levelDB中的所有操作的返回值都用一个名为Status的类来存储, Status有什么特别的呢？
#Status 介绍
leveldb::Status表示levelDB的一个返回状态，通常的错误处理（如:errno）是返回一个错误号，然后根据错误号可以获得出错的描述信息。 leveldb将错误号和错误信息封装成Status类，来统一进行处理。
#Status 声明
leveldb::Status 的声明如下：
class 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);
}
static Status Corruption(const Slice& msg, const Slice& msg2 = Slice()) {
return Status(kCorruption, msg, msg2);
}
static Status NotSupported(const Slice& msg, const Slice& msg2 = Slice()) {
return Status(kNotSupported, msg, msg2);
}
static Status InvalidArgument(const Slice& msg, const Slice& msg2 = Slice()) {
return Status(kInvalidArgument, msg, msg2);
}
static Status IOError(const Slice& msg, const Slice& msg2 = Slice()) {
return Status(kIOError, 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; }
// Returns true iff the status indicates a Corruption error.
bool IsCorruption() const { return code() == kCorruption; }
// Returns true iff the status indicates an IOError.
bool IsIOError() const { return code() == kIOError; }
// Return a string representation of this status suitable for printing.
// Returns the string "OK" for success.
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(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
#Status 定义
leveldb::Status 的定义如下：
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;
case kCorruption:
type = "Corruption: ";
break;
case kNotSupported:
type = "Not implemented: ";
break;
case kInvalidArgument:
type = "Invalid argument: ";
break;
case kIOError:
type = "IO error: ";
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;
}
}
#Status 解析
可以看到Status就只有一个成员变量const char *state_，是一个指向字符串的指针，对该字符串进行了封装：
// 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_;
对Status 定义了一堆的静态成员函数，通过调用这些静态成员函数来构造Status的对象，Status 的使用方法如下：
Status WriteBatch::Iterate(Handler* handler) const {
Slice input(rep_);
if (input.size() < kHeader) {
return Status::Corruption("malformed WriteBatch (too small)");
}
input.remove_prefix(kHeader);
Slice key, value;
int found = 0;
while (!input.empty()) {
found++;
char tag = input[0];
input.remove_prefix(1);
switch (tag) {
case kTypeValue:
if (GetLengthPrefixedSlice(&input, &key) &&
GetLengthPrefixedSlice(&input, &value)) {
handler->Put(key, value);
} else {
return Status::Corruption("bad WriteBatch Put");
}
break;
case kTypeDeletion:
if (GetLengthPrefixedSlice(&input, &key)) {
handler->Delete(key);
} else {
return Status::Corruption("bad WriteBatch Delete");
}
break;
default:
return Status::Corruption("unknown WriteBatch tag");
}
}
if (found != WriteBatchInternal::Count(this)) {
return Status::Corruption("WriteBatch has wrong count");
} else {
return Status::OK();
}
}
Status中通过静态成员函数构造对象，通过ToString()成员函数来获取错误消息。
leveldb::Status status = leveldb::DB::Open(options, "/tmp/testdb", &db);
if (!status.ok())
{
cerr << status.ToString() << endl;
}
#从Status中学到如下知识
- 使用enum 来对错误信息编码
- 将消息长度，消息类型和消息一起编码到一个char *数组，使用memcpy 复制数组，加快程序的运行速度
- sprintf不安全，请用snprintf
- Slice::compare 比较两个已知长度的字符串，依然使用memcmp
- 将赋值构造函数和拷贝构造函数的共同部分定义成一个辅助函数
- 通过静态成员函数来返回对象