[Muduo网络库源码分析] (8) base/StringPiece.h_字符串参数传递类型

字符串参数传递类型

实现：StringPiece、StringArg类

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功能：实现字符串类型、字符串参数传递类型

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知识点：

• __type_traits
  对一个类型进行描述，从而对其运算速度进行优化
• reinterpret_cast
  其可以进行指针间的转换，并且其可以实现指针和整数的相互转换

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用途：

可用于字符串传参，和其他字符串处理

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代码及分析：

StringPiece.h

// Taken from PCRE pcre_stringpiece.h//// Copyright (c) 2005, Google Inc.// All rights reserved.//// Redistribution and use in source and binary forms, with or without// modification, are permitted provided that the following conditions are// met:////     * Redistributions of source code must retain the above copyright// notice, this list of conditions and the following disclaimer.//     * Redistributions in binary form must reproduce the above// copyright notice, this list of conditions and the following disclaimer// in the documentation and/or other materials provided with the// distribution.//     * Neither the name of Google Inc. nor the names of its// contributors may be used to endorse or promote products derived from// this software without specific prior written permission.//// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.//// Author: Sanjay Ghemawat//// A string like object that points into another piece of memory.// Useful for providing an interface that allows clients to easily// pass in either a "const char*" or a "string".//// Arghh!  I wish C++ literals were automatically of type "string".#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:  //构造函数  StringArg(const char* str)    : str_(str)  {     }  //构造函数 string类型  StringArg(const string& str)    : str_(str.c_str())  { }#ifndef MUDUO_STD_STRING  StringArg(const std::string& str)    : str_(str.c_str())  { }#endif  //返回对应指针  const char* c_str() const { return str_; } private:  //字符指针  const char* str_;};class StringPiece { private:  //存储指针  const char*   ptr_;  //字符串长度  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.  //构造函数，初始化  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.  //返回字符串指针   const char* data() const { return ptr_; }  //返回字符串长度  int size() const { return length_; }  //判断是否为空  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_; }  //清空  void clear() { ptr_ = NULL; length_ = 0; }  //重置  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;  }  //重载[]运算符  char operator[](int i) const { return ptr_[i]; }  //截掉前n个字符  void remove_prefix(int n) {    ptr_ += n;    length_ -= n;  }  //截掉后n个字符  void remove_suffix(int n) {    length_ -= n;  }  //重载==运算符  bool operator==(const StringPiece& x) const {    return ((length_ == x.length_) &&            (memcmp(ptr_, x.ptr_, length_) == 0));  }  //重载不等于运算符  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  //比较两个对象是否相等，利用memcmp进行比较  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;  }  //以 string 类型返回  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 {    //将string 内容清空，并将ptr_字符串赋值给string    target->assign(ptr_, length_);  }#endif  //将StringPiece与当前字符串比较是否相同  // 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// 经过特化的StringPiece // This makes vector<StringPiece> really fast for some STL implementationstemplate<> struct __type_traits<muduo::StringPiece> {  typedef __true_type    has_trivial_default_constructor;//有构造函数  typedef __true_type    has_trivial_copy_constructor;   //有复制构造函数  typedef __true_type    has_trivial_assignment_operator;//有赋值函数  typedef __true_type    has_trivial_destructor;         //有析构函数  typedef __true_type    is_POD_type;                    //是C++的内建类型或传统的C结构体类型};#endif// allow StringPiece to be logged// 重载运算符，以便写日志std::ostream& operator<<(std::ostream& o, const muduo::StringPiece& piece);#endif  // MUDUO_BASE_STRINGPIECE_H