muduo : StringPiece

http://www.zhihu.com/question/34499426/answer/58891014
为什么要有这个类?
C++里面有string和char*，如果你用const string &s 做函数形参，可以同时兼容两种字符串。但当你传入一个很长的char * 时，会生成一个较大的string对象，开销比较大。
如果你的目的仅仅是读取字符串的值，用这个StringPiece的话，仅仅是4+一个指针的内存开销，而且也保证了兼容性。
所以这个类的目的是传入字符串的字面值，它内部的ptr_ 这块内存不归他所有。所以不能做任何改动。

#ifndef MUDUO_BASE_STRINGPIECE_H
#define MUDUO_BASE_STRINGPIECE_H

#include 
#include     // for ostream forward-declaration

#include 
#ifndef MUDUO_STD_STRING
#include 
#endif

namespace muduo {

// For passing C-style string argument to a function.
class StringArg // copyable
{
 public:
  StringArg(const char* str)
    : str_(str)
  { }

  StringArg(const string& str)
    : str_(str.c_str())
  { }

#ifndef MUDUO_STD_STRING
  StringArg(const std::string& str)
    : str_(str.c_str())
  { }
#endif

  const char* c_str() const { return str_; }

 private:
  const char* str_;
};

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

 public:
  // We provide non-explicit singleton constructors so users can pass
  // in a "const char*" or a "string" wherever a "StringPiece" is
  // expected.
  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())) { }
#ifndef MUDUO_STD_STRING
  StringPiece(const std::string& str)
    : ptr_(str.data()), length_(static_cast<int>(str.size())) { }
#endif
  StringPiece(const char* offset, int len)
    : ptr_(offset), length_(len) { }

  // data() may return a pointer to a buffer with embedded NULs, and the
  // returned buffer may or may not be null terminated.  Therefore it is
  // typically a mistake to pass data() to a routine that expects a NUL
  // terminated string.  Use "as_string().c_str()" if you really need to do
  // this.  Or better yet, change your routine so it does not rely on NUL
  // termination.
  const char* data() const { return ptr_; }
  int size() const { return length_; }
  bool empty() const { return length_ == 0; }
  const char* begin() const { return ptr_; }
  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]; }

  void remove_prefix(int n) {
    ptr_ += n;
    length_ -= n;
  }

  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_);
  }

#ifndef MUDUO_STD_STRING
  void CopyToStdString(std::string* target) const {
    target->assign(ptr_, length_);
  }
#endif

  // 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 really fast for some STL implementations
template<> struct __type_traits {
  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

注意体会这两个函数：

  void remove_prefix(int n) {
    ptr_ += n;
    length_ -= n;
  }

  void remove_suffix(int n) {
    length_ -= n;
  }