldb源码分析之Slice

class Slice {
 public:
  // Create an empty slice.
  //创建空的字符串，用法Slice slice;
  Slice() : data_(""), size_(0) { }
  // Create a slice that refers to d[0,n-1].
  //用一个字符串指针和字符串长度初始化一个Slice
  Slice(const char* d, size_t n) : data_(d), size_(n) { }
  // Create a slice that refers to the contents of "s"，
  //用C++字符串初始化Slice
  Slice(const std::string& s) : data_(s.data()), size_(s.size()) { }
  // Create a slice that refers to s[0,strlen(s)-1]
  //用一个字符串指针初始化Slice，
  Slice(const char* s) : data_(s), size_(strlen(s)) { }
  // Return a pointer to the beginning of the referenced data
  // 获取Slice字符串，不能改变值

const char* data() const { return data_; }
// Return the length (in bytes) of the referenced data
// 获取字符串的长度
size_t size() const { return size_; }
// Return true iff the length of the referenced data is zero
// 判断Slice是否为空，如果为空，返回true，如果不为空，则返回false；
bool empty() const { return size_ == 0; }
// Return the ith byte in the referenced data.
// REQUIRES: n < size()
// 重载[]操作符，用户通过slice[n]获取第n个字符 

char operator[](size_t n) const {
    assert(n < size());
    return data_[n];
  }
  // Change this slice to refer to an empty array
  //将这个Slice清空
  void clear() { data_ = ""; size_ = 0; }
  // Drop the first "n" bytes from this slice.
  //去除Slice前缀n个字符，例如为了取出Status信息，需要去除前5个前缀字符
  void remove_prefix(size_t n) {
    assert(n <= size());
    data_ += n;//指针向前移动n个字符
    size_ -= n;//长度减n
  }
  // Return a string that contains the copy of the referenced data.
  //返回Slice的string形式的副本
  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"
  // 判断这个Slice是否以字符串x为前缀
  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_;
  // Intentionally copyable
};
// 重载==操作符，用于判断Slice==Slice
inline bool operator==(const Slice& x, const Slice& y) {
  return ((x.size() == y.size()) &&
          (memcmp(x.data(), y.data(), x.size()) == 0));
}
// 重载不等于操作符
inline bool operator!=(const Slice& x, const Slice& y) {
  return !(x == y);
}
//字符串比较函数
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) {//字符串相等的情况下，可能长度不一样，所以也要进行判断。
    if (size_ < b.size_) r = -1;
    else if (size_ > b.size_) r = +1;
  }
  return r;
}

#include <cassert>
#include <iostream>
#include <string>
#include <leveldb/db.h>
using namespace std;
int main() {
  const char* str = "hello world!";
  leveldb::Slice slice(str);
  cout << slice.data() << endl;
  cout << slice.size() << endl;
  return 0;
}