[TOC]

回顾上一篇文章《[leveldb] 初步探索 leveldb》的样例代码，是不是发现有个类比例简单：leveldb::Status，你发现它有status.ok()和status.ToString()方法。那怎么找到Status类的定义和成员方法的实现呢？

1、 第一步：把相关代码找出来

1-1 用grep 大法过滤内容

我们知道C++的类一般是这样定义的：class Status {...},我们grep在当前目前递归-r .全词匹配-w和半词匹配都搜不到，最后grep "Status {"出来了，并显示代码所在行数-n。

wu_being@UbuntuKylin1704:~/Github/leveldb$ grep "class Status" -rnw .
wu_being@UbuntuKylin1704:~/Github/leveldb$ grep "class Status" -rn .
wu_being@UbuntuKylin1704:~/Github/leveldb$ grep "Status{" -rn .
wu_being@UbuntuKylin1704:~/Github/leveldb$ grep "Status {" -rn .
./include/leveldb/status.h:22:class LEVELDB_EXPORT Status {
wu_being@UbuntuKylin1704:~/Github/leveldb$ 

这个就是Status类定义的头文件：

./include/leveldb/status.h:22:class LEVELDB_EXPORT Status {

1-2 用find 大法找到文件

我们发现上面grep大法连文件名都过滤出来，下面用find大法通过文件名匹配到相关文件，找到一个Status类方法实现的文件./util/status.cc。

wu_being@UbuntuKylin1704:~/Github/leveldb$ 
wu_being@UbuntuKylin1704:~/Github/leveldb$ find . -name *status.*
./util/status.cc
./util/status.o
./out-static/util/status.o
./include/leveldb/status.h
./out-shared/util/status.o
wu_being@UbuntuKylin1704:~/Github/leveldb$ 

2、 第二步：RTFSC (Read the fucking source code )

wu_being@UbuntuKylin1704:~/Github/leveldb$ 
wu_being@UbuntuKylin1704:~/Github/leveldb$ gedit include/leveldb/status.h &
[2] 3936
wu_being@UbuntuKylin1704:~/Github/leveldb$ gedit util/status.cc 
[2]+  已完成               gedit include/leveldb/status.h
wu_being@UbuntuKylin1704:~/Github/leveldb$ 

2-1 include/leveldb/status.h

#ifndef STORAGE_LEVELDB_INCLUDE_STATUS_H_
#define STORAGE_LEVELDB_INCLUDE_STATUS_H_

#include <string>
#include "leveldb/export.h"
#include "leveldb/slice.h"

namespace leveldb {

class LEVELDB_EXPORT Status {
 public:
  // Create a success status.
  Status() : state_(NULL) { }
  ~Status() { delete[] state_; }

  // Copy the specified status.
  Status(const Status& s);
  void operator=(const Status& s);

  // Return a success status.
  static Status OK() { return Status(); }

  // Return error status of an appropriate type.
  static Status NotFound(const Slice& msg, const Slice& msg2 = Slice()) {
    return Status(kNotFound, msg, msg2);
  }
...

  // Returns true iff the status indicates success.
  bool ok() const { return (state_ == NULL); }

  // Returns true iff the status indicates a NotFound error.
  bool IsNotFound() const { return code() == kNotFound; }
...

  // Return a string representation of this status suitable for printing.
  // Returns the string "OK" for success.
  // status.cc
  std::string ToString() const;

 private:
  // OK status has a NULL state_.  Otherwise, state_ is a new[] array
  // of the following form:
  //    state_[0..3] == length of message
  //    state_[4]    == code
  //    state_[5..]  == message
  const char* state_;

  enum Code {
    kOk = 0,
    kNotFound = 1,
    kCorruption = 2,
    kNotSupported = 3,
    kInvalidArgument = 4,
    kIOError = 5
  };

  Code code() const {
    return (state_ == NULL) ? kOk : static_cast<Code>(state_[4]);
  }

  // status.cc
  Status(Code code, const Slice& msg, const Slice& msg2);
  static const char* CopyState(const char* s);
};

inline Status::Status(const Status& s) {
  state_ = (s.state_ == NULL) ? NULL : CopyState(s.state_);
}
inline void Status::operator=(const Status& s) {
  // The following condition catches both aliasing (when this == &s),
  // and the common case where both s and *this are ok.
  if (state_ != s.state_) {
    delete[] state_;
    state_ = (s.state_ == NULL) ? NULL : CopyState(s.state_);
  }
}

}  // namespace leveldb

#endif  // STORAGE_LEVELDB_INCLUDE_STATUS_H_

2-1-1 类Status公有成员变量和方法

构造函数

 public:
  // Create a success status.
  Status() : state_(NULL) { }
  
  // Copy the specified status.
  Status(const Status& s);
  void operator=(const Status& s);

inline Status::Status(const Status& s) {
  state_ = (s.state_ == NULL) ? NULL : CopyState(s.state_);
}

inline void Status::operator=(const Status& s) {
  // The following condition catches both aliasing (when this == &s),
  // and the common case where both s and *this are ok.
  if (state_ != s.state_) {
    delete[] state_;
    state_ = (s.state_ == NULL) ? NULL : CopyState(s.state_);
  }
}

析构函数

  ~Status() { delete[] state_; }

2-1-2 类Status私有成员变量和方法

私有成员变量

下面这个state_设计很不好调试，在后面的CopyState方法就是了，没有手工调试配制state_。

 // OK status has a NULL state_.  Otherwise, state_ is a new[] array
  // of the following form:
  //    state_[0..3] == length of message
  //    state_[4]    == code
  //    state_[5..]  == message
  const char* state_;

  enum Code {
    kOk = 0,
    kNotFound = 1,
    kCorruption = 2,
    kNotSupported = 3,
    kInvalidArgument = 4,
    kIOError = 5
  };

私有成员方法

  Code code() const {
    return (state_ == NULL) ? kOk : static_cast<Code>(state_[4]);
  }
  // status.cc
  Status(Code code, const Slice& msg, const Slice& msg2);
  static const char* CopyState(const char* s);

2-2 util/status.cc

#include <stdio.h>
#include "port/port.h"
#include "leveldb/status.h"

namespace leveldb {

const char* Status::CopyState(const char* state) {
  uint32_t size;
  memcpy(&size, state, sizeof(size));

  char* result = new char[size + 5];
  memcpy(result, state, size + 5);

  return result;
}

Status::Status(Code code, const Slice& msg, const Slice& msg2) {
  assert(code != kOk);
  const uint32_t len1 = msg.size();
  const uint32_t len2 = msg2.size();
  const uint32_t size = len1 + (len2 ? (2 + len2) : 0);
  char* result = new char[size + 5];
  memcpy(result, &size, sizeof(size));
  result[4] = static_cast<char>(code);
  memcpy(result + 5, msg.data(), len1);
  if (len2) {
    result[5 + len1] = ':';
    result[6 + len1] = ' ';
    memcpy(result + 7 + len1, msg2.data(), len2);
  }
  state_ = result;
}

std::string Status::ToString() const {
  if (state_ == NULL) {
    return "OK.";
  } else {
    char tmp[30];
    const char* type;
    switch (code()) {
      case kOk:
        type = "OK...";
        break;
      case kNotFound:
        type = "NotFound: ";
        break;
...
      default:
        snprintf(tmp, sizeof(tmp), "Unknown code(%d): ",
                 static_cast<int>(code()));
        type = tmp;
        break;
    }
    std::string result(type);
    uint32_t length;
    memcpy(&length, state_, sizeof(length));
    result.append(state_ + 5, length);
    return result;
  }
}

}  // namespace leveldb

Status::CopyState

 // OK status has a NULL state_.  Otherwise, state_ is a new[] array
  // of the following form:
  //    state_[0..3] == length of message
  //    state_[4]    == code
  //    state_[5..]  == message
  
const char* Status::CopyState(const char* state) {
  uint32_t size; //sizeof(size)为4
  memcpy(&size, state, sizeof(size));//把state前面4个length of message字符给size

  char* result = new char[size + 5]; 
  memcpy(result, state, size + 5);

  return result;
}

Status::ToString() const

std::string Status::ToString() const {
  if (state_ == NULL) {
    return "OK.";
  } else {
    char tmp[30];
    const char* type;
    switch (code()) {
      case kOk:
        type = "OK...";
        break;
      case kNotFound:
        type = "NotFound: ";
        break;
...
      default:
        snprintf(tmp, sizeof(tmp), "Unknown code(%d): ",
                 static_cast<int>(code()));
        type = tmp;
        break;
    }
    std::string result(type);
    uint32_t length;
    memcpy(&length, state_, sizeof(length));
    result.append(state_ + 5, length);
    return result;
  }
}

私有构造函数

Status::Status

Status::Status(Code code, const Slice& msg, const Slice& msg2) {
  assert(code != kOk);
  const uint32_t len1 = msg.size();
  const uint32_t len2 = msg2.size();
  const uint32_t size = len1 + (len2 ? (2 + len2) : 0);
  char* result = new char[size + 5];
  memcpy(result, &size, sizeof(size));
  result[4] = static_cast<char>(code);
  memcpy(result + 5, msg.data(), len1);
  if (len2) {
    result[5 + len1] = ':';
    result[6 + len1] = ' ';
    memcpy(result + 7 + len1, msg2.data(), len2);
  }
  state_ = result;
}

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《[leveldb] 学习leveldb第一个类Status》： https://yq.aliyun.com/articles/241407/

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