我正在开展一项需要多项计划能力的科学项目。在浏览了可用的工具后,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,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,自制的纯文本文件数据库的大量数据:导入,转换,分析,报告。
现在我达到了我需要持久性的程度。所以我包含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)。
我想到了一些东西: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(总是存在),但我认为它可以是一个适合我需要的好解决方案。
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为blob。
在填写INSERT查询时,如何继续使用标准流?
答案 0 :(得分:10)
哈。我之前从未使用过sqlite3 C API。我从未编写过输出streambuf
实现。但看到我将来可能会在c ++代码库中使用sqlite3,我以为我会花一些时间与
cppreference http://en.cppreference.com/w/cpp/io/basic_streambuf
事实证明你可以 open a blob field for incremental IO。但是,尽管您可以读取/写入BLOB,但您无法更改大小(通过单独的UPDATE语句除外)。
因此,我演示的步骤变为:
blob_buf
的自定义std::basic_streambuf<>
对象中,并可与std::ostream
一起使用以写入该blob ostream
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
}
您将认识到所概述的步骤。
要测试它,假设您创建一个新的sqlite数据库:
sqlite3 test.sqlite3 <<< "CREATE TABLE DEMO(ID INTEGER PRIMARY KEY AUTOINCREMENT, FILE BLOB);"
现在,运行程序后,您可以查询它:
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
如果启用测试代码(放置的数据多于blob_size允许的数据),您将看到blob被截断:
contents truncated at 256 bytes
#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...非常粗暴。您将要引入一个帮助函数来检查sqlite3错误代码并转换为异常。 :)