如何将::序列化为 sqlite::blob?
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【中文标题】如何将::序列化为 sqlite::blob?【英文标题】:How to boost::serialize into a sqlite::blob? 【发布时间】:2013-12-05 20:21:23 【问题描述】:我正在从事一个需要多种程序能力的科学项目。在四处寻找可用工具后,我决定使用 Boost 库,它为我提供了 C++ 标准库不提供的所需功能,例如日期/时间管理等。
我的项目是一组命令行,它处理来自旧的、自制的、基于纯文本文件的数据库中的一堆数据:导入、转换、分析、报告。
现在我已经到了需要坚持的地步。所以我加入了我发现非常有用的 boost::serialization。我能够存储和恢复“中等”数据集(不是那么大但也不是那么小),它们大约是 (7000,48,15,10)-dataset。
我还使用 SQLite C API 来存储和管理命令默认值、输出设置和变量元信息(单位、比例、限制)。
我想到了一些事情:序列化为 blob 字段而不是单独的文件。可能有一些我还没有看到的缺点(总是有),但我认为这可能是一个很好的解决方案,可以满足我的需求。
我能够将文本序列化为 std::string,所以我可以这样做:没有困难,因为它只使用普通字符。但我想二进制序列化成一个blob。
在填写我的 INSERT 查询时,我应该如何继续使用标准流?
【问题讨论】:
【参考方案1】:哈。我以前从未使用过 sqlite3 C API。而且我从未编写过输出streambuf
实现。但是看到我将来可能会如何在 c++ 代码库中使用 sqlite3,我想我已经花了一些时间在
cppreference http://en.cppreference.com/w/cpp/io/basic_streambuf
原来你可以open a blob field for incremental IO。但是,尽管您可以读/写 BLOB,但不能更改大小(通过单独的 UPDATE 语句除外)。
所以,我的演示步骤变成了:
-
将记录插入表中,绑定特定(固定)大小的“零块”
在新插入的记录中打开 blob 字段
将 blob 句柄包装在派生自
std::basic_streambuf<>
的自定义 blob_buf
对象中,并可与 std::ostream
一起使用以写入该 blob
将一些数据序列化到ostream
刷新
销毁/清理
它有效:)
main
中的代码:
int main()
sqlite3 *db = NULL;
int rc = sqlite3_open_v2("test.sqlite3", &db, SQLITE_OPEN_READWRITE, NULL);
if (rc != SQLITE_OK)
std::cerr << "database open failed: " << sqlite3_errmsg(db) << "\n";
exit(255);
// 1. insert a record into a table, binding a "zero-blob" of a certain (fixed) size
sqlite3_int64 inserted = InsertRecord(db);
// 2. open the blob field in the newly inserted record
// 3. wrap the blob handle in a custom `blob_buf` object that derives from `std::basic_streambuf<>` and can be used with `std::ostream` to write to that blob
blob_buf buf(OpenBlobByRowId(db, inserted));
std::ostream writer(&buf); // this stream now writes to the blob!
// 4. serialize some data into the `ostream`
auto payload = CanBeSerialized "hello world", 1, 2, 3.4, 1e7, -42.42 ;
boost::archive::text_oarchive oa(writer);
oa << payload;
#if 0 // used for testing with larger data
std::ifstream ifs("test.cpp");
writer << ifs.rdbuf();
#endif
// 5. flush
writer.flush();
// 6. destruct/cleanup
sqlite3_close(db);
// ==7653== HEAP SUMMARY:
// ==7653== in use at exit: 0 bytes in 0 blocks
// ==7653== total heap usage: 227 allocs, 227 frees, 123,540 bytes allocated
// ==7653==
// ==7653== All heap blocks were freed -- no leaks are possible
您会认出概述的步骤。
为了测试它,假设你创建了一个新的 sqlite 数据库:
sqlite3 test.sqlite3 <<< "CREATE TABLE DEMO(ID INTEGER PRIMARY KEY AUTOINCREMENT, FILE BLOB);"
现在,一旦你运行了程序,你就可以查询它了:
sqlite3 test.sqlite3 <<< "SELECT * FROM DEMO;"
1|22 serialization::archive 10 0 0 11 hello world 5 0 1 2 3.3999999999999999 10000000 -42.420000000000002
如果您启用测试代码(放置的数据超过 blob_size 允许的数量),您将看到 blob 被截断:
contents truncated at 256 bytes
完整程序
#include <sqlite3.h>
#include <string>
#include <iostream>
#include <ostream>
#include <fstream>
#include <boost/serialization/vector.hpp>
#include <boost/archive/text_oarchive.hpp>
template<typename CharT, typename TraitsT = std::char_traits<CharT> >
class basic_blob_buf : public std::basic_streambuf<CharT, TraitsT>
sqlite3_blob* _blob; // owned
int max_blob_size;
typedef std::basic_streambuf<CharT, TraitsT> base_type;
enum BUFSIZE = 10 ; // Block size - tuning?
char buf[BUFSIZE+1/*for the overflow character*/];
size_t cur_offset;
std::ostream debug;
// no copying
basic_blob_buf(basic_blob_buf const&) = delete;
basic_blob_buf& operator= (basic_blob_buf const&) = delete;
public:
basic_blob_buf(sqlite3_blob* blob, int max_size = -1)
: _blob(blob),
max_blob_size(max_size),
buf 0,
cur_offset(0),
// debug(std::cerr.rdbuf()) // or just use `nullptr` to suppress debug output
debug(nullptr)
debug.setf(std::ios::unitbuf);
if (max_blob_size == -1)
max_blob_size = sqlite3_blob_bytes(_blob);
debug << "max_blob_size detected: " << max_blob_size << "\n";
this->setp(buf, buf + BUFSIZE);
int overflow (int c = base_type::traits_type::eof())
auto putpointer = this->pptr();
if (c!=base_type::traits_type::eof())
// add the character - even though pptr might be epptr
*putpointer++ = c;
if (cur_offset >= size_t(max_blob_size))
return base_type::traits_type::eof(); // signal failure
size_t n = std::distance(this->pbase(), putpointer);
debug << "Overflow " << n << " bytes at " << cur_offset << "\n";
if (cur_offset+n > size_t(max_blob_size))
std::cerr << "contents truncated at " << max_blob_size << " bytes\n";
n = size_t(max_blob_size) - cur_offset;
if (SQLITE_OK != sqlite3_blob_write(_blob, this->pbase(), n, cur_offset))
debug << "sqlite3_blob_write reported an error\n";
return base_type::traits_type::eof(); // signal failure
cur_offset += n;
if (this->pptr() > (this->pbase() + n))
debug << "pending data has not been written";
return base_type::traits_type::eof(); // signal failure
// reset buffer
this->setp(buf, buf + BUFSIZE);
return base_type::traits_type::not_eof(c);
int sync()
return base_type::traits_type::eof() != overflow();
~basic_blob_buf()
sqlite3_blob_close(_blob);
;
typedef basic_blob_buf<char> blob_buf;
struct CanBeSerialized
std::string sometext;
std::vector<double> a_vector;
template<class Archive>
void serialize(Archive & ar, const unsigned int version)
ar & boost::serialization::make_nvp("sometext", sometext);
ar & boost::serialization::make_nvp("a_vector", a_vector);
;
#define MAX_BLOB_SIZE 256
sqlite3_int64 InsertRecord(sqlite3* db)
sqlite3_stmt *stmt = NULL;
int rc = sqlite3_prepare_v2(db, "INSERT INTO DEMO(ID, FILE) VALUES(NULL, ?)", -1, &stmt, NULL);
if (rc != SQLITE_OK)
std::cerr << "prepare failed: " << sqlite3_errmsg(db) << "\n";
exit(255);
else
rc = sqlite3_bind_zeroblob(stmt, 1, MAX_BLOB_SIZE);
if (rc != SQLITE_OK)
std::cerr << "bind_zeroblob failed: " << sqlite3_errmsg(db) << "\n";
exit(255);
rc = sqlite3_step(stmt);
if (rc != SQLITE_DONE)
std::cerr << "execution failed: " << sqlite3_errmsg(db) << "\n";
exit(255);
rc = sqlite3_finalize(stmt);
if (rc != SQLITE_OK)
std::cerr << "finalize stmt failed: " << sqlite3_errmsg(db) << "\n";
exit(255);
return sqlite3_last_insert_rowid(db);
sqlite3_blob* OpenBlobByRowId(sqlite3* db, sqlite3_int64 rowid)
sqlite3_blob* pBlob = NULL;
int rc = sqlite3_blob_open(db, "main", "DEMO", "FILE", rowid, 1/*rw*/, &pBlob);
if (rc != SQLITE_OK)
std::cerr << "blob_open failed: " << sqlite3_errmsg(db) << "\n";
exit(255);
return pBlob;
int main()
sqlite3 *db = NULL;
int rc = sqlite3_open_v2("test.sqlite3", &db, SQLITE_OPEN_READWRITE, NULL);
if (rc != SQLITE_OK)
std::cerr << "database open failed: " << sqlite3_errmsg(db) << "\n";
exit(255);
// 1. insert a record into a table, binding a "zero-blob" of a certain (fixed) size
sqlite3_int64 inserted = InsertRecord(db);
// 2. open the blob field in the newly inserted record
// 3. wrap the blob handle in a custom `blob_buf` object that derives from `std::basic_streambuf<>` and can be used with `std::ostream` to write to that blob
blob_buf buf(OpenBlobByRowId(db, inserted));
std::ostream writer(&buf); // this stream now writes to the blob!
// 4. serialize some data into the `ostream`
auto payload = CanBeSerialized "hello world", 1, 2, 3.4, 1e7, -42.42 ;
boost::archive::text_oarchive oa(writer);
oa << payload;
#if 0 // used for testing with larger data
std::ifstream ifs("test.cpp");
writer << ifs.rdbuf();
#endif
// 5. flush
writer.flush();
// 6. destruct/cleanup
sqlite3_close(db);
PS。我一直在处理错误……非常粗糙。您可能需要引入一个辅助函数来检查 sqlite3 错误代码并转换为异常。 :)
【讨论】:
您的回复只是强调了我错过的所有内容:(i)没有办法构建包含原始二进制数据的经典查询字符串(我应该怎么错过它,很明显),(ii)blob 有预定义大小(STL 容器让您忘记您必须关心数据实际占用的位置),(iii)SQlite C++ API 中有专用函数与 blob 字段交互(不知道从哪里开始,这密切相关(i))。我阅读了您的参考资料,我将在本周末实施。您的回答对我帮助很大,感谢您回复我的第一篇 Stack Overflow 帖子。 我今天早上做了睾丸,它就像一个魅力,开箱即用。再次感谢您,我现在将开始在我的界面中实现它。以上是关于如何将::序列化为 sqlite::blob?的主要内容,如果未能解决你的问题,请参考以下文章