muduo_base代码剖析之StringPiece

为什么要有这个StringPiece类?

1. StringPiece类是google提供的一个类。在网络传输中，如果传递一个很大的char*类型的字符串，那么这个很大的内存块需要在网络上进行传输；这么一块字符串虽然小，但是对于网络上高性能传输上，小小的因素都将会带来网络负载的加重，因此，使用StringPiece类对string/char*进行封装，封装成const char* ptr_ + int length，并且ptr始终是一个指针，ptr的赋值不是通过strcpy进行拷贝，而仅仅是改变ptr的指针。这样StringPiece类的大小始终都是const char* ptr_ + int length，大小为4字节+4字节。
2. StringPiece类使用的场景：目的仅仅是读取字符串的值，所以这个类的目的是传入字符串的字面值，它内部的ptr_ 这块内存不归它所有，所以不能做任何改动。
3. 归根结底，是处于性能的考虑，用以实现高效的字符串传递，这里既可以用const char*，也可以用std::string类型作为参数，并且不涉及内存拷贝。

#ifndef MUDUO_BASE_STRINGPIECE_H
#define MUDUO_BASE_STRINGPIECE_H

#include <string.h>
#include <iosfwd>    // for ostream forward-declaration

#include <muduo/base/Types.h>

namespace muduo
{
class StringPiece {
 private:
  const char*   ptr_;
  int           length_;

 public:
  //构造函数：让ptr_指针指向形参const char* str，不存在内存拷贝
  StringPiece()
    : ptr_(NULL), length_(0) { }
  StringPiece(const char* str)
    : ptr_(str), length_(static_cast<int>(strlen(ptr_))) { }
  StringPiece(const unsigned char* str)
    : ptr_(reinterpret_cast<const char*>(str)),
      length_(static_cast<int>(strlen(ptr_))) { }
  StringPiece(const string& str)
    : ptr_(str.data()), length_(static_cast<int>(str.size())) { }
  StringPiece(const char* offset, int len)
    : ptr_(offset), length_(len) { }

  const char* data() const { return ptr_; }
  int size() const { return length_; }
  bool empty() const { return length_ == 0; }
  //起始指针
  const char* begin() const { return ptr_; }
  //终止指针
  const char* end() const { return ptr_ + length_; }

  void clear() { ptr_ = NULL; length_ = 0; }
  void set(const char* buffer, int len) { ptr_ = buffer; length_ = len; }
  void set(const char* str) {
    ptr_ = str;
    length_ = static_cast<int>(strlen(str));
  }
  void set(const void* buffer, int len) {
    ptr_ = reinterpret_cast<const char*>(buffer);
    length_ = len;
  }

  char operator[](int i) const { return ptr_[i]; }

  //移除ptr_的前n个字节
  void remove_prefix(int n) {
    ptr_ += n;
    length_ -= n;
  }
  //移除ptr_的后n个字节
  void remove_suffix(int n) {
    length_ -= n;
  }

  bool operator==(const StringPiece& x) const {
    return ((length_ == x.length_) &&
            (memcmp(ptr_, x.ptr_, length_) == 0));
  }
  bool operator!=(const StringPiece& x) const {
    return !(*this == x);
  }

#define STRINGPIECE_BINARY_PREDICATE(cmp,auxcmp)                             \
  bool operator cmp (const StringPiece& x) const {                           \
    int r = memcmp(ptr_, x.ptr_, length_ < x.length_ ? length_ : x.length_); \
    return ((r auxcmp 0) || ((r == 0) && (length_ cmp x.length_)));          \
  }
  STRINGPIECE_BINARY_PREDICATE(<,  <);
  STRINGPIECE_BINARY_PREDICATE(<=, <);
  STRINGPIECE_BINARY_PREDICATE(>=, >);
  STRINGPIECE_BINARY_PREDICATE(>,  >);
#undef STRINGPIECE_BINARY_PREDICATE

  int compare(const StringPiece& x) const {
    int r = memcmp(ptr_, x.ptr_, length_ < x.length_ ? length_ : x.length_);
    if (r == 0) {
      if (length_ < x.length_) r = -1;
      else if (length_ > x.length_) r = +1;
    }
    return r;
  }

  string as_string() const {
    return string(data(), size());
  }

  void CopyToString(string* target) const {
    target->assign(ptr_, length_);
  }

  // Does "this" start with "x"
  bool starts_with(const StringPiece& x) const {
    return ((length_ >= x.length_) && (memcmp(ptr_, x.ptr_, x.length_) == 0));
  }
};

}  // namespace muduo

// ------------------------------------------------------------------
// Functions used to create STL containers that use StringPiece
//  Remember that a StringPiece's lifetime had better be less than
//  that of the underlying string or char*.  If it is not, then you
//  cannot safely store a StringPiece into an STL container
// ------------------------------------------------------------------

#ifdef HAVE_TYPE_TRAITS
// This makes vector<StringPiece> really fast for some STL implementations
template<> struct __type_traits<muduo::StringPiece> {
  typedef __true_type    has_trivial_default_constructor;
  typedef __true_type    has_trivial_copy_constructor;
  typedef __true_type    has_trivial_assignment_operator;
  typedef __true_type    has_trivial_destructor;
  typedef __true_type    is_POD_type;
};
#endif

// allow StringPiece to be logged
std::ostream& operator<<(std::ostream& o, const muduo::StringPiece& piece);

#endif  // MUDUO_BASE_STRINGPIECE_H