Reading BLOB from SQLite3 no longer working

[Tectu]

Hi,

I've updated an application using SOCI from 9bcc5f8a9181886f4c73ea5b4671b35d8722cb3a to a55c80984a447bf8c788a7d6a23ff93c022069fa.

The only problem I encountered so far is with regards of reading a BLOB from an SQLite3 database.
Previously, the application did this (inside of a custom/user type conversion):

std::string str = v.get<std::string>("my_column");

Where:

• my_column is an SQLite3 column of type BLOB
• v is an lvalue reference to soci::base_type

This used to work well. However, after the update the code is throwing an std::bad_cast in soci::details::holder::get() (called by soci::row::get()).

According to the git log there have been some activities around blob support.

What is the new, correct way of reading (and writing!) a blob to/from SQLite3?

[vadz]

Thanks for reporting this, it's definitely a regression and should be fixed because I don't see anything wrong with this code.

I thought we had some testing code doing essentially the same thing, but I was clearly wrong. It would be great if you could please make a patch adding the (now) failing test case, this will help debug/bisect it and will ensure that it remains fixed in the future.

[Tectu]

Sure, I'll look into adding a test. Will take a while to get familiar with your testing setup tho.

I think the underlying "issue" or "cause" is that soci::details::holder::get() is clearly intentionally throwing when the type is db_blob:

soci/include/soci/type-holder.h

Lines 338 to 351 in a55c809

	case db_double:
	return soci_cast<T, double>::cast(*val_.d);
	case db_date:
	return soci_cast<T, std::tm>::cast(*val_.t);
	case db_blob:
	// blob is not copyable
	break;
	case db_xml:
	case db_string:
	return soci_cast<T, std::string>::cast(*val_.s);
	}
	throw std::bad_cast();
	}

There's a comment indicating that this is due to blob being non-copyable.

Seems like this was introduced in commit ae07cd5.