leveldb学习笔记一 SLICE

Slice是leveldb项目中重写的string,很多谷歌项目都没有使用自带的string,而是重写,Slice功能简单,只有正常的赋值比较,去前缀.

#include <assert.h>#include <stddef.h>#include <string.h>#include <string>namespace leveldb {class 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)) { }  // 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_;  // Intentionally copyable};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 int 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

以上是源代码.
1.4个构造函数实现了基本赋值操作,和string向slice的转换.
2.data(),size()函数用于取出字符串
3.remove_prefix用于去掉前缀,通过地址向后移动需要去除的字符长度来去除前缀.
4.重载比较符号
5.memcmp函数,前两个参数是字符指针,第三个参数为比较长度,用于判定一个字符串是否是另一个的前缀.