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 <string.h>#include <iosfwd>    // for ostream forward-declaration#include <muduo/base/Types.h>#ifndef MUDUO_STD_STRING#include <string>#endifnamespace 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<StringPiece> really fast for some STL implementationstemplate<> 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 loggedstd::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;  }